The effects of poliomyelitis on motor unit behavior during repetitive muscle actions: a case report
BMC Research Notes
The effects of poliomyelitis on motor unit behavior during repetitive muscle actions: a case report
Michael A Trevino 0
Trent J Herda 0
Michael A Cooper 1
0 Neuromechanics Laboratory, Department of Health, Sport, and Exercise Sciences, University of Kansas , 1301 Sunnyside Ave, Room 101BE, Lawrence, KS , USA
1 Dept. of Anatomy and Cell Biology, University of Kansas Medical Center , Kansas City, KS , USA
Background: Acute paralytic poliomyelitis is caused by the poliovirus and usually results in muscle atrophy and weakness occurring in the lower limbs. Indwelling electromyography has been used frequently to investigate the denervation and innervation characteristics of the affected muscle. Recently developed technology allows the decomposition of the raw surface electromyography signals into the firing instances of single motor units. There is limited information regarding this electromyographic decomposition in clinical populations. In addition, regardless of electromyographic methods, no study has examined muscle activation parameters during repetitive muscle actions in polio patients. Therefore, the purpose of this study was to examine the motor unit firing rates and electromyographic amplitude and center frequency of the vastus lateralis during 20 repetitive isometric muscle actions at 50% maximal voluntary contraction in healthy subjects and one patient that acquired acute paralytic poliomyelitis. Case presentation: One participant that acquired acute type III spinal poliomyelitis (Caucasian male, age = 29 yrs) at 3 months of age and three healthy participants (Caucasian females, age = 19.7 ± 2.1 yrs) participated in this study. The polio participant reported neuromuscular deficiencies as a result of disease in the hips, knees, buttocks, thighs, and lower legs. None of the healthy participants reported any current or ongoing neuromuscular diseases or musculoskeletal injuries. Conclusion: An acute bout of poliomyelitis altered motor unit behavior, such as, healthy participants displayed greater firing rates than the polio patient. The reduction in motor unit firing rates was likely a fatigue protecting mechanism since denervation via poliomyelitis results in a reduction of motorneurons. In addition, the concurrent changes in motor unit firing rates, electromyography amplitude and frequency for the polio participant would suggest that the entire motorneuron pool was utilized in each contraction unlike for the healthy participants. Finally, healthy participants exhibited changes in all electromyographic parameters during the repetitive muscle actions despite successfully completing all contractions with only a slight reduction in force. Thus, caution is warranted when quantifying muscular fatigue via motor unit firing rates and other electromyographic parameters since the parameters changed despite successful completing of all contractions with only a moderate reduction in strength in healthy subjects.
Electromyography; Firing rates; Isometric; Motorneuron; Vastus lateralis
Acute paralytic polio is a disease caused by the poliovirus
that typically occurs in young children. The poliovirus
spreads along nerve pathways and primarily destroys
motorneurons of the anterior horns [
]. The poliovirus
usually results in asymmetrical paralysis with muscle
atrophy and weakness occurring in the lower limbs more so
than the upper limbs [
]. Numerous studies have used
indwelling electromyography (EMG) to investigate the
denervation and reinnervation characteristics of the affected
muscle (i.e., motor unit number and amplitude, motor
unit firings, conduction velocity, etc.) [
]. For example,
Larsson et al.  reported that healthy participants had
greater motor unit (MU) firing rates of the tibialis anterior
muscle than the poliomyelitis participants during slowly
increasing isometric voluntary muscle actions. In addition,
McComas et al. [
] reported a reduction in motor unit
number after an acute bout of poliomyelitis at twice the
rate than healthy subjects aged > 60 years, whereas, Herda
and Cooper [
] reported a reduction in MUs recruited
above 20% of maximal voluntary contraction (MVC) for a
poliomyelitis patient in comparison to healthy participants.
Recently, De Luca et al. [
] developed technology
able to decompose the raw surface EMG signals recorded
during isometric muscle actions into its constituent MU
action potential trains with the Precision Decomposition
(PD) III algorithm. The resulting output allows for the
examination of the firing instances for each detected MU.
The PD III algorithm has typically detected 30-40 MUs
and occasionally up to 60 MUs during a single isometric
trapezoid muscle action at various force levels (5 – 100%
MVC). Previously, De Luca et al. [
] and Nawab et al.
] validated the PD III algorithm and
reconstruct-andtest (used for testing accuracy of MU action potential
trains) algorithms. This decomposition technology has
been used to examine the MU control scheme, which
states that lower threshold MUs have higher firing rates
than higher threshold MUs during isometric muscle
]. However, there is limited information
available regarding the use of EMG decomposition in
clinical populations [
]. It would be expected that an
acute bout of paralytic polio would influence the overall
MU control scheme and be detectable by the EMG
decomposition technique developed by De Luca et al.
