Estimation from surface EMG of the distribution of motor unit sizes in the m. vastus lateralis

We examined the means, medians, and variability for motor-unit interpulse intervals (IPIs) during voluntary, high force contractions. Eight men (mean age = 22 years) attempted to perform isometric contractions at 90% of their maximal voluntary contraction force while bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis and vastus medialis muscles. Surface EMG signal decomposition was used to determine the recruitment thresholds and IPIs of motor units that demonstrated accuracy levels ≥ 96.0%. Motor units with high recruitment thresholds demonstrated longer mean IPIs, but the coefficients of variation were similar across all recruitment thresholds. Polynomial regression analyses indicated that for both muscles, the relationship between the means and standard deviations of the IPIs was linear. The majority of IPI histograms were positively skewed. Although low-threshold motor units were associated with shorter IPIs, the variability among motor units with differing recruitment thresholds was comparable.

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Synchronization of Lower Limb Motor Unit Activity During Walking in Human Subjects

Journal of Neurophysiology ◽

10.1152/jn.2001.86.3.1266 ◽

2001 ◽

Vol 86 (3) ◽

pp. 1266-1276 ◽

Cited By ~ 49

Author(s):

N. L. Hansen ◽

S. Hansen ◽

L.O.D. Christensen ◽

N. T. Petersen ◽

J. B. Nielsen

Keyword(s):

Motor Unit ◽

Unit Activity ◽

Lower Limb ◽

Human Subjects ◽

Vastus Lateralis ◽

Surface Emg ◽

Medial Gastrocnemius ◽

Human Gait ◽

Knee Extensors ◽

Motor Unit Activity

Synchronization of motor unit activity was investigated during treadmill walking (speed: 3–4 km/h) in 25 healthy human subjects. Recordings were made by pairs of wire electrodes inserted into the tibialis anterior (TA) muscle and by pairs of surface electrodes placed over this muscle and a number of other lower limb muscles (soleus, gastrocnemius lateralis, gastrocnemius medialis, biceps femoris, vastus lateralis, and vastus medialis). Short-lasting synchronization (average duration: 9.6 ± 1.1 ms) was observed between spike trains generated from multiunit electromyographic (EMG) signals recorded by the wire electrodes in TA in eight of nine subjects. Synchronization with a slightly longer duration (12.8 ± 1.2 ms) was also found in 13 of 14 subjects for paired TA surface EMG recordings. The duration and size of this synchronization was within the same range as that observed during tonic dorsiflexion in sitting subjects. There was no relationship between the amount of synchronization and the speed of walking. Synchronization was also observed for pairs of surface EMG recordings from different ankle plantarflexors (soleus, medial gastrocnemius, and lateral gastrocnemius) and knee extensors (vastus lateralis and medialis of quadriceps), but not or rarely for paired recordings from ankle and knee muscles. The data demonstrate that human motor units within a muscle as well as synergistic muscles acting on the same joint receive a common synaptic drive during human gait. It is speculated that the common drive responsible for the motor unit synchronization during gait may be similar to that responsible for short-term synchronization during tonic voluntary contraction.

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Surface electromyographic amplitude does not identify differences in neural drive to synergistic muscles

Journal of Applied Physiology ◽

10.1152/japplphysiol.01115.2017 ◽

2018 ◽

Vol 124 (4) ◽

pp. 1071-1079 ◽

Cited By ~ 44

Author(s):

Eduardo Martinez-Valdes ◽

Francesco Negro ◽

Deborah Falla ◽

Alessandro Marco De Nunzio ◽

Dario Farina

Keyword(s):

