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Neurophysiological bases of the strength‐dexterity paradigm with the use of H‐reflex

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Neurophysiological bases of the strength‐dexterity paradigm with the use of H‐reflex

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Content Neurophysiological Bases of the Strength-
Dexterity Paradigm with the Use of H-reflex
Master’s Thesis
Degree Conferral Month:
May 2015

Submitted by:
Akira Nagamori
Biomedical Engineering
University of Southern California

Committee Members:
Prof. Francisco Valero-Cuevas
Prof. Gerald Loeb
Prof. James Finley

2

Contents
Chapter 1 ....................................................................................................................................... 4
Abstract ......................................................................................................................................... 4
1.1 Introduction ............................................................................................................................. 7
1.2 Methods .................................................................................................................................... 9
Subjects ................................................................................................................................... 9
Testing Measurements .......................................................................................................... 10
MVIC and Eccentric Contractions ........................................................................................ 14
Fatigue Assessment ............................................................................................................... 15
Testing Protocol .................................................................................................................... 15
Data Analysis and Statistics .................................................................................................. 16
1.3 Results .................................................................................................................................... 16
LED Test Performance ......................................................................................................... 17
H-reflex and EMG Variables ................................................................................................ 18
Fatigue ................................................................................................................................... 19
1.4 Discussion .............................................................................................................................. 19
Chapter 2 ..................................................................................................................................... 30
2.1 H-reflex .................................................................................................................................. 30
2.1.1 Scientific Bases of H-reflex ......................................................................................... 30
2.1.2 Methodological Considerations of H-reflex testing ..................................................... 33
References: ................................................................................................................................... 40
Acknowledgement ....................................................................................................................... 45

3

List of Figures
Chapter 1
Figure 1.1………………………………………………………………………………....6
Figure 1.2………………………………………………………………………………..,11
Figure 1.3………………………………………………………………………………...16
Figure 1.4………………………………………………………………………………...18
Figure 1.5………………………………………………………………………………...22
Chapter 2
Figure 2.1………………………………………………………………………………...36
Figure 2.2………………………………………………………………………………...38
Figure 2.3………………………………………………………………………………...41

List of Tables
Chapter 1
Table 1.1………………………………………………………………………………....13
Table 1.2………………………………………………………………………………....18
Table 1.3………………………………………………………………………………....19

4

Chapter 1
Eccentric contractions impede the ability of the leg to
regulate dynamic instabilities.

Abstract
Controlling instabilities with the legs (as per the Lower Extremity Strength-Dexterity (LED)
paradigm) is likely a product of a hierarchical organization of neural control, in agreement with
current thinking. A pilot study showed that LED performance deteriorates with 20 minutes of
downhill walking in the absence of fatigue. This suggests that the control of leg dexterity might
degrade with eccentric contractions—and may be a plausible mechanism for non-contact sports
injuries. However, this effect remains ambiguous because concentric and eccentric phases
coexist in the stance phase of downhill walking. Hierarchical control of instabilities involves
“low level” sub-cortical or spinal mechanisms. Such short-latency responses are often mediated
by tunable spindle afferents. The ability to perform short-latency corrections should, therefore,
be dependent on the gain and gaiting of spindle afferent signals. Therefore, we hypothesized that
purely eccentric contractions of the soleus muscle would affect spindle afferent gains; and
therefore preferentially affect a subjects’ ability to stabilize the leg. Hoffman’s reflex (H-reflex)
is a measure of spinally modulated gating of afferent signals onto alpha-motoneurone excitability
that is independent of fusimotor gain. We further hypothesized that effect of eccentric

5

contractions on spindle gating would be manifested in changes in H-reflex. We compared resting
H-reflex excitability and LED performance bilaterally in nine young adult male subjects
(25.4±3.8 yrs) before and after exposure to unilateral eccentric contractions at 15% maximal
voluntary contraction (MVC) vs. level treadmill walking. There was no measurable fatigue as
per objective MVC measurements (mean relative increase of 18.8±23.6%) and the subjective
Borg scale (mean value of 2.0±0.7 out of 10).
Despite an increase in MVC and no measurable fatigue, we found that eccentric
contractions had an adverse effect on the LED compression force (i.e., a reduction in the
maximal level of controllable instability) when compared with contralateral control and walking
condition. Thus, exposure to non-fatiguing, purely eccentric contractions disrupted the ability to
dynamically stabilize the leg. It did not, however, affect resting H-reflex. These results,
alongside with retained neural drive to the involved muscles measured by EMG signals, suggest
the decreased LED performance did not stem from deficit of the central neural drive or spinally
modulated gating of afferent signals. We speculate eccentric contractions lead to possible
fusimotor reprogramming of spindle sensitivity and therefore afferent signals. Our results compel
further studies to test these different levels in hierarchical organization of neural control of
instabilities. Moreover, such an adverse effect of eccentric contraction on the ability to control
instabilities may provide new insights into sensorimotor processing of proprioceptive signals in
the context of neuromuscular performance and injury mechanisms.

6

Figure 1.1: Schematic Representation of our study.

Eccentric
Concentric
Soleus
LED Test
Electrical Stimulation
Ia afferent
Presynaptic
Inhibition

Descending Command
Underlying Neurophysiological Mechanisms
? Fusimotor System
- Fusimotor reprogramming of muscle spindle
sensitivity and therefore afferent signals
Testing Paradigm
× Resting Spinal Excitability
- Presynaptic inhibition via group III and IV
afferents sensitive to muscle damage and fatigue
movements
? Corticospinal Pathway

Eccentric contractions impede the ability of the leg to regulate dynamic instabilities
? Muscle spindle sensitibity
muscle damage
Effects of eccentric contractions on LED test performance
Results
Hypothesis: purely eccentric contractions of the soleus
muscle would affect spindle afferent gains; and therefore
preferentially affect a subjects’ ability to stabilize the leg
Adverse effect on
LED test performance
Electrical Stimulation
Ia afferent
Presynaptic
Inhibition
γ
No effect on H-reflex
× Neural Drive to the muscle

7

1.1 Introduction
The ability to dynamically control instabilities with the isolated leg (as per the Lower
Extremity Strength Dexterity (LED) paradigm) (Lyle et al., 2013b) is a measure of an integrated
behavioral, sensorimotor performance. The LED test is specifically designed to require low force
and become increasingly unstable as a subject compresses the spring further. Furthermore, LED
test performance has shown to be independent of strength (Lyle et al. 2013b). Therefore, the
maximal level of LED compression force that subjects achieve is analogous to maximal
instabilities they can sustain (i.e., leg dexterity). Prior work shows that reduced lower limb
dexterity is indicative of decreased whole body agility in athletes (Lyle et al. 2013a) and may be
informative of unfavorable landing biomechanics often observed in female athletes (Lyle et al.
2014), which are though to increase the change of non-contact injuries (Laughlin et al., 2011).
Therefore, these results suggest that deficits in this particular sensorimotor performance might
provide plausible mechanisms of non-contact lower extremity injuries such as anterior cruciate
ligament (ACL) tears.
Lower limb dexterity seems highly susceptible to disruptions to the neuromuscular
system. A prior pilot study investigated effects of level and downhill walking on LED test
performance, showing decreased LED test compression force with 20 minutes of downhill
walking in the absence of fatigue. We speculated that the exposure to eccentric contractions
during downhill walking might have been the main cause of the decreased sensorimotor
performance. However, this effect remains ambiguous since there exist both concentric and
eccentric phases during the stance phase of downhill walking. Therefore, in this study, we seek
to substantiate this effect by employing purely eccentric contractions of the soleus muscle.