, which was supported by Herda and Cooper [
Previous studies have utilized repetitive isometric
muscle actions to further elucidate mechanisms of fatigue
on MU behavior [
]. For example, Carpentier et al.
 indicated that central drive intensified, as measured
by EMG amplitude, while MU discharge rates decreased
progressively during the repetitive muscle actions of the
first dorsal interosseous at 50% MVC. In addition, the
authors reported that the level of change in MU
behavior was related to recruitment threshold (low- versus
high-threshold), which was also supported by Farina et al.
]. To date, it is unknown the influence of poliomyelitis
on MU firing rates and other EMG parameters (amplitude
and center frequency) during repetitive muscle actions.
Since polio patients have far less motorneurons as a result
of the denervation and reinnervation process, it is
conceivable that their MU behavior during the repetitive
tasks would be altered in comparison to healthy subjects.
Therefore, the purpose of this study was to examine the
MU control scheme and EMG amplitude and frequency
of the vastus lateralis (VL) during 20 repetitive isometric
muscle actions at 50% MVC for healthy individuals and
one individual that acquired acute paralytic poliomyelitis.
In this case report, we present one participant that acquired
acute type III spinal poliomyelitis (mean ± standard
deviation [SD], Caucasian male, age = 29 yrs, weight = 63.4 kg,
height = 150.7 cm) at 3 months of age and three healthy
participants (Caucasian females, age = 19.7 ± 2.1 yrs,
weight = 67.5 ± 9.4 kg, height = 170.7 ± 2.3 cm). Previous
research has reported no sex-related differences in EMG
amplitude, EMG frequency, or MU behavior during
fatiguing contractions [
The participant with acute type III spinal poliomyelitis
(PO) reported neuromuscular deficiencies as a result of
disease in the hips, knees, buttocks, thighs, and lower
legs. None of the healthy (HE) participants reported any
current or ongoing neuromuscular diseases or
musculoskeletal injuries. Each participant signed a written informed
consent document. This study was approved by the Human
Subjects Committee – Lawrence at the University of
Each participant was seated with restraining straps over
the pelvis, trunk, and contralateral thigh, and the lateral
condyle of the femur was aligned with the input axis of
the Biodex System 3 isokinetic dynamometer (Biodex
Medical Systems, Inc., Shirley, NY, USA). All isometric
leg extensor strength assessments were performed on the
right leg at a flexion of 90°. Isometric strength for the leg
extensors muscles was measured using the force signal
from a load cell (LC402, Omegadyne, Inc., Sunbury, OH,
USA) that was fitted to the isokinetic dynamometer.
During the experimental trial, participants completed
a warm-up consisting of three to five voluntary isometric
muscle actions from 30%-80% MVC. Participants then
performed three isometric MVCs with strong verbal
encouragement for motivation followed by twenty
repetitive submaximal isometric trapezoid muscle actions at
50% MVC. The trapezoid trajectory contained five
segments: a 5 sec quiescent period for baseline noise
calculation, an up-ramp increased at a rate of 10% MVC/sec, a
constant force of 50% MVC for 12 sec, a down-ramp
decreased at 10% MVC/sec, and a 3 sec quiescent period.
Therefore, the duration of each repetitive trial lasted
22 sec and subjects were given a rest period of 8 – 9 sec
between trials. Each participant was instructed to maintain
their force output as close as possible to the target force
presented digitally in real time on a computer monitor.
Following the repetitive muscle actions, participants
performed an additional MVC.
Electromyography signal detection and processing
Surface EMG signals were recorded from the VL using a
5 pin surface array sensor (Delsys, Boston, MA). Prior to
sensor placement, the surface of the skin was prepared
by shaving, removing superficial dead skin with adhesive
tape, and sterilized with an alcohol swab. In addition,
EMG signals must successfully pass the Delsys Signal
Quality Check (SQC) prior to the recording of EMG for
decomposition. The SQC requires the signal to noise
ratio to be > 2, baseline noise < 4.8 μV root mean square,
and line interference < 1.0 (normalized magnitude power
spectrum at 50/60 Hz) (Delsys, Inc., User’s Guide). The
surface EMG sensor was placed over the belly of the VL
and fixed with adhesive tape while the reference electrode
was placed over the patella. Refer to De Luca et al. [
for detailed information regarding the signal processing of
the EMG signals. Decomposition techniques (PD III
algorithm) were applied to the surface EMG signals to extract
action potentials and the firing events of single MUs
]. Furthermore, the accuracy level of each MU
was assessed by a reconstruct-and-test procedure .