Motor Unit ◽

Action Potentials ◽

Discharge Rate ◽

Vastus Lateralis ◽

Signal Amplitude ◽

Surface Emg ◽

Neural Drive ◽

Emg Amplitude ◽

Emg Signal ◽

Synergistic Muscles

Surface electromyographic (EMG) signal amplitude is typically used to compare the neural drive to muscles. We experimentally investigated this association by studying the motor unit (MU) behavior and action potentials in the vastus medialis (VM) and vastus lateralis (VL) muscles. Eighteen participants performed isometric knee extensions at four target torques [10, 30, 50, and 70% of the maximum torque (MVC)] while high-density EMG signals were recorded from the VM and VL. The absolute EMG amplitude was greater for VM than VL ( P < 0.001), whereas the EMG amplitude normalized with respect to MVC was greater for VL than VM ( P < 0.04). Because differences in EMG amplitude can be due to both differences in the neural drive and in the size of the MU action potentials, we indirectly inferred the neural drives received by the two muscles by estimating the synaptic inputs received by the corresponding motor neuron pools. For this purpose, we analyzed the increase in discharge rate from recruitment to target torque for motor units matched by recruitment threshold in the two muscles. This analysis indicated that the two muscles received similar levels of neural drive. Nonetheless, the size of the MU action potentials was greater for VM than VL ( P < 0.001), and this difference explained most of the differences in EMG amplitude between the two muscles (~63% of explained variance). These results indicate that EMG amplitude, even following normalization, does not reflect the neural drive to synergistic muscles. Moreover, absolute EMG amplitude is mainly explained by the size of MU action potentials. NEW & NOTEWORTHY Electromyographic (EMG) amplitude is widely used to compare indirectly the strength of neural drive received by synergistic muscles. However, there are no studies validating this approach with motor unit data. Here, we compared between-muscles differences in surface EMG amplitude and motor unit behavior. The results clarify the limitations of surface EMG to interpret differences in neural drive between muscles.

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Amplitude cancellation reduces the size of motor unit potentials averaged from the surface EMG

Journal of Applied Physiology ◽

10.1152/japplphysiol.01282.2005 ◽

2006 ◽

Vol 100 (6) ◽

pp. 1928-1937 ◽

Cited By ~ 78

Author(s):

Kevin G. Keenan ◽

Dario Farina ◽

Roberto Merletti ◽

Roger M. Enoka

Keyword(s):

Motor Unit ◽

Conduction Velocity ◽

Motor Units ◽

Surface Emg ◽

Excitation Level ◽

Bipolar Electrode ◽

Emg Signal ◽

Motor Unit Synchronization ◽

Rate Coding ◽

Simulated Surface

The purpose of the study was to evaluate the influence of selected physiological parameters on amplitude cancellation in the simulated surface electromyogram (EMG) and the consequences for spike-triggered averages of motor unit potentials derived from the interference and rectified EMG signals. The surface EMG was simulated from prescribed recruitment and rate coding characteristics of a motor unit population. The potentials of the motor units were detected on the skin over a hand muscle with a bipolar electrode configuration. Averages derived from the EMG signal were generated using the discharge times for each of the 24 motor units with lowest recruitment thresholds from a population of 120 across three conditions: 1) excitation level; 2) motor unit conduction velocity; and 3) motor unit synchronization. The area of the surface-detected potential was compared with potentials averaged from the interference, rectified, and no-cancellation EMGs. The no-cancellation EMG comprised motor unit potentials that were rectified before they were summed, thereby preventing cancellation between the opposite phases of the potentials. The percent decrease in area of potentials extracted from the rectified EMG was linearly related to the amount of amplitude cancellation in the interference EMG signal, with the amount of cancellation influenced by variation in excitation level and motor unit conduction velocity. Motor unit synchronization increased potentials derived from both the rectified and interference EMG signals, although cancellation limited the increase in area for both potentials. These findings document the influence of amplitude cancellation on motor unit potentials averaged from the surface EMG and the consequences for using the procedure to characterize motor unit properties.

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Endurance training alters motor unit activation strategies for the vastus lateralis, yet sex-related differences and relationships with muscle size remain

European Journal of Applied Physiology ◽

10.1007/s00421-021-04622-7 ◽

2021 ◽

Vol 121 (5) ◽

pp. 1367-1377

Author(s):

Stephanie A. Sontag ◽

Michael A. Trevino ◽

Trent J. Herda ◽

Adam J. Sterczala ◽

Jonathan D. Miller ◽

...