8

Effects of eccentric contractions on individual components of sensorimotor performance
have been studied extensively using the contralateral limb position- or force-matching method
(Brockett et al., 1997, Carson et al., 2002, Saxton et al., 1995, Weerakkody et al., 2003). It has
been shown that eccentric contractions induce deficits in the proprioceptive system (error in
limb-positioning estimation and in force estimation). Brockett and colleagues showed that these
deficits were induced by low-intensity eccentric contractions (120 repetitions at 20% MVC)
without a significant level of fatigue. These results have been attributed to peripheral
mechanisms, most likely in muscle spindle sensitivity (Brockett et al., 1997).
Controlling instabilities as per LED paradigm is likely a product of a hierarchical
organization of neural control (Mosier et al., 2011), in agreement of current thinking (Loeb et al.,
1999, Lawrence et al., 2014). The hierarchical organization of neural control represents
communications between different levels of sensorimotor processing and integration of those
signals. Therefore, it likely involves “low level”, sub-cortical or spinal mechanisms. Such short-
latency responses are often mediated by tunable spindle afferents. The ability of short-latency
corrections required in this task would, therefore, be dependent on afferent signals and their gain,
which is often modulated by the spinal cord. We hypothesized that these mechanisms
preferentially affect a subject’s ability to control instabilities. In this regard, it is highly likely
that disruption of proprioceptive inputs due to eccentric contractions previously reported
(Brockett et al., 1997, Carson et al., 2002, Saxton et al., 1995, Weerakkody et al., 2003) would
likely lead to the reduced LED performance. It has not been well documented, however, if non-
fatiguing eccentric contractions will cause modulation of gain of afferent signals.
The Hoffman reflex (H-reflex) is a measure of gain of Ia afferent signals onto alpha-
motoneurones modulated by spinal interneurons (spinal excitability). In the presence of fatigue

9

and muscle damage, indicated by lower force-generating capability of muscle and delayed-onset
muscle soreness (DOMS), spinal excitability would likely be depressed (Avela et al., 1999,
Vangsgaard et al., 2013). However, numerous factors introduced by fatigue and DOMS due to
the high-intensity eccentric exercise would not explain possible changes in gain of afferent
signals after low-force eccentric contractions employed in our study. Therefore, in this study, we
seek to articulate effects of non-fatiguing eccentric contractions on spinal excitability, which
might explain mechanisms related to possible deterioration of LED test performance.
The primary goals of this study were to articulate effects of eccentric contractions on the
sensorimotor ability to dynamically regulate instabilities and to identify possible neural
mechanisms responsible for expected disruption of such ability. We hypothesized that repetitive
low-force eccentric contractions would disrupt spindle afferent gains and such a disruption
would be manifested in H-reflex measurements; and therefore preferentially affect subjects’
ability to stabilize the leg. To our knowledge, this is the first study to investigate effects of
eccentric contractions on the integrated sensorimotor performance that has been proven to be an
important indicator of neuromuscular performance of athletes and possible injury risks.
Therefore, we hope that this study can provide implications of sensorimotor processing of
proprioceptive information to neuromuscular performance and non-contact injury mechanisms.
1.2 Methods
Subjects
Nine healthy male subjects (age 25.4 ± 3.8 years, height 1.77 ± 0.1 m, bodyweight 82.2
±16.2kg) participated in this study. The Health Science Campus Institutional Review Board at

10

the University of Southern California approved this study and all the subjects gave informed
consent upon participation.

Testing Measurements
Lower Extremity Dexterity (LED) Test
The LED test is described in detail elsewhere (Lyle et al., 2013b). In short, this test is
designed to test the ability of subjects to dynamically control instability with the isolated leg. It
requires low force (20% of bodyweight) and becomes increasingly difficult as being compressed.
A helical compression spring (Century Springs Corp., Los Angeles, CA) was mounted on a
uniaxial force sensor (Transducer Techniques, Temecula, CA) affixed to a stable base with a
platform affixed to the free end. The force sensor was connected to USB-DAQ (National
Instruments, Austin, TX) and the compression force was sampled at 2000Hz using a custom-
built program in MATLAB (The Mathworks, Natick, MA). Visual feedback of the compression
force was displayed on a computer screen in order for subjects to evaluate their performance
after each trial.
Standardized posture consistent with previous studies was assured for all subjects
throughout the experiment (Lyle et al. 2013, Lawrence et al. 2014). Subjects were partly seated
on a bicycle seat while equally distributing their bodyweight between the contralateral limb and
the bicycle seat (Figure 1.2). Subjects were allowed to lean on a strap at the height of the xiphoid
process and to hold safety bars on a squat rack for additional support. This positing was
employed to minimize the use of the contralateral leg and trunk and to provide additional
stability. The height of the bicycle seat was adjusted so that the foot of the contralateral limb
made a complete contact with the ground with the knee completely extended (0˚).

11

Subjects were asked to compress the spring with their maximal effort and to sustain the
highest achievable force level at least 5 seconds. Subjects were allowed as many practice trials as
needed for familiarization then they performed at least 10 trials for each test limb. Point-to-point
running averaging was applied to the raw compression force data during the hold phase of each
trial and the three maximal mean compression force trials were then averaged to represent the
LED test compression force (F
LED
). The LED test performance was also quantified by the
compression dynamics during the sustained compression as described in previous literature
(Lawrence et al., 2014) and pertaining dependent variables are summarized in Table 1.2. Means
of each variable from the three maximal mean compression force trials were used for further
analysis. We further analyzed the compression dynamics using the phase portraits (F
LED
vs. F vs.
F). The mean Euclidean distance from the origin during the sustained compression was
calculated. A greater Euclidean distance is indicative of corrective actions by the neuromuscular
controller enforcing the sustained compression (Dayanidhi et al., 2013, Lawrence et al., 2014).

Figure 1.2 Top: Resting H-reflex measurements were recorded in the soleus with bipolar surface electrodes.
Peripheral nerve stimulation was applied to the posterior tibial nerve in the popliteal fossa. For between subjects

12

comparisons, H-reflex amplitudes were normalized to the maximal M-wave. Lower Left: Participants were
positioned in a dynamometer with the hip and knee of the tested leg at 90˚ and the seat back was adjusted to a
comfortable position. They completed 500 eccentric contractions of the soleus at 15% MVC with the right leg; the
left was considered a within subjects control. Lower Right: Subject positioning and testing protocol during the
LED test were consistent with previous work.