Only MUs with > 90% accuracy were included for analysis.
For each MU, the recruitment thresholds (expressed
relative to MVC) and the mean firing rate (MFR) at the
targeted contraction level (pulses per second [pps]) were
analyzed. The MFR was calculated as the average value
of the mean firing rate trajectory during steady force
]. Linear regressions were performed on the MFR
versus recruitment threshold relationships for each
muscle action. Slopes and y-intercepts were calculated
for each linear regression model. The y-intercept values
specify a maximal firing rate (pps) sustainable by the
lowest threshold MUs.
In addition, EMG amplitude and mean power frequency
(MPF) was investigated. The signals were collected at a
frequency of 20 kHz and bandpass filtered (fourth-order
Butterworth) at 10-500 Hz. Signal processing was
performed with custom programs written with LabVIEW
software (Version 11.0, National Instruments, Austin,
TX). The amplitude for the EMG signal was calculated
with root mean square (RMS). For the isometric muscle
actions, EMG RMS and MPF were calculated by averaging
the values across the four EMG channels during the
entire 12 sec targeted contraction force. These data
were normalized to the EMG RMS and MPF values
corresponding to the first 50% repetitive muscle action.
There was a total of 383 MUs analyzed for the HE
participants with 80 MUs analyzed for the PO participant
(Table 1). For the pre- and post-MVCs, peak force values
decreased 10.3% and 28.0% for the HE (548.67 ± 121.01 N,
492.00 ± 147.15 N) and PO (37.80 N, 27.20 N),
respectively. The normalized (i.e., 1st repetition = 100%) EMG
RMS was 117.68 ± 9.36% and 126.97 ± 23.13% while the
MPF was 89.90 ± 1.90% and 88.19 ± 4.06% for the HE
during the 10th and 20th repetition, respectively (Figure 1).
For the PO, the normalized EMG RMS was 93.37% and
101.71% while the MPF was 101.78% and 100.00% during
the 10th and 20th repetitive contraction (Figure 1). For the
y-intercepts, the mean ± SD for the HE were 33.42 ± 2.41
pps, 29.27 ± 2.12 pps, and 29.12 ± 1.83 pps for the 1st, 10th,
and 20th repetition and the y-intercepts for the PO were
25.64 pps, 23.48 pps, and 25.26 pps for the 1st, 10th, and
20th repetition (Figure 2). For the slopes, the mean ± SD
for the HE were -0.47 ± 0.05, -0.34 ± 0.02, and -0.41 ± 0.11
for the 1st, 10th, and 20th repetition and the slopes for the
PO for the 1st, 10th, and 20th repetition were -0.42, -0.26,
and -0.41 (Figure 2).
Previously, Larsson et al. [
] reported differences in MU
firing rates between healthy and prior polio patients during
“slowly increasing isometric voluntary contractions”(p.190).
The y-intercept from the MFR versus recruitment
threshold relationships theoretically represents a
maximal firing rate sustainable by the lowest threshold
]. In the present study, the MFRs for the HE
were higher (y-intercept range: 33.42 ± 2.41 – 29.12 ±
1.83 pps) than the PO (y-intercept range: 25.64 - 23.48
pps) for each repetition (Figure 2). Negative slopes
were reported from the linear regressions for each
participant and contraction and, thus, earlier recruited
MUs tended to have greater firing rates than the later
recruited MUs. Referring to Table 1 and Figure 2, there
were no observed differences between the slopes
between PO and HE participants. Therefore, the inverse
relationship between the firing rate and recruitment
threshold typically reported in healthy muscle was
present for the PO muscle [
]. The overall MU control
scheme (i.e., negative slopes) was similar between the PO
and HE participants, however, the MFRs were less (i.e.,
y-intercepts) for the PO than the HE participants.
Highthreshold MUs are typically described as fast fatiguing
for the VL. De Luca and Hostage [
] suggested that if
the high-threshold MUs discharged at relatively higher
rates, high-threshold MUs would fatigue more quickly
and force will not be maintained. Since it has been
reported that there are far less motorneurons in the PO
than HE participants as a result of denervation [
lower MFRs throughout the force spectrum for the PO
is possibly a fatigue protecting mechanism.
