Motor Unit Synchronization Increases EMG Amplitude and Force Fluctuations in Simulated Isometric Contractions.

The purpose of this study was to examine changes in motor unit activity in the biceps brachii muscle after an initial ( Bout 1) and repeated ( Bout 2) session of eccentric exercise separated by 1 wk. Eight subjects (aged 22 ± 2 yr) participated in experimental assessments of neuromuscular function obtained before, immediately after, 24 h after, and 7 days after each exercise bout. Each experimental session involved assessments of elbow-flexor force and biceps and triceps brachii electromyography during maximum voluntary isometric contractions (MVCs) and constant-force isometric contractions at five contraction intensities (5–50% MVC), along with indicators of muscle damage (muscle pain and passive tension). In addition, motor unit recordings were obtained before exercise, 7 days after Bout 1, and 24 h after Bout 2 to assess motor unit synchronization and recruitment thresholds. Following a single eccentric exercise session that elicited significant indicators of muscle damage, we found a 57% increase in motor unit synchronization 7 days later compared with before exercise, despite the recovery of maximal strength, soreness, and relaxed elbow-joint angle at this time. Furthermore, a second bout of the same eccentric exercise resulted in reduced indicators of muscle damage and a decline in the strength of motor unit synchronization (24 h after Bout 2) toward levels observed before both exercise sessions. In contrast, no changes in motor unit recruitment thresholds were observed 7 days after Bout 1 or 24 h after Bout 2 compared with before exercise. The increased motor unit synchronization 7 days after a single eccentric exercise session provides new evidence of changes in motor unit activity during the putative repair and regeneration phase following eccentric muscle damage.

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Neuromuscular adjustments that constrain submaximal EMG amplitude at task failure of sustained isometric contractions

Journal of Applied Physiology ◽

10.1152/japplphysiol.00186.2011 ◽

2011 ◽

Vol 111 (2) ◽

pp. 485-494 ◽

Cited By ~ 62

Author(s):

Jakob L. Dideriksen ◽

Roger M. Enoka ◽

Dario Farina

Keyword(s):

Motor Unit ◽

Action Potentials ◽

Muscle Activation ◽

Isometric Contractions ◽

Neural Drive ◽

Task Failure ◽

Emg Amplitude ◽

Motor Unit Action Potentials ◽

Motor Unit Action ◽

Unit Action

The amplitude of the surface EMG does not reach the level achieved during a maximal voluntary contraction force at the end of a sustained, submaximal contraction, despite near-maximal levels of voluntary effort. The depression of EMG amplitude may be explained by several neural and muscular adjustments during fatiguing contractions, including decreased net neural drive to the muscle, changes in the shape of the motor unit action potentials, and EMG amplitude cancellation. The changes in these parameters for the entire motor unit pool, however, cannot be measured experimentally. The present study used a computational model to simulate the adjustments during sustained isometric contractions and thereby determine the relative importance of these factors in explaining the submaximal levels of EMG amplitude at task failure. The simulation results indicated that the amount of amplitude cancellation in the simulated EMG (∼40%) exhibited a negligible change during the fatiguing contractions. Instead, the main determinant of the submaximal EMG amplitude at task failure was a decrease in muscle activation (number of muscle fiber action potentials), due to a reduction in the net synaptic input to motor neurons, with a lesser contribution from changes in the shape of the motor unit action potentials. Despite the association between the submaximal EMG amplitude and reduced muscle activation, the deficit in EMG amplitude at task failure was not consistently associated with the decrease in neural drive (number of motor unit action potentials) to the muscle. This indicates that the EMG amplitude cannot be used as an index of neural drive.

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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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Neuromuscular drive and force production are not altered during bilateral contractions

Journal of Applied Physiology ◽

10.1152/jappl.1998.84.1.200 ◽

1998 ◽

Vol 84 (1) ◽

pp. 200-206 ◽

Cited By ~ 56

Author(s):

J. M. Jakobi ◽

E. Cafarelli

Keyword(s):

Motor Unit ◽

Force Production ◽

Young Male ◽

Neuromuscular Control ◽

Force Generation ◽

Isometric Contractions ◽

Firing Rates ◽

Significant Limitation ◽

Motor Unit Firing ◽

Male Subjects

Jakobi, J. M., and E. Cafarelli. Neuromuscular drive and force production are not altered during bilateral contractions. J. Appl. Physiol. 84(1): 200–206, 1998.—Several investigators have studied the deficit in maximal voluntary force that is said to occur when bilateral muscle groups contract simultaneously. A true bilateral deficit (BLD) would suggest a significant limitation of neuromuscular control; however, some of the data from studies in the literature are equivocal. Our purpose was to determine whether there is a BLD in the knee extensors of untrained young male subjects during isometric contractions and whether this deficit is associated with a decreased activation of the quadriceps, increased activation of the antagonist muscle, or an alteration in motor unit firing rates. Twenty subjects performed unilateral (UL) and bilateral (BL) isometric knee extensions at 25, 50, 75, and 100% maximal voluntary contraction. Total UL and BL force (Δ3%) and maximal rate of force generation (Δ2.5%) were not significantly different. Total UL and BL maximal vastus lateralis electromyographic activity (EMG; 2.7 ± 0.28 vs. 2.6 ± 0.24 mV) and coactivation (0.17 ± 0.02 vs. 0.20 ± 0.02 mV) were also not different. Similarly, the ratio of force to EMG during submaximal UL and BL contractions was not different. Analysis of force production by each leg in UL and BL conditions showed no differences in force, rate of force generation, EMG, motor unit firing rates, and coactivation. Finally, assessment of quadriceps activity with the twitch interpolation technique indicated no differences in the degree of voluntary muscle activation (UL: 93.6 ± 2.51 Hz, BL: 90.1 ± 2.43 Hz). These results provide no evidence of a significant limitation in neuromuscular control between BL and UL isometric contractions of the knee extensor muscles in young male subjects.