EMG measurement
The electromyographic (EMG) activity of the soleus and tibialis anterior muscle was
recorded using self-adhesive surface electrodes (Myotronics Inc., Kent, WA). Electrode
placement for each muscle was as follows: 1 – 2 cm below the inferior aspect of the medial head
of the gastrocnemius (soleus) and 1 cm lateral to the tibia at one third of the line connecting the
fibula head and lateral malleolus (tibialis anterior). EMG signals were sampled at 2000Hz using
the same system used for the LED test. EMG signals were band-pass filtered (10 to 1000 Hz)
using 4
th
order Butterworth filter and rectified. The amplitude of EMG signals (EMG
AMP
)
calculated by root mean square (RMS) value of EMG signals during the sustained LED
compression as well as mean (MNF) and median power frequencies (MDF) of the EMG signals
was calculated.
H-reflex measurements
The Hoffman reflex (H-reflex) was obtained from the soleus by applying 1ms square
electrical pulses to the posterior tibial nerve through a bipolar stimulating electrode by a constant
current stimulator (Digitimer Ltd., Hertfordshire, England). The H-reflex response was attained
from raw EMG signals using the EMG acquisition system described above. The optimal
stimulation site was found by palpating the popliteal fossa and confirmed by appearance of H-

13

reflex response in the absence of direct motor response (M-wave). All the measurements were
taken from subjects in a seated position with the knee of the tested leg completely extended and
the foot on a step box (Figure 1.1). Throughout H-reflex measurements, subjects were asked to
remain in a consistent posture and to completely relax. Resting condition was chosen to
minimize effects of inputs from different pathways (e.g. the descending pathway and recurrent
inhibition from the homonymous muscle) during contraction on spinal excitability.
The recruitment curves for H and M waves were obtained by calculating their respective
peak-to-peak amplitudes with increasing stimulus intensities up to an intensity that induces the
maximal motor response (M
MAX
). M
MAX
was determined if an increase in stimulus intensity did
not produce a further increase in M-wave amplitude. This procedure was repeated for every time
and condition in order to keep track on possible changes in M
MAX
values (Pierrot-Deseilligny and
Burke, 2010). Test H-reflex was then measured in the presence of a small M wave, whose peak-
to-peak amplitude corresponded to 10% M
MAX
, in order to assure stimulus consistency. Stimulus
current was applied manually with random intervals greater than 2s to minimize the effect of
post-activation depression on H-reflex size (Pierrot-Deseilligny and Burke, 2010). The
acceptance range of the target M-wave was set to 10%+/-1% M
MAX
. The measurements were
repeated so as to obtain 20 stimulations that produced acceptable M-wave amplitudes. The test
H-reflex responses were then averaged and normalized to M
MAX
(H/ M
MAX
).

Variable Symbol Description
LED compression
force
F
LED
Mean compression force during the sustained compression
Force velocity F Mean of absolute value of the first derivative of LED compression force per
unit time
Force acceleration F Mean of absolute value of the second derivative of LED compression force per

14

unit time
Force RMS RMS
F
Mean amplitude of dispersion of LED compression force
Euclidean Distance ED Mean distance from the origin in the phase portraits
H-reflex amplitude H/M
MAX
Mean H-reflex amplitude normalized to M
MAX

EMG amplitude EMG
AMP
Mean RMS of EMG signals during the sustained compression
EMG Mean power
frequency
MNF Mean of mean power frequency of EMG signals during the sustained
compression
EMG Median power
frequency
MDF Mean of median power frequency of EMG signals during the sustained
compression
Table 1.1: Definition of dependent variables
MVIC and Eccentric Contractions
Plantarflexion maximal voluntary isometric contraction (MVIC) and eccentric
contraction of the soleus muscle were performed using a Humac Norm Dynamometer (CSMi,
Stoughton, MA). Subjects were seated with the hip and knee joint of the tested leg at 90˚ and
against an incline set to a comfortable position (Figure 1.1). Both of their thighs were secured to
the dynamometer with straps.
MVIC was measured at the neutral position of the ankle. Subjects were allowed five
practice trials and then executed three 5-second MVIC trials. Subjects were instructed to ramp up
torque as quickly as possible and hold the torque level for about three seconds while verbally
encouraged. The highest torque value was taken from the three trials. After familiarization trails
(10 repetitions), subjects completed 500 eccentric contractions at 15% MVIC torque in the same
posture as in the MVIC trails.

15

Fatigue Assessment
Ratio of pre- to post- MVIC torque values was calculated to assess fatigue level of
subjects after eccentric contractions. Also, subjective assessment of fatigue (Borg scale) was
obtained after eccentric contractions and walking. The Borg’s scale was scored between 0 – 10
(Grant et al., 1999).
Testing Protocol
Subjects attended two separate sessions in which they were exposed to three different
conditions: eccentric contraction, contralateral control and walking conditions. In eccentric
contraction condition, subjects performed the non-fatiguing eccentric contractions with their
right leg as described above and their left leg was used as a contralateral control without any
exposure. The LED test performance and H-reflex measurements were obtained bilaterally
before and after the exposure. In walking condition, subjects walked on a level ground for the
same duration needed to complete 500 eccentric contractions. Only the right leg was tested for
the LED test and H-reflex measurements before and after walking. The order of testing
measurements is summarized in Figure 1.3. This order was kept consistent across subjects in
order to minimize the difference in intervals between exercise and each testing measurement
across subjects. After eccentric contraction, H-reflex measurement and LED test on the exposed
leg were performed first.

16

Figure 1.3: Order of testing measurements. Durations of each testing measurement required to test for each leg are also noted.
Data Analysis and Statistics
Two by three repeated measures analysis of variance (ANOVA) was performed to
examine significant main effects of time and conditions and interactions between time (pre/post
exposure) and condition on LED test performance and H-reflex. If a statistically significant main
effects or level of interaction was observed, post hoc analysis was performed to compare
different combinations of main effects. Specifically, one-way repeated measures ANOVA or
paired t-test was repeated at each level of each factor (time and condition). In the case of one-
way repeated measures ANOVA, pair-wise comparisons with Bonferroni correction were further
performed. Statistical significance was set for p ≤ 0.05. All of these statistical analyses were
performed using R software (v 3.1.1, R Development Core Team, 2010).

1.3 Results
One subject was excluded from further data analyses because his change score
(difference between pre and post values) in the LED compression force exceeded 2.2 times the
inter-quartile range in the eccentric contraction condition.
LED test
H-reflex
Eccentric contractions/Walking
LED test
H-reflex
15min
20 - 30 min
10 - 15 min
12 - 15 min
20 - 30 min
10 - 15 min

17

LED Test Performance
Two by three repeated measures ANOVA shows a significant level of interaction
between time and condition in the LED compression force (p=0.006), force velocity (p=0.035),
the mean Euclidean distance (p=0.040) and a measure of force variability defined as the root-
mean-square (RMS
F
) of the force signal (p=0.015). The results of post hoc analysis on the LED
compression force are shown in Figure 1.4. The significant simple main effect of condition was
observed at both points in time. While the LED compression force was significantly lower in the
contralateral control compared to the other two conditions before exposure (p < 0.05), a
significantly higher LED compression force in walking condition compared to the other two
conditions was observed after exposure (p < 0.05). The significant simple main effect of time
was only seen in the contralateral control condition (p < 0.05).