Keyword(s):

Endurance Training ◽

Motor Unit ◽

Vastus Lateralis ◽

Muscle Size ◽

Motor Unit Activation

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Statistically rigorous calculations do not support common input and long-term synchronization of motor-unit firings

Journal of Neurophysiology ◽

10.1152/jn.00725.2013 ◽

2014 ◽

Vol 112 (11) ◽

pp. 2729-2744 ◽

Cited By ~ 11

Author(s):

Carlo J. De Luca ◽

Joshua C. Kline

Keyword(s):

Motor Unit ◽

Statistical Tests ◽

Vastus Lateralis ◽

Common Input ◽

Data Set ◽

Order Of Magnitude ◽

Score Method ◽

Surface Electromyographic Signal ◽

Measure Synchronization

Over the past four decades, various methods have been implemented to measure synchronization of motor-unit firings. In this work, we provide evidence that prior reports of the existence of universal common inputs to all motoneurons and the presence of long-term synchronization are misleading, because they did not use sufficiently rigorous statistical tests to detect synchronization. We developed a statistically based method (SigMax) for computing synchronization and tested it with data from 17,736 motor-unit pairs containing 1,035,225 firing instances from the first dorsal interosseous and vastus lateralis muscles—a data set one order of magnitude greater than that reported in previous studies. Only firing data, obtained from surface electromyographic signal decomposition with >95% accuracy, were used in the study. The data were not subjectively selected in any manner. Because of the size of our data set and the statistical rigor inherent to SigMax, we have confidence that the synchronization values that we calculated provide an improved estimate of physiologically driven synchronization. Compared with three other commonly used techniques, ours revealed three types of discrepancies that result from failing to use sufficient statistical tests necessary to detect synchronization. 1) On average, the z-score method falsely detected synchronization at 16 separate latencies in each motor-unit pair. 2) The cumulative sum method missed one out of every four synchronization identifications found by SigMax. 3) The common input assumption method identified synchronization from 100% of motor-unit pairs studied. SigMax revealed that only 50% of motor-unit pairs actually manifested synchronization.

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Rectification is required to extract oscillatory envelope modulation from surface electromyographic signals

Journal of Neurophysiology ◽

10.1152/jn.00296.2014 ◽

2014 ◽

Vol 112 (7) ◽

pp. 1685-1691 ◽

Cited By ~ 19

Author(s):

Christopher J. Dakin ◽

Brian H. Dalton ◽

Billy L. Luu ◽

Jean-Sébastien Blouin

Keyword(s):

Motor Unit ◽

Stimulus Frequency ◽

Motor Units ◽

Surface Emg ◽

Medial Gastrocnemius ◽

Single Motor Unit ◽

Single Motor ◽

Single Motor Units ◽

Emg Signal ◽

Envelope Modulation

Rectification of surface electromyographic (EMG) recordings prior to their correlation with other signals is a widely used form of preprocessing. Recently this practice has come into question, elevating the subject of EMG rectification to a topic of much debate. Proponents for rectifying suggest it accentuates the EMG spike timing information, whereas opponents indicate it is unnecessary and its nonlinear distortion of data is potentially destructive. Here we examine the necessity of rectification on the extraction of muscle responses, but for the first time using a known oscillatory input to the muscle in the form of electrical vestibular stimulation. Participants were exposed to sinusoidal vestibular stimuli while surface and intramuscular EMG were recorded from the left medial gastrocnemius. We compared the unrectified and rectified surface EMG to single motor units to determine which method best identified stimulus-EMG coherence and phase at the single-motor unit level. Surface EMG modulation at the stimulus frequency was obvious in the unrectified surface EMG. However, this modulation was not identified by the fast Fourier transform, and therefore stimulus coherence with the unrectified EMG signal failed to capture this covariance. Both the rectified surface EMG and single motor units displayed significant coherence over the entire stimulus bandwidth (1–20 Hz). Furthermore, the stimulus-phase relationship for the rectified EMG and motor units shared a moderate correlation ( r = 0.56). These data indicate that rectification of surface EMG is a necessary step to extract EMG envelope modulation due to motor unit entrainment to a known stimulus.

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