Comparison of contractile responses of single human motor units in the toe extensors during unloaded and loaded isotonic and isometric conditions

Much of the repertoire of muscle function performed in everyday life involves isotonic dynamic movements, either with or without an additional load, yet most studies of single motor units measure isometric forces. To assess the effects of muscle load on the contractile response, we measured the contractile properties of single motor units supplying the toe extensors, assessed by intraneural microstimulation of single human motor axons, in isotonic, loaded isotonic, and isometric conditions. Tungsten microelectrodes were inserted into the common peroneal nerve, and single motor axons ( n = 10) supplying the long toe extensors were electrically stimulated through the microelectrode. Displacement was measured from the distal phalanx of the toe with either an angular displacement transducer for the unloaded (i.e., no additional load) and loaded (addition of a 4-g mass) isotonic conditions or a force transducer for the isometric conditions. Mean twitch profiles were measured at 1 Hz for all conditions: rise time, fall time, and duration were shortest for the unloaded isotonic conditions and longest for the isometric conditions. Peak displacements were lower in the loaded than unloaded isotonic conditions, and the half-maximal response in the loaded condition was achieved at lower frequencies than in the unloaded isotonic condition. We have shown that the contractile responses of single motor units supplying the human toe extensors are influenced by how they are measured: twitches are much slower when measured in loaded than unloaded isotonic conditions and slowest when measured in isometric conditions.

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Comparison of the contractile responses to irregular and regular trains of stimuli during microstimulation of single human motor axons

Journal of Neurophysiology ◽

10.1152/jn.00835.2013 ◽

2014 ◽

Vol 111 (7) ◽

pp. 1499-1506 ◽

Cited By ~ 4

Author(s):

Michael Leitch ◽

Vaughan G. Macefield

Keyword(s):

Angular Displacement ◽

Motor Units ◽

Common Peroneal Nerve ◽

Displacement Transducer ◽

Motor Axons ◽

Interspike Intervals ◽

Contractile Responses ◽

Single Motor ◽

Single Motor Units ◽

Voluntary Contractions

During voluntary contractions, human motoneurons discharge with a physiological variability of ∼20%. However, studies that have measured the contractile responses to microstimulation of single motor axons have used regular trains of stimuli with no variability. We tested the hypothesis that irregular (physiological) trains of stimuli produce greater contractile responses than regular (nonphysiological) trains of identical mean frequency but zero variability. High-impedance tungsten microelectrodes were inserted into the common peroneal nerve and guided into fascicles supplying a toe extensor muscle. Selective microstimulation was achieved for 14 single motor axons. Contractile responses were measured via an angular displacement transducer over the relevant toe. After the responses to regular trains of 10 stimuli extending from 2 to 100 Hz were recorded, irregular trains of 10 stimuli, based on the interspike intervals recorded from single motor units during voluntary contractions, were delivered. Finally, the stimulation sequences were repeated following a 2-min period of continuous stimulation at 10 Hz to induce muscle fatigue. Regular trains of stimuli generated a sigmoidal increase in displacement with frequency, whereas irregular trains, emulating the firing of volitionally driven motoneurons, displayed significantly greater responses over the same frequency range (8–24 Hz). This was maintained even in the presence of fatigue. We conclude that physiological discharge variability, which incorporates short and long interspike intervals, offers an advantage to the neuromuscular system by allowing motor units to operate on a higher level of the contraction-frequency curve and taking advantage of catch-like properties in skeletal muscle.

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Intraneural microstimulation of motor axons in the study of human single motor units

Muscle & Nerve ◽

10.1002/mus.20324 ◽

2005 ◽

Vol 32 (2) ◽

pp. 119-139 ◽

Cited By ~ 11

Author(s):

Penelope A. McNulty ◽

Vaughan G. Macefield

Keyword(s):

Motor Units ◽

Motor Axons ◽

Single Motor ◽

Single Motor Units

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In vivo magnetic and electric recordings from nerve bundles and single motor units in mammalian skeletal muscle. Correlations with muscle force.