Figure 1.4: Pre and post LED compression force for each condition. The asterisk above a line connecting pre and post data
points represents a significant effect of time.
140
145
150
Pre Post
Time
LED Compression Force (N)
Condition
Eccentric Contraction
Contralateral Control
Walking
*
*
*
* p < 0.05
*
*

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Table 1.2: Result of post hoc analysis on the LED compression dynamics. * denotes a statistical significance.
The results of post hoc analysis on the compression dynamics are summarized in Table
1.2. All the variables of the compression dynamics followed the same trend that those in
eccentric contraction condition was significantly higher than walking condition before exposure
and their values tend to decrease after exposure to eccentric contractions.
H-reflex and EMG Variables
There was no significant level of interaction between time and condition in H-reflex
EMG variables except for the RMS of the EMG signal of the soleus with a marginally significant
interaction between time and condition (p=0.099). The RMS of the EMG signal of the soleus in
eccentric contraction condition showed a trend of a significant decrease after the exposure
(p=0.056). Table 1.3 summarizes relative changes of each variable for the soleus muscle.
Contralateral Control Eccentric Contraction Walking
Variable Time Condition
𝐅
Eccentric: p=0.097
Control: p=0.191
Walking: p=0.637
Pre: Eccentric vs. Walking
*p=0.045
ED
Eccentric: p=0.154
Control: p=0.249
Walking: p=0.775
Pre: Eccentric vs. Walking
*p=0.039
RMS
F

Eccentric: *p=0.031
Control: p=0.421
Walking: p=0.563
Pre: Eccentric vs. Walking
*p=0.019

19

H/M
MAX

76.6±83.1 59.7±18.7 -12.6±27.6
EMG
AMP
4.1±19.4 -15.2±20.1 -5.6±14.8
MDF 9.5±16.9 12.2±22.4 10.6±17.3
MNF 7.7±12.6 9.3±13.3 -0.3±12.4
Table 1.3: Relative changes (post/pre*100) of H-reflex and EMG variables of the soleus
In order to elucidate the relationship between the decreased EMG amplitude of the soleus
and the decreased LED compression force, Pearson’s product-moment correlation between
relative changes (Post/Pre*100) of EMG amplitude of the soleus and LED compression force in
eccentric contraction condition was computed. We found no correlation between those two
values (ρ=-0.01, p=0.986).
Fatigue
The Borg scale and change in MVIC torques pre- and post-eccentric contraction were
used to assess presence of fatigue after eccentric contraction and walking. The average Borg
scale score from 0 to 10 was 2.0±0.7. Relative increase in plantarflexion MVIC torque was
18.8±23.6%. Both subjective and objective assessment of fatigue showed no indication of fatigue
after eccentric contractions, except for one subject whose MVIC decreased (relative change of -
16.2%). None of the subjects reported soreness in the soleus after eccentric contractions.
1.4 Discussion
The primary purpose of this study was to disambiguate the adverse effects of eccentric
contractions on LED test performance. The decrease in the LED compression force (2.84N) due
to eccentric contractions was not statistically significant (p=0.127). A statistically significant
interaction between time and condition on the LED compression force, alongside the improved

20

performance in the contralateral control condition (p<0.05), suggested disruption of the ability to
dynamically control instabilities due to eccentric contractions overrode an expected exposure
effect observed in the contralateral control condition. Moreover, reduced LED compression
dynamics after eccentric contractions indicated that the decreased extent of instabilities that
subjects can control was accompanied by less variability during the test.
It is important to note that some confounding factors may have affected our results. First,
the exposure effect observed in the contralateral control condition (p<0.05) might suggest
possibility of an exposure effect in the eccentric contraction condition, which could have
obscured the true effect of eccentric contractions on the LED compression force. This exposure
effect contradicted the consistency of LED performance previously reported in the literature
(Lyle et al., 2013). This phenomenon might be due to the fact that eight out of nine subjects had
no experience with the LED test prior to the eccentric contraction session. This particular
population might have required more practice trials than those who have prior experience.
Although their LED performance appeared to be saturated, the number of trials allowed in the
first session might not be sufficient to reach their best performance. Second, the use of the
contralateral limb as a control condition might have contributed to this phenomenon as well.
Contrary to previously published literature (Lawrence et al., 2014), there was a significant
difference in LED compression force between limbs as indicated by the differences we report
between eccentric contraction and contralateral control conditions. It might be possible that it
takes more practice to learn how to perform the LED test with the non-dominant limb than the
dominant limb. This difference might also be attributed to characteristics of the population
tested. While the study by Lawrence et al. (2014) employed more active population (former and
current elite skiers), vast majority of the subjects (5 out of 8) who participated in this study was

21

sedentary and had no prior active involvement with sport activities. However, further
investigations are needed to validate these arguments. Third, lack of a statistical significance for
the reduced LED compression after eccentric contractions might be due to our choice of
modality of eccentric exercise. The primary objective of the eccentric exercise was to not induce
any fatigue effect. The intensity and the number of repetitions used, therefore, might not have
been strenuous enough to induce the same level of disruption as downhill walking did
previously, indicated by the change in LED compression force lower than the previously
reported detectable difference of 5.5N (Lyle et al., 2013b). Finally, lack of statistical power
(n=8) might also have contributed to these results.
Building upon observation of the deleterious effect of eccentric contractions on LED
performance, we further attempted to interrogate its neurophysiological mechanisms with the use
of H-reflex as we hypothesized that changes in spinal excitability due to eccentric contractions
would preferentially affect the ability to perform short-latency corrections required in the tasks.
We showed that modulation of spinal excitability was not affected by eccentric exercise,
indicating dissociation between the adverse effect of eccentric contractions on LED test
performance and modulation of spinal excitability. Neurophysiological mechanisms responsible
for these results are summarized in Figure 1.5 and will be discussed further below.

22

Figure 1.5: Neurophysiological mechanisms that might have been responsible for the adverse effect of eccentric contractions on
LED performance as well as H-reflex are shown in expected time-course of each effect in relation to our testing protocol. These
mechanisms are discussed in detail in the text.

