Recording Electromyographic Activity (EMG) from Single Motor Units in Human Subjects

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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Voluntary hyperventilation changes recruitment order of parasternal intercostal motor units

Journal of Applied Physiology ◽

10.1152/jappl.1987.62.1.187 ◽

1987 ◽

Vol 62 (1) ◽

pp. 187-193 ◽

Cited By ~ 11

Author(s):

T. W. Watson ◽

W. A. Whitelaw

Keyword(s):

Human Subjects ◽

Motor Units ◽

Quiet Breathing ◽

Rib Cage ◽

Voluntary Hyperventilation ◽

Single Motor ◽

Intercostal Muscles ◽

Single Motor Units ◽

Normal Human ◽

Recruitment Order

The order of recruitment of single-motor units in parasternal intercostal muscles during inspiration was studied in normal human subjects during quiet breathing and voluntary hyperventilation. Electromyograms were recorded from the second and third intercostal spaces by means of bipolar fine wire electrodes. Flow at the mouth, volume, end-expired CO2, and rib cage and abdominal anterior-posterior diameters were monitored. Single-motor units were identified using criteria of amplitude and shape, and the time of first appearance of each unit in each inspiration was noted. Hyperventilation was performed with visual feedback of the display of rib cage and abdomen excursions, keeping the ratio of rib cage to abdominal expansion. Subjects were normocapnic in quiet breathing and developed hypocapnia during hyperventilation. Recruitment order was stable in quiet breathing, but in some cases was altered during voluntary hyperventilation. Some low threshold units that fired early in the breath in quiet breathing fired earlier at the beginning of a period of voluntary hyperventilation but progressively later in the breath as hyperventilation went on, whereas later firing units moved progressively toward the early part of inspiration. This suggests that different groups of motoneurons in the pool supplying parasternal intercostal muscles receive different patterns of synaptic input.

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Human stretch reflex pathways reexamined

Journal of Neurophysiology ◽

10.1152/jn.00295.2013 ◽

2014 ◽

Vol 111 (3) ◽

pp. 602-612 ◽

Cited By ~ 12

Author(s):

Ş. Utku Yavuz ◽

Natalie Mrachacz-Kersting ◽

Oğuz Sebik ◽

M. Berna Ünver ◽

Dario Farina ◽

...

Keyword(s):

Stretch Reflex ◽

Discharge Rate ◽

Human Subjects ◽

Silent Period ◽

Motor Units ◽

Single Motor Unit ◽

Single Motor ◽

Reflex Responses ◽

Single Motor Units ◽

Discharge Rates

Reflex responses of tibialis anterior motor units to stretch stimuli were investigated in human subjects. Three types of stretch stimuli were applied (tap-like, ramp-and-hold, and half-sine stretch). Stimulus-induced responses in single motor units were analyzed using the classical technique, which involved building average surface electromyogram (SEMG) and peristimulus time histograms (PSTH) from the discharge times of motor units and peristimulus frequencygrams (PSF) from the instantaneous discharge rates of single motor units. With the use of SEMG and PSTH, the tap-like stretch stimulus induced five separate reflex responses, on average. With the same single motor unit data, the PSF technique indicated that the tap stimulus induced only three reflex responses. Similar to the finding using the tap-like stretch stimuli, ramp-and-hold stimuli induced several peaks and troughs in the SEMG and PSTH. The PSF analyses displayed genuine increases in discharge rates underlying the peaks but not underlying the troughs. Half-sine stretch stimuli induced a long-lasting excitation followed by a long-lasting silent period in SEMG and PSTH. The increase in the discharge rate, however, lasted for the entire duration of the stimulus and continued during the silent period. The results are discussed in the light of the fact that the discharge rate of a motoneuron has a strong positive linear association with the effective synaptic current it receives and hence represents changes in the membrane potential more directly and accurately than the other indirect measures. This study suggests that the neuronal pathway of the human stretch reflex does not include inhibitory pathways.

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Spontaneous Electromyographic Activity in Adult Rat Soleus Muscle

Journal of Neurophysiology ◽

10.1152/jn.1998.80.1.365 ◽

1998 ◽

Vol 80 (1) ◽

pp. 365-376 ◽

Cited By ~ 49

Author(s):

Torsten Eken

Keyword(s):

Motor Unit ◽

Soleus Muscle ◽

Motor Units ◽

Firing Frequency ◽

Electromyographic Activity ◽

Single Motor Unit ◽

Single Motor ◽

Adult Rat ◽

Single Motor Units ◽

Rat Soleus Muscle

Eken, Torsten. Spontaneous electromyographic activity in adult rat soleus muscle. J. Neurophysiol. 80: 365–376, 1998. Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16–25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20–30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.

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Muscle spindle discharge in response to contraction of single motor units

Journal of Neurophysiology ◽

10.1152/jn.1983.49.2.291 ◽

1983 ◽

Vol 49 (2) ◽

pp. 291-302 ◽

Cited By ~ 119

Author(s):

B. McKeon ◽

D. Burke

Keyword(s):

Muscle Spindle ◽

Discharge Rate ◽

Human Subjects ◽

Motor Units ◽

Muscle Spindles ◽

Single Motor Unit ◽

Single Motor ◽

Twitch Contraction ◽

Single Motor Units ◽

Tendon Organ

1. In human subjects, microelectrode recordings were made from 25 muscle spindle afferents and two tendon organ afferents coming from muscles innervated by the peroneal nerve. 2. Stimulation at low intensity through the recording microelectrode activated efferent axons innervating motor units in close proximity to the muscle spindle or tendon organ. There was a clear alteration in the discharge of 17 afferents (15 muscle spindle, 2 tendon organ) in response to twitch contractions that involved only one, two, or three motor units. With three other afferents there was a less overt but statistically significant alteration in discharge rate by the twitch contraction of a single motor unit. 3. The sensitivity of 21 receptors (20 spindles, 1 tendon organ) to twitch contractions of anatomically close motor units was contrasted with their sensitivity to twitches of more remote motor units in the muscle. In no instance was the sensitivity to the contraction of remote motor units greater than that to the contraction of local motor units stimulated through the microelectrode; with remote stimulation many units usually had to be activated before the resulting twitch contraction altered the discharge of an afferent. 4. It is concluded that muscle spindles as well as tendon organs can play a role in monitoring the activity of motor units anatomically close to the receptor.

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Spike train characteristics of single motor units in the human masseter muscle

Experimental Neurology ◽

10.1016/0014-4886(78)90026-2 ◽

1978 ◽

Vol 61 (3) ◽

pp. 592-608 ◽

Cited By ~ 26

Author(s):

Bess Derfler ◽

Louis J. Goldberg

Keyword(s):

Spike Train ◽

Masseter Muscle ◽

Motor Units ◽

Single Motor ◽

Single Motor Units

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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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