EMG RMS is a global measure of MU recruitment
and firing rates while EMG MPF reflects the average
muscle fiber conduction velocity [
]. Classically, muscle
fatigue has been monitored with these EMG parameters
and indicators of fatigue are believed to be evident by
increases in RMS and decreases in MPF [
]. An increase
in EMG RMS is the result of synchronization of active
MUs and the recruitment of new MUs while the decrease
in MPF has been primarily attributed to a reduction in
muscle fiber conduction velocity [
]. For the 1st to 10th
repetition, the HE had an increase (17.7%) in EMG RMS
while EMG MPF declined (10.0%). Concurrently, the
y-intercepts from the MFR versus recruitment threshold
relationships decreased (12.4%) for the HE participants
and, thus, would suggest that the HE participants recruited
additional MUs and/or had greater MU synchronization
during the 10th repetition in comparison to the 1st
repetition. In contrast, the y-intercepts from the MFR versus
recruitment threshold relationships and EMG RMS
decreased (8.4% and 6.6%) simultaneously while EMG
MPF (+1.8%) remained stable from the 1st to 10th
repetition for the PO participant. For the 10th to 20th repetition,
there was relatively no change in the y-intercepts (0.5%)
or EMG MPF (-1.7%), however, EMG RMS increased
9.3% for the HE. For the PO, the y-intercept from the
MFR versus recruitment threshold and EMG RMS
returned to baseline levels (i.e., increased) during the 20th
repetition. The EMG parameters indicated that MU
behavior differed between the PO and HE during the
repetitive muscle actions. Subsequently, MU firing rates and
EMG RMS responded in a similar manner for the PO, in
contrast, MU firing rates and EMG MPF decreased while
EMG RMS increased during the repetitive muscle actions
for the HE. Future studies may want to consider
normalizing these parameters to an MVC rather than a
submaximal muscle action as in the present study to
further elucidate neural mechanisms of fatigue.
Previously, Larsson et al. [
] reported that polio patients
used muscle fibers in an all or none manner during slowly
increasing isometric voluntary muscle actions. In the
present study, the changes in EMG RMS mirrored the
changes in the y-intercepts from the MFR versus
recruitment threshold relationships with relatively no
change in MPF for the PO, which tentatively suggested
that all MUs were recruited during each muscle action
at 50% MVC. Since EMG RMS is the concurrent
representation of MU recruitment and firing rates, the increase
in EMG RMS with a corresponding decrease in MFR
would suggest that HE relied on recruitment of
additional MUs and/or synchronization of MU during the
repetitive tasks. However, the mirrored changes between
EMG RMS and MFR for the PO suggested that no
additional MUs were recruited nor was there an increase in
MU synchronization during the repetitive tasks.
Interestingly, HE participants were able to complete all 20
contractions with a moderate (10.3%) reduction in post-MVC
strength. Although the PO had fewer changes in all
parameters measured (MU MFR, EMG RMS and MPF), the
decrease in post MVC for the PO was much greater than
the HE (28.0% vs. 10.3%). Thus, EMG parameters changed
during the repetitive tasks despite a lack of fatigue in the
HE as indicated by the successful completion of the tasks
with only a moderate reduction in strength.
In summary, the denervation and reinnervation process
as a result of an acute bout of poliomyelitis altered MU
behavior in comparison to the HE participants. Specifically,
the PO participants had lower MFRs and the y-intercepts
from the MFR versus recruitment threshold relationships
mirrored changes in EMG RMS from the 1st to 20th
repetition, unlike the HE participants. Thus, suggesting that all
MUs were recruited during each contraction for the PO
participant. In addition, caution is warranted when
examining EMG RMS and MPF and MU MFR behavior
as indicators of muscular fatigue during repetitive muscle
actions since the parameters changed in the HE despite
successfully completing all muscle actions with only a
moderate decrease in strength. Future research should
examine the influence of high-intensity exercise training
on the fatigability of polio patients as it is evident that the
alterations in MU control scheme provide a mechanism
to respond to such training despite the denervation and
Written informed consent was obtained from all
participants for publication of this Case Report. A copy of the
written consent is available for review by the
Editor-inChief of this journal.
EMG: Electromyography; HE: Healthy participant; MFR: Mean firing rate; MPF: Mean
power frequency; MU: Motor Unit; MVC: Maximal voluntary contraction; PO: Polio
participant; PD: Precision decomposition; PPS: Pulses per second; RMS: Root mean
square; SQC: Signal quality check; SD: Standard deviation; VL: Vastus lateralis.
The authors declare that they have no competing interests.
MT carried out the statistical analysis and interpretation of data, and drafted
the manuscript. TH participated in the design of the study, data collection,
assisted in the analyses and interpretation of data, and helped revise the
manuscript. MC assisted in the design of the study, data collection, and
interpretation of data. All authors read and approved the final manuscript.
No funding was provided for the authors or for the manuscript.
We are grateful for the contributions of the undergraduate research assistants
in the Neuromechanics Laboratory for their assistance in performing the
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