Previous studies have revealed unique mechanisms of eccentric contractions and
neuromuscular control during the movement (see review by Duchateau and Baudry, 2014).
Therefore, eccentric contractions would likely have a multitude of unique neurophysiological
consequences. Especially, higher degree of fatigue after high-intensity eccentric exercise has
received a great attention among literature. The low-force generating capability and
accompanying changes in EMG activity have often been observed after repetitive eccentric
contractions, presumably due to the use of high intensity modality (e.g., 50 repetitions at
85%MVC) (Carson et al., 2002, Weerakkody et al., 2003). This lower force-generating capacity
has been considered as a potential mechanism for deficit in motor performance introduced by
eccentric contractions (Carson et al., 2002, Weerakkody et al., 2003). Moreover, it has been
End of Eccentric Exercise H-reflex Testing LED test
15min
30min
Post-activation depres-
sion
2 - 3min
Fusimotor after-effects 10min
Presynaptic inhibition
of Ia terminals
20 - 30min
Activity depedent hyper-
polarization
1 hour
20 - 30min
Reduced stretch reflex
response
20 - 30min

23

reported that reduced spinal excitability, presumably presynaptic in its origin, occurs after
eccentric exercise (Vangsgaard et al., 2013). It has been attributed to an inhibitory effect of
chemosensitive group III and IV muscle afferents activated by by-products of fatigue (Pettorossi
et al., 1999, Vangsgaard et al., 2013).
In this study, we designed the protocol of eccentric contractions such that they would not
induce peripheral or central fatigue. Evidence provided in this study suggests that fatigue and
pertaining modulation of spinal excitability cannot contribute to the reduced sensorimotor
performance in the LED test. Subjects were able to sustain, or even increase, plantarflexor MVIC
torque level after eccentric contractions. The increased platarflexor torque might be indicative of
the enhanced isometric torque production after muscle stretch or an eccentric contraction
reported previously (Lee et al., 2002, Hahn et al., 2010 & 2012). However, the residual force
enhancement would not affect the LED performance, as it has been observed only immediately
following muscle stretch or eccentric contraction. Moreover, properties of EMG signals during
the LED test (EMG
AMP
, MNF, MDF) did not change with eccentric contractions. This result
implies that levels of neural drive to the soleus and tibials anterior during the LED test were
sufficient to perform the test as equally well as before eccentric contractions. We also showed
that modulation of spinal excitability was not affected by eccentric exercise, indicating
dissociation between the reduced LED performance and modulation of spinal excitability. Since
we did not observe a sign of fatigue after eccentric contractions, presynaptic inhibition of alpha-
motoneurone excitability through group III and IV afferents might have been absent. Also, even
if residual effects of fatigue on H-reflex modulation might have been present immediately after
eccentric contractions, it would have decayed away as it took about 15 minutes to initiate H-
reflex measurements after eccentric contractions.

24

Given the same level of fusimotor activity between contraction modalities, eccentric
contractions involve augmented activation of spindle afferents compared to concentric
contractions or isometric contractions (Hulliger et al., 1985). Repetitive activation of spindle
afferents through cyclic movements could induce post-activation depression of H-reflex (Trimble
and Harp, 1998), which can last up to a few minutes. Considering the interval between the end of
eccentric contractions and initiation of H-reflex testing in this study, this effect could not
influence our result (Figure 1.5). Another mechanism that could potentially affect H-reflex
response, unrelated to gain of Ia afferent signals onto alpha-motoneurones (presynaptic), might
be activity-dependent hyperpolarization of the Ia afferent and motor axons (Kuwabara et al.,
2002). Although this effect could be long lasting (tens of minutes), changes in electrical
threshold of Ia afferent and motor axons should have been addressed through the standardized H-
reflex testing procedure to determine stimulus intensity used to elicit H-reflex response. Longer-
lasting depression of H-reflex (up to 20 - 30 minutes), presumably presynaptic in its origin, has
been reported after 30 minutes of rhythmic arm movements (Javan and Zehr, 2008) and one hour
of muscle or tendon vibration (Lapole et al., 2012, Uchiyama et al., 2005). This mechanism
might have sustained during the H-reflex testing in our study but might have decayed away
before the LED test and therefore cannot explain the decreased LED test performance.
Furthermore, the absence of H-reflex modulation after eccentric contractions suggested that
duration (15 minutes) of the exercise might have been insufficient to induce such an effect in this
study, granted that neither cyclic movements tendon nor muscle vibration might be analogous to
eccentric contractions in terms of their neurophysiological effects.
It can be speculated that the adverse effect of eccentric contractions on LED performance
could have been originated at mechanisms involved in muscle spindle and fusimotor system. The

25

ability to perform short-latency corrections required in the LED test would likely depend on
afferent signals from peripheral receptors as much as their gain on alpha-motoneurones. It has
been postulated that repetitive eccentric contractions or stretching of muscle could increase the
compliance of extrafusal fibers and possibly intrafusal fibers (Pusson et al. 1990, Wood et al.,
1993) and thereby decrease the sensitivity of muscle spindles to muscle stretch (Avela et al.,
1999 & 2004, Morgan and Allen, 1999, Proske and Morgan, 2014, Whitehead et al., 2001). The
exact mechanism of the change in compliance has been under debate. Proske and colleagues
have shown that repetitive eccentric contractions, or even a single bout of eccentric contraction,
can lead to muscle damage (Gregory et al. 2003) and they attributed it to the cause of increased
compliance. They further showed that errors in proprioception could be introduced by even
moderate-intensity eccentric contractions similar to our study (Brockett et al. 1997). According
to this study, when subjects were asked to match the position of both arms, which were exposed
to either eccentric or concentric contraction at 20%MVC, they placed the eccentrically-exercised
arm into more extended position compared to the concentrically exercised arm. However, it is
unlikely that eccentric contractions at such a low force used in our study could have led to
muscle damage as none of the subjects reported noticeable symptoms such as stiffness and
soreness (Clarkson et al., 1991). Avela and colleagues (1999) demonstrated that prolonged
passive stretching (1 hour) could induce depression of stretch reflex response presumably due to
increased compliance of extrafusal and possible intrafusal fibers without having indirect
evidence of muscle damage. They speculated that structures other than extrafusal fibers (e.g.,
connective tissues such as titin) would be responsible for the increased compliance (Avela et al.,
1999 & 2004). Despite the lack of evidence for the exact mechanisms, their results have great
implications to our study. Importantly, the reduction in stretch reflex response persisted after

26

fatigue effect has abolished (at least 30 minutes after passive stretching) while decrease in H-
reflex response was only observed immediately after. The residual effect on H-reflex could be
attributed to presynaptic inhibition of Ia terminals due to fatigue as it disappeared with
abolishment of fatigue. Although there is an important distinction between this study and our
study in that passive stretching does not involve active contraction of muscle, these observations
would help explain our findings. Also, based on the study by Avela et al. (1999), it remains
speculative that the decreased stretch reflex response might have been present during the LED
test in this study. The adverse effect of eccentric contractions on LED test performance might be
due to lowered spindle sensitivity and therefore we did not observe any change in H-reflex
response.
Another possible mechanism that could change muscle spindle sensitivity is fusimotor
system as activation of gamma motoneurones directly influences sensitivity of primary and
secondary spindle endings (Hulliger et al., 1977). Co-activation of alpha and gamma
motoneurones exists during muscle contraction and the pattern varies depending on movement
patterns (Murphy and Martin, 1993). It is reasonable to assume, therefore, that eccentric
contractions involve a distinct pattern of alpha-gamma co-activation. Fusimotor aftereffects,
residual effects of stimulation of fusimotor axons on discharge of muscle spindle endings, and
therefore reprogramming of fusimotor system have been observed (Emonet-Denand and Laporte,
1984). However, this aftereffect can last for less than 10 minutes and would likely be diminished
by the time of the LED test in this study (Emonet-Denand and Laporte, 1984). Also, it is
important to note that MVC trails followed by eccentric contractions and a number of trials
required to obtain LED test performance might have reset fusimotor system once again.

