scholarly journals Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles

2009 ◽  
Vol 107 (1) ◽  
pp. 80-89 ◽  
Author(s):  
T. M. Altenburg ◽  
A. de Haan ◽  
P. W. L. Verdijk ◽  
W. van Mechelen ◽  
C. J. de Ruiter
Keyword(s):  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Knee Extensors ◽  
Emg Activity ◽  
Single Motor Unit ◽  
Recruitment Threshold ◽  
Single Motor ◽  
Discharge Rates ◽  
Isometric Torque

Single motor unit electromyographic (EMG) activity of the knee extensors was investigated at different knee angles with subjects ( n = 10) exerting the same absolute submaximal isometric torque at each angle. Measurements were made over a 20° range around the optimum angle for torque production (AngleTmax) and, where feasible, over a wider range (50°). Forty-six vastus lateralis (VL) motor units were recorded at 20.7 ± 17.9 %maximum voluntary contraction (%MVC) together with the rectified surface EMG (rsEMG) of the superficial VL muscle. Due to the lower maximal torque capacity at positions more flexed and extended than AngleTmax, single motor unit recruitment thresholds were expected to decrease and discharge rates were expected to increase at angles above and below AngleTmax. Unexpectedly, the recruitment threshold was higher ( P < 0.05) at knee angles 10° more extended (43.7 ± 22.2 N·m) and not different ( P > 0.05) at knee angles 10° more flexed (35.2 ± 17.9 N·m) compared with recruitment threshold at AngleTmax (41.8 ± 21.4 N·m). Also, unexpectedly the discharge rates were similar ( P > 0.05) at the three angles: 11.6 ± 2.2, 11.6 ± 2.1, and 12.3 ± 2.1 Hz. Similar angle independent discharge rates were also found for 12 units ( n = 5; 7.4 ± 5.4 %MVC) studied over the wider (50°) range, while recruitment threshold only decreased at more flexed angles. In conclusion, the similar recruitment threshold and discharge behavior of VL motor units during submaximal isometric torque production suggests that net motor unit activation did not change very much along the ascending limb of the knee-angle torque relationship. Several factors such as length-dependent twitch potentiation, which may contribute to this unexpected aspect of motor control, are discussed.

Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

2008 ◽  
Vol 33 (6) ◽  
pp. 1086-1095 ◽  
Author(s):  
Teatske M. Altenburg ◽  
Cornelis J. de Ruiter ◽  
Peter W.L. Verdijk ◽  
Willem van Mechelen ◽  
Arnold de Haan
Keyword(s):  
Motor Unit ◽  
Muscle Activation ◽  
Discharge Rate ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Voluntary Contraction ◽  
Knee Extensors ◽  
Single Motor Unit ◽  
Discharge Rates ◽  
Active Motor

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20° at 10°/s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%–47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 ± 0.19) and discharge rate (1.11 ± 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 ± 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 ± 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

Nonlinear behaviour of human motoneurons

1995 ◽  
Vol 73 (1) ◽  
pp. 113-123 ◽  
Author(s):  
Louise Smith ◽  
Tao Zhong ◽  
Parveen Bawa
Keyword(s):  
Steady State ◽  
Firing Rate ◽  
Motor Unit ◽  
Human Subjects ◽  
Motor Units ◽  
Rate Of Change ◽  
Emg Activity ◽  
Nonlinear Behaviour ◽  
Single Motor Unit ◽  
Single Motor

When ramp-and-hold currents are injected into a motoneuron of an anesthetized cat, the motoneuron responds with a high initial firing rate (dynamic phase), which then adapts to a lower steady-state firing rate. The firing rates during the dynamic and the steady-state phases are linearly related to the rate of change and the magnitude of the injected current, respectively. In human subjects, where inputs to the motoneurons are not accessible, force parameters are used to describe motoneuron behaviour. Population responses of human motoneurons, measured in terms of gross electromyographic (EMG) activity, increase linearly with the magnitude and the rate of change of force. No study has attempted to examine the question of linearity of single motor units during the dynamic as well as the steady-state phases. The following study recorded single motor unit and EMG activities simultaneously from the flexor carpi radialis muscle in human subjects completing ramp-and-hold force trajectories. Although the results confirmed the linear relationship between EMG activities and the rate and magnitude of the force, a nonlinear activity pattern was observed between the single motor unit firing and the force parameters, suggesting that recruitment must be responsible for the linear behaviour of EMG activity. Comparisons of different background activity levels on the firing patterns of a given motor unit, as well as comparisons of two simultaneously recorded units, further supported nonlinear response patterns of single motor units.Key words: human, motoneurons, motor units, nonlinearity, force trajectory, repetitive firing.