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Variability of motor unit discharge and force fluctuations across a range of muscle forces in older adults

Muscle & Nerve ◽

10.1002/mus.20392 ◽

2005 ◽

Vol 32 (4) ◽

pp. 533-540 ◽

Cited By ~ 122

Author(s):

Brian L. Tracy ◽

Katrina S. Maluf ◽

Jennifer L. Stephenson ◽

Sandra K. Hunter ◽

Roger M. Enoka

Keyword(s):

Older Adults ◽

Motor Unit ◽

Muscle Forces ◽

Unit Discharge ◽

Force Fluctuations ◽

Motor Unit Discharge

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Recurrence quantification analysis of surface EMG detects changes in motor unit synchronization induced by recurrent inhibition

Experimental Brain Research ◽

10.1007/s00221-006-0734-x ◽

2006 ◽

Vol 178 (3) ◽

pp. 308-315 ◽

Cited By ~ 17

Author(s):

F. Del Santo ◽

F. Gelli ◽

R. Mazzocchio ◽

A. Rossi

Keyword(s):

Motor Unit ◽

Surface Emg ◽

Recurrence Quantification Analysis ◽

Recurrent Inhibition ◽

Recurrence Quantification ◽

Quantification Analysis ◽

Motor Unit Synchronization

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Absence of lateral gastrocnemius activity and differential motor unit behavior in soleus and medial gastrocnemius during standing balance

Journal of Applied Physiology ◽

10.1152/japplphysiol.00906.2013 ◽

2014 ◽

Vol 116 (2) ◽

pp. 140-148 ◽

Cited By ~ 44

Author(s):

Martin E. Héroux ◽

Christopher J. Dakin ◽

Billy L. Luu ◽

John Timothy Inglis ◽

Jean-Sébastien Blouin

Keyword(s):

Firing Rate ◽

Motor Unit ◽

Standing Position ◽

Standing Balance ◽

Triceps Surae ◽

Medial Gastrocnemius ◽

Isometric Contractions ◽

Functional Requirements ◽

Vertical Projection ◽

Lateral Gastrocnemius

In a standing position, the vertical projection of the center of mass passes in front of the ankle, which requires active plantar-flexor torque from the triceps surae to maintain balance. We recorded motor unit (MU) activity in the medial (MG) and lateral (LG) gastrocnemius muscle and the soleus (SOL) in standing balance and voluntary isometric contractions to understand the effect of functional requirements and descending drive from different neural sources on motoneuron behavior. Single MU activity was recorded in seven subjects with wire electrodes in the triceps surae. Two 3-min standing balance trials and several ramp-and-hold contractions were performed. Lateral gastrocnemius MU activity was rarely observed in standing. The lowest thresholds for LG MUs in ramp contractions were 20–35 times higher than SOL and MG MUs ( P < 0.001). Compared with MUs from the SOL, MG MUs were intermittently active ( P < 0.001), had higher recruitment thresholds ( P = 0.022), and greater firing rate variability ( P < 0.001); this difference in firing rate variability was present in standing balance and isometric contractions. In SOL and MG MUs, both recruitment of new MUs ( R2 = 0.59–0.79, P < 0.01) and MU firing rates ( R2 = 0.05–0.40, P < 0.05) were associated with anterior-posterior and medio-lateral torque in standing. Our results suggest that the two heads of the gastrocnemius may operate in different ankle ranges with the larger MG being of primary importance when standing, likely due to its fascicle orientation. These differences in MU discharge behavior were independent of the type of descending neural drive, which points to a muscle-specific optimization of triceps surae motoneurons.

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EMG Changes in Human Thenar Motor Units With Force Potentiation and Fatigue

Journal of Neurophysiology ◽

10.1152/jn.00924.2005 ◽

2006 ◽

Vol 95 (3) ◽

pp. 1518-1526 ◽

Cited By ~ 22

Author(s):

C. K. Thomas ◽

R. S. Johansson ◽

B. Bigland-Ritchie

Keyword(s):

Motor Unit ◽

Motor Units ◽

Emg Activity ◽

Single Motor Unit ◽

Tetanic Force ◽

Twitch Force ◽

Emg Amplitude ◽

Before And After ◽

Induced Changes ◽

Motor Unit Emg

Few studies have analyzed activity-induced changes in EMG activity in individual human motor units. We studied the changes in human thenar motor unit EMG that accompany the potentiation of twitch force and fatigue of tetanic force. Single motor unit EMG and force were recorded in healthy subjects in response to selective stimulation of their motor axons within the median nerve just above the elbow. Twitches were recorded before and after a series of pulse trains delivered at frequencies that varied between 5 and 100 Hz. This stimulation induced significant increases in EMG amplitude, duration, and area. However, in relative terms, all of these EMG changes were substantially smaller than the potentiation of twitch force. Another 2 min of stimulation (13 pulses at 40 Hz each second) induced additional potentiation of EMG amplitude, duration, and area, but the tetanic force from every unit declined. Thus activity-induced changes in human thenar motor unit EMG do not indicate the alterations in force or vice versa. These data suggest that different processes underlie the changes in EMG and force that occur during human thenar motor unit activity.

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