27

Therefore, this effect might not be a feasible candidate responsible for the adverse effect of
eccentric contractions on LED test performance.
It is important to note that sources of afferent signals comprise two types of muscle
spindle endings (primary and secondary) and Golgi tendon organs. Traditionally, these receptors
are categorized based on their responses to different stimuli. However, recent evidence suggests
that their responses are rather complicated and would not be a 1:1 relationship to corresponding
stimuli (Fallon and Macefield, 2007, Gregory et al., 1977). To our particular interest, Golgi
tendon organs also found to be sensitive to muscle stretch. Moreover, it has been shown that
Golgi tendon organs could provide feedback regarding changes due to muscle damage from
eccentric contractions (Gregory et al., 2001 and 2003). Although muscle damage would likely be
minimal, possible contribution of Golgi tendon organs as a monitor of tension during the LED
test and effects of eccentric contractions on this system need to be considered in the future study.
An intriguing aspect of the results of our study is that isolated eccentric contractions of
one muscle group (the soleus and gastrocnemius) and presumably associated disruption of the
pertaining afferent pathways resulted in the reduced sensorimotor ability in the behavioral task
that involves multiple muscles at multiple joints. The extent of disruption in this study is likely
smaller than more extensive disruption of the multiple afferent pathways expected from downhill
walking because eccentric contractions occur not only in the calf muscles but also in the
quadriceps muscles. High sensitivity of this particular behavioral performance to eccentric
contractions might reflect the highly integrated sensorimotor ability required in this task. The
LED test requires continuous calibration of efferent signals based on proprioceptive information
signaled through afferent pathways originated in multiple muscles. Deliberate modification of
afferent signals with the use of thixotropic effect of intrafusal and extrafusal fibers has suggested

28

that the CNS does not process afferent information in terms of individual muscles, but rather it
concerns difference in afferent signals within a functional unit (e.g. agonist vs. antagonist or
right arm vs. left arm) (Proske et al., 2014). Following this line of thought, the CNS might
consider from all the involved muscles at multiple joints as one functional unit and process the
integrated afferent signals as a proprioceptive input. Therefore, although disruption of afferent
pathways would likely be limited to one of the muscle groups, it is sufficient to disturb the
functional unit as a whole, which was then manifested in the deteriorated LED performance.
We believe that the reduced ability to control instabilities reported in this study would
provide new insights into non-contact injury mechanisms. To best of our knowledge, this is the
first time to document an adverse effect of low-intensity eccentric exercise on sensorimotor
performance in a behavioral task. Previous studies investigating individual components of
sensorimotor performance or specifically examining afferent signals from muscle proprioceptive
systems in relation to eccentric exercise (Brockett et al., 1997, Carson et al., 2002, Gregory et al.,
2001, Gregory et al., 2003, Saxton et al., 1995, Weerakkody et al., 2003) have helped us identify
plausible mechanisms of disruptive effects of eccentric exercise. However, they have inherent
limitations in their applicability to clinical problems such as non-contact injuries because clinical
problems are often a manifestation of an integrated sensorimotor behavior. Prior work has
proven the validity of the LED test as a useful technique to quantify differences in the lower
extremity dexterity among different populations and possible relation of leg dexterity to non-
contact injuries. Lyle et al. (2013b) proposed the reduced lower extremity dexterity observed in
female soccer players as a potential contributor to their use of undesirable landing biomechanics,
which could be one of the possible explanations for higher incident rate of the anterior cruciate
ligament (ACL) tears in female athletes (Agel et al., 2005, Borowski et al., 2008, Yard et al.,

29

2008). The adverse effect of low-force, repetitive eccentric contractions on the LED test shown
in this study might highlight the importance of corrective movements in LED test performance
based on afferent signals originated in proprioceptive receptors in muscles. Therefore, it might
have implications about potential contributions of sensorimotor processing of proprioceptive
information to mechanisms of non-contact injuries.
Repetitive eccentric contractions are often considered as an important mechanism
involved in strain injuries (Opar et al., 2012). It has been thought that both accumulation of
muscle damage and an incident where supramaximal mechanical stress is applied to the muscle
result in strain injuries. The finding of this study might add a potential role of neuromuscular
control in strain injuries. The intensity of eccentric contractions employed in this study was
designed to be low, so that even 500 eccentric contractions maintain the muscle damage at the
minimal level. As discussed above, however, these repetitive eccentric contractions might lead to
the reduced leg dexterity and therefore unfavorable biomechanical strategy in a situation where
an athlete is at higher risk of such injuries. The unfavorable biomechanical strategy would lead to
the supramaximal mechanical stress on the muscle or connective tissues. Therefore, it can be
speculated that repetitive eccentric contractions not only induce microscopic muscle damage, but
also increase the chance of such an inimical incident.

30

Chapter 2
Additional Materials
The first chapter is written in a manuscript format regarding our study that investigated effects of
eccentric contractions on the ability of control instabilities and pertaining changes in
neurophysiological mechanisms. The aim of this chapter is to provide more in-depth descriptions
of the H-reflex technique in the hope to provide an aid for future experimenters. This chapter
will discuss about methodological considerations of the H-reflex as well as brief overview of its
scientific base.
2.1 H-reflex
Our study presented in the first chapter is originally derived from an ongoing research project
where we examine neurophysiological bases of the Strength-Dexterity (SD) paradigm. The
research project involves both upper and lower extremities, and therefore this section goes
beyond the scope of our study and covers methodological considerations regarding both upper-
and lower-extremity H-reflex measurement. For those who are interested in the use of H-reflex
technique, it is highly encouraged to see Knikou (2008), Palmieri et al. (2004) and Pierrot-
Deseilligny and Burke (2012) for more detailed explanations. The information contained in this
section is obtained from those three references if not specified.

2.1.1 Scientific Bases of H-reflex
H-reflex is a monosynaptic reflex response of homonymous motoneurones induced by an
electrical stimulation of Ia afferent fibers located in a pertaining peripheral nerve (Figure 2.1).

31

This response is akin to mechanically induced stretch reflex but different in that H-reflex
bypasses the muscle spindle and therefore the influence of fusimotor activity. Therefore,
modulation of H-reflex is considered to occur primarily at the spinal cord. The H-reflex is a
measure of gain of Ia afferent signals onto alpha-motoneurone excitability (spinal excitability).
The reflex pathway seems simple at the first glance, as the reflex pathway is often shown as in
Figure 2.1 for the sake of simplicity. However, this pathway involves many pathways that could
potentially influence H-reflex response, such as presynaptic inhibition of Ia terminals, recurrent
inhibition and reciprocal inhibition. Therefore, when interpreting changes in H-reflex size, all the
possible mechanisms have to be taken into account.