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

2014 ◽  
Vol 112 (7) ◽  
pp. 1685-1691 ◽  
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.

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

2006 ◽  
Vol 95 (3) ◽  
pp. 1518-1526 ◽  
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.

VASTUS LATERALIS MOTOR UNIT RECRUITMENT THRESHOLDS ARE COMPRESSED TOWARDS LOWER FORCES IN OLDER MEN

2020 ◽  
pp. 1-6
Author(s):  
R.M. Girts ◽  
J.A. Mota ◽  
K.K. Harmon ◽  
R.J. MacLennan ◽  
M.S. Stock
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Older Men ◽  
Force Level ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
Younger Men ◽  
The Mean

Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle.

Organization of single motor units in feline sartorius

1994 ◽  
Vol 72 (4) ◽  
pp. 1885-1896 ◽  
Author(s):  
E. Smits ◽  
P. K. Rose ◽  
T. Gordon ◽  
F. J. Richmond
Keyword(s):  
Motor Unit ◽  
Cross Section ◽  
Cross Sections ◽  
Muscle Fibers ◽  
Motor Units ◽  
Single Motor Unit ◽  
Single Motor ◽  
Medial Head ◽  
Single Motor Units ◽  
Fast Twitch

1. We depleted single motor units in feline sartorius muscles of glycogen by stimulating their motoneurons intracellularly. We mapped the intramuscular distribution of depleted fibers by inspecting histological cross-sections throughout the length of sartorius. 2. We selected ten depleted motor units for detailed study and quantitative analysis. Nine motor units were located in the anterior head of sartorius. One was located in a muscle whose distal half appeared to have been damaged some time before the acute experiment. A single motor unit was located in the medial head of sartorius. 3. Five motor units were composed of fast-twitch glycolytic (FG) muscle fibers, two of fast-twitch oxidative glycolytic (FOG) muscle fibers, and three of slow-twitch oxidative (SO) muscle fibers. Estimates of the numbers of depleted fibers in motor units of anterior sartorius indicated that FG motor units were larger (mean 566 fibers) than FOG and SO motor units (SO mean 190, FOG mean 156 fibers). The SO motor unit in the damaged muscle had 550 fibers. One motor unit depleted in the medial head of sartorius had 270 fibers with FG profiles. 4. Muscle fibers belonging to each anterior motor unit were never distributed throughout the whole cross-section of anterior sartorius at any proximodistal level. Furthermore, fibers were distributed nonuniformly along the proximodistal axis of the muscle. In most muscles at least a few depleted fibers were found at all proximodistal levels. However, in one normal muscle and the damaged muscle, depleted fibers were confined to the proximal end. 5. The fibers in the medial motor unit were confined to a strip that did not extend across the whole cross-section of the muscle head. Fibers within this strip were scattered quite evenly from origin to insertion. This medial FG motor unit occupied a smaller territory and contained fewer fibers than anterior motor units of the same histochemical type. 6. These results show that sartorius motor units are not distributed uniformly in the mediolateral plane; those in anterior sartorius were distributed asymmetrically in the proximodistal axis as well. This finding has important functional implications for the way in which we model force development and transmission in sartorius and other long muscles.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

1994 ◽  
Vol 76 (6) ◽  
pp. 2411-2419 ◽  
Author(s):  
S. J. Garland ◽  
R. M. Enoka ◽  
L. P. Serrano ◽  
G. A. Robinson
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Human Subjects ◽  
Absolute Threshold ◽  
Motor Units ◽  
Single Motor ◽  
Threshold Force ◽  
Single Motor Units ◽  
Discharge Rates

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

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