Figure 2.1: Schematic representation of H-reflex pathway. Obtained from Pierrot-Deseilligny and Burke (2012).
Electrical stimulation of peripheral nerves will excite both Ia afferent axons and efferent
axons. Excitation of efferent axons produces two responses, one propagating to the muscle
(orthodromic) and the other propagating to the spinal cord (antidromic) (Figure 2.1). The
orthodromic response appears on an EMG trace with a latency shorter than the H-reflex because
of the shorter distance of efferent pathway from stimulating site to the muscle. This is called
muscle response (M-wave). Amplitude of M-wave increases with increasing stimulus intensity
Electrical Stimulation
Ia afferent
Presynaptic
Inhibition
γ

Antidromic

32

and reaches a plateau, which is called maximal muscle response (M
MAX
). M
MAX
represents
excitation of the entire alpha-motoneuone pool. The antidromic response travels in a “wrong”
direction to the spinal cord and collides with reflex response along the efferent pathway
(antidromic collision). This will cause a decrease in H-reflex response with increasing stimulus
intensities. Since Ia afferent fibers have lower threshold to electrical stimulation than efferent
fibers, Ia afferents are excited first at lower stimulus intensity and therefore there is a range
where only H-reflex can be observed in EMG trace. While excitation of alpha-motoneurones by
afferent signals follows the orderly recruitment (i.e., from small motor units to large motor
units), the direct electrical stimulation of motor axons is thought to be reverse in the order (i.e.,
from large motor units to small motor units). Therefore, above certain stimulus intensity, H-
reflex response continues to increase in its magnitude despite appearance of M-wave and that of
antidromic response, as they do not share the same motor units. Once the magnitude of an
antridromic volley reaches a certain level, H-reflex response will be completely abolished. These
phenomena can be visualized in the recruitment curves of H-reflex and M-wave, where their
amplitudes are plotted against stimulus intensities (Figure 2.2).

33

Figure 2.2: H-reflex and M-wave recruitment curves. Amplitudes of both responses at each stimulus intensity were plotted here.
2.1.2 Methodological Considerations of H-reflex testing
H-reflex recordings
Preparation of H-reflex recording follows a standard procedure for the electromyogram
(EMG) technique. An adhesive surface electrode is placed over the muscle of interest after
securing proper impedance by cleaning the skin with abrasive gel and alcohol. In our study,
EMG signals were obtained at 2000Hz. H-reflex response can be identified from raw EMG
signals.
Square pulses of 1ms duration to the peripheral nerve through a bipolar stimulating
electrode by a constant current stimulator (Digitimer Ltd., Hertfordshire, England) are used to
induce H-reflex response of the homonymous muscle. The duration of the pulses might need
some adjustment depending on the muscle of interest. The duration of 1ms and 0.5ms is
recommended for the lower-extremity muscles and the upper-extremity muscles, respectively.
0
1
2
3
0 10 20 30 40 50
Stimulus Intensity(mA)
Amplitude(V)
Mwave
Hreflex

34

Standard procedure for H-reflex recordings consists of locating proper stimulating site,
identification of M
MAX
, and H-reflex testing. The location of proper stimulating site is found
through palpation of the parent peripheral nerve and observation of reflex response from the
muscle. For example, if a stimulating electrode is correctly placed over the posterior tibial nerve,
its stimulation produces a clear plantarflexion movement at the ankle joint through activation of
the soleus muscle. If not, it might produce eversion of the ankle joint by stimulating the deep
fibular nerve to the anterior tibialis muscle. It is further ensured by identification of H-reflex
response in the absence of M wave from EMG signals. Although reflex response of the lower-
extremity muscles tend to be easily identifiable, that of the wrist muscles is not always so for
several reasons, 1) latency of H-reflex response and M-wave, 2) difference in patterns of H-
reflex response, and 3) requirement of background EMG. These points are discussed later in
detail.
Once the stimulating site is determined, the recruitment curves for H-reflex and M-wave
are obtained by calculating their respective peak-to-peak amplitudes (Figure 2.2). Although this
process is not necessary to identify M
MAX
, the recruitment curves are a valuable tool to
distinguish H-reflex response from EMG artifact and M-wave and to estimate the location of
ascending limb of the H-reflex recruitment curve in relation to stimulus intensity and M-wave. In
our study, stimulus intensity is increased from 2 to 20mA with increment of 1mA and from
20mA up to the intensity required to induce M
MAX
with increment of 2mA. M
MAX
is determined
if increase in stimulus intensity does not produce further increase in M-wave amplitude.
Identification of M
MAX
needs to be performed before every H-reflex testing to accommodate a
possible change in M
MAX
most likely due to change in position of stimulating electrode with
respect to the peripheral nerve.

35

Test H-reflex is measured in the presence of a small M wave, whose peak-to-peak
amplitude corresponds to 10% M
MAX
, in order to assure stimulus consistency. The acceptance
range of the target M-wave is set to +/-10% of the 10% M
MAX
. It is assumed that, when the
constant number of motor axons is stimulated by the test stimuli, the constant number of afferent
axons would likely be so. Therefore, changes in H-reflex size due to variability in composition of
afferent signals can be minimized. The measurements are repeated so as to obtain 20
stimulations that produced the acceptable M-wave amplitudes. The test H-reflex responses are
then averaged and normalized to M
MAX
(H/ M
MAX
). This allows for comparisons of H-reflex
response within and across subjects and between sessions.
Latency
H-reflex response and M-wave assume a tri-phasic pattern and appear in certain latencies
(duration of response to appear on EMG trace after stimulation) (Figure 2.3). Latencies of H-
reflex response and M-wave depend on the distance of their respective pathway and therefore
limb length of a subject. Although this would not be an issue when H-reflex is obtained from the
soleus as done in our study, it can be for other muscles such as ones in the upper extremity.
Shorter distance of M-wave pathway could make it difficult to distinguish stimulation artifact
and M-wave response. Also, shorter reflex pathway of the upper extremity might cause H-reflex
to reach to the muscle before after-hyperpolarization of M-wave ends.

36

Figure 2.3: Typical H-reflex and M-wave responses obtained from the soleus.

We developed custom-built MATLAB (The Mathworks, Natick, MA) program to
identify those responses and calculate their peak-to-peak amplitudes, which can also generate the
recruitment curve of H-reflex and M-wave. This requires manually selecting windows for each
response to minimize errors in calculation due to stimulus artifact and difference in latencies
between subjects.
Pattern of H-reflex response
Another factor that one needs to take into consideration when obtaining H-reflex
response from the upper extremity muscles is that it does not always assume the same tri-phasic
pattern as one from the lower extremity muscles. Therefore, identification of H-reflex response
from EMG signals might require tracking changes in amplitude of EMG signals with changing
stimulus intensity at the latency expected for the muscle of interest (Miller et al., 1995). Also, H-
-0.5
0.0
0.5
0 20 40 60
Time(msec)
Amplitude(V)
M-wave
H-re!ex

37

reflex of the upper extremity muscles might not have distinct peaks and therefore needs be
calculated otherwise, such as distance from a peak to baseline.
Effects of sub-threshold excitability of alpha-motoneurones
One of the advantages of H-reflex is that the response can be elicited in various
conditions (e.g., at rest or during movement) and therefore it can be used as a valuable tool to
investigate how the reflex pathway is modulated according to different task requirements.
However, one needs to pay a special attention to effects of sub-threshold excitability of alpha-
motoneurones when testing H-reflex modulation in different conditions.
H-reflex of the soleus can easily be obtained from a subject at rest. In our study, we chose
to test H-reflex modulation at rest as it can minimize influences of the supraspinal centers and
inhibitory spinal interneurons induced by contraction of homonymous muscle. However, there is
a drawback to resting H-reflex that sub-threshold excitability of alpha-motoneurones cannot be
controlled. The sub-threshold excitability of alpha-motoneurones has a significant effect on H-
reflex size and can vary within and between subjects. It has been recommended, therefore, to
have a weak voluntary contraction of a homonymous muscle (10%MVC) during H-reflex
testing, so that the sub-threshold excitability of alpha-motoneurones can be maintained.
The sub-threshold excitability of alpha-motoneurones also needs to be taken into account
when one seeks to compare H-reflex modulation between various conditions. It is sometimes the
case that there is a difference in levels of background EMG activity between conditions (e.g.,
walking vs. running). This difference would not only affect H-reflex size but also alter influences
of supraspinal centers and inhibitory spinal interneurons as mentioned previously. These
mechanisms would likely obscure effect of different task requirement between conditions (e.g.

38

stable vs. unstable) on H-reflex modulation. Therefore, it is generally recommended to match the
level of background EMG activity between conditions if possible.
H-reflex response on the ascending limb
One of the limitations of our study was that H-reflex response might not have been
controlled to occur on the ascending limb of the recruitment curve. Measuring H-reflex response
on the ascending limb has been strongly suggested, as the slow motoneurones primarily
responsible for H-reflex are less sensitive to excitation and inhibition and can be influenced by Ib
and recurrent inhibitory mechanisms. For some subjects in our study, the stimulus intensity that
produced a small M-wave whose amplitude corresponds to 10%M
MAX
did not fall over the
ascending limb (e.g., Figure 2.2). Therefore, changes in test H-reflex size before and after
exposure to different conditions might not have reflected possible changes in excitability that
might have been observed otherwise. However, it was also sometimes the case that a small M-
wave whose amplitude is less than 10% M
MAX
(e.g., 4-8% M
MAX
as recommended by Knikou
(2008)) might not be identifiable from EMG signals of some subjects due to resolution of our
system.
This limitation would have something to do with a methodological difference between
our study and Pierrot-Deseilligny and Burke (2012). In our study, the stimulus intensity was
adjusted to maintain the constant amplitude of a small M-wave (10% M
MAX
) within and across
subjects. Changes in H-reflex size at this particular stimulus intensity before and after exposure
to different conditions were then compared to elucidate possible effects of eccentric contractions
on H-reflex modulation. On the contrary, Knikou (2008) and Pierrot-Deseilligny and Burke
(2012) recommended controlling the control H-reflex response to be on the ascending limb of the
recruitment curve (20 to 40% M
MAX
, or at least 10% M
MAX
) to which test H-reflex response in

39

the presence of a conditioning volley that has either excitatory or inhibitory effects on the test H-
reflex is compared. The amplitude of a small M-wave is then controlled to ensure the stimulus
consistency. In this case, the difference between control and test H-reflex represents changes in
H-reflex modulation between different conditions. This might have had a better control on H-
reflex modulation to be measured on the ascending limb for all the subjects. An additional
advantage of this method is that it can be utilized to examine effects of specific mechanisms
acting on H-reflex modulation (e.g., presynaptic inhibition or motoneurone excitability) (see
Knikou (2008) and Pierrot-Deseilligny and Burke (2012) for more detail).
Stimulus Frequency
One needs to pay a close attention to frequency of stimulation that elicits H-reflex
response. When the interval between stimulations is too short (less than 2 sec), attenuation of
reflex response due to homosynaptic depression would be significant. Therefore, it is
recommended that stimulations be separated by more than 3 sec. The interval of stimulations
depends on a condition in which H-reflex is obtained. A shorter interval would be sufficient
during a contraction of the tested muscle.

40

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45

Acknowledgement
I would like to express the deepest gratitude to my Master’s thesis committee chair, Professor
Francisco Valero-Cuevas. He provided me with the opportunity to obtain valuable research
experience under his supervision and continuously and enthusiastically supported me throughout
the course of Master’s thesis. I would also like to thank my committee members, Professor
Gerald Loeb and Professor James Finely for answering to my questions with detailed insight.
Their valuable advice and support have been a great help to understand and utilize H-reflex
methodology. This thesis would not have been possible without the help and support of Emily
Lawrence, a lab manager at the Brain-Body Dynamics Lab. She assisted me in all aspects of my
Master’s thesis from development of a MATLAB GUI to data collection and analysis. Thanks
also go to all members of the Brain-Body Dynamics Lab for their valuable feedbacks to my
presentation and consistent support throughout the course work. I am also thankful for all the
subjects who generously shared their time and participated in my study. Lastly, I am grateful to
my parents for their financial support and allowing me to have such precious experience in the
United States.

Abstract (if available)

Abstract Controlling instabilities with the legs (as per the Lower Extremity Strength‐Dexterity (LED) paradigm) is likely a product of a hierarchical organization of neural control, in agreement with current thinking. A pilot study showed that LED performance deteriorates with 20 minutes of downhill walking in the absence of fatigue. This suggests that the control of leg dexterity might degrade with eccentric contractions—and may be a plausible mechanism for non‐contact sports injuries. However, this effect remains ambiguous because concentric and eccentric phases coexist in the stance phase of downhill walking. Hierarchical control of instabilities involves “low level” sub‐cortical or spinal mechanisms. Such short‐latency responses are often mediated by tunable spindle afferents. The ability to perform short‐latency corrections should, therefore, be dependent on the gain and gaiting of spindle afferent signals. Therefore, we hypothesized that purely eccentric contractions of the soleus muscle would affect spindle afferent gains

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Asset Metadata

Creator Nagamori, Akira (author)

Core Title Neurophysiological bases of the strength‐dexterity paradigm with the use of H‐reflex

School Viterbi School of Engineering

Degree Master of Science

Degree Program Biomedical Engineering

Publication Date 02/23/2015

Defense Date 01/30/2015

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag eccentric contractions,H‐reflex,lower extremity dexterity,OAI-PMH Harvest,sensorimotor function

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Valero-Cuevas, Francisco J. (committee chair), Finley, James (committee member), Loeb, Gerald E. (committee member)

Creator Email a.nagamori0517@gmail.com,nagamori@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c3-535511

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