The Journal of General Physiology ◽

10.1085/jgp.98.5.1043 ◽

1991 ◽

Vol 98 (5) ◽

pp. 1043-1061 ◽

Cited By ~ 14

Author(s):

F L Gielen ◽

R N Friedman ◽

J P Wikswo

Keyword(s):

Motor Unit ◽

Motor Units ◽

Common Peroneal Nerve ◽

Single Motor Unit ◽

Twitch Force ◽

Single Motor ◽

Single Motor Units ◽

Compound Action Potentials ◽

Compound Action

Recent advances in the technology of recording magnetic fields associated with electric current flow in biological tissues have provided a means of examining action currents that is more direct and possibly more accurate than conventional electrical recording. Magnetic recordings are relatively insensitive to muscle movement, and, because the recording probes are not directly connected to the tissue, distortions of the data due to changes in the electrochemical interface between the probes and the tissue are eliminated. In vivo magnetic recordings of action currents of rat common peroneal nerve and extensor digitorum longus (EDL) muscle were obtained by a new magnetic probe and amplifier system that operates within the physiological temperature range. The magnetically recorded waveforms were compared with those obtained simultaneously by conventional, extracellular recording techniques. We used the amplitude of EDL twitch force (an index of stimulus strength) generated in response to graded stimulation of the common peroneal nerve to enable us to compare the amplitudes of magnetically recorded nerve and muscle compound action currents (NCACs and MCACs, respectively) with the amplitudes of electrically recorded nerve compound action potentials (NCAPs). High, positive correlations to stimulus strength were found for NCACs (r = 0.998), MCACs (r = 0.974), and NCAPs (r = 0.998). We also computed the correlations of EDL single motor unit twitch force with magnetically recorded single motor unit compound action currents (SMUCACs) and electrically recorded single motor unit compound action potentials (SMUCAPs) obtained with both a ring electrode and a straight wire serving as a point electrode. Only the SMUCACs had a relatively strong positive correlation (r = 0.768) with EDL twitch force. Correlations for ring and wire electrode-recorded SMUCAPs were 0.565 and -0.366, respectively. This study adds a relatively direct examination of action currents to the characterization of the normal biophysical properties of peripheral nerve, muscle, and muscle single motor units.

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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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The effects of double activations of single motor units on discharge patterns of primary muscle spindle endings in the cat

Neuroscience Letters ◽

10.1016/0304-3940(82)90075-1 ◽

1982 ◽

Vol 28 (3) ◽

pp. 303-307 ◽

Cited By ~ 2

Author(s):

U. Windhorst ◽

R. Schwestka ◽

W. Koehler

Keyword(s):

Muscle Spindle ◽

Motor Units ◽

Single Motor ◽

Single Motor Units ◽

Primary Muscle Spindle Endings ◽

Discharge Patterns ◽

Muscle Spindle Endings

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Biomechanical organization of single motor units in two multi-tendoned muscles of the cat distal forelimb

Experimental Brain Research ◽

10.1007/bf02259116 ◽

1992 ◽

Vol 88 (2) ◽

pp. 411-421 ◽

Cited By ~ 6

Author(s):

N. Fritz ◽

C. Schmidt ◽

T. Yamaguchi

Keyword(s):

Motor Units ◽

Single Motor ◽

Single Motor Units

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Threshold variations of medial pterygoid single motor units during vertical or horizontal force tasks

Journal of Oral Rehabilitation ◽

10.1111/joor.13257 ◽

2021 ◽

Author(s):

Yalda Nozad Mojaver ◽

Paul Tawadros ◽

Polyana Moura Ferreira ◽

Terry Whittle ◽

Greg M. Murray

Keyword(s):

Motor Units ◽

Horizontal Force ◽

Single Motor ◽

Single Motor Units ◽

Medial Pterygoid

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Functional Properties of Single Motor Units in the Inferior Head of Human Lateral Pterygoid Muscle: Task Firing Rates

Journal of Neurophysiology ◽

10.1152/jn.2002.88.2.751 ◽

2002 ◽

Vol 88 (2) ◽

pp. 751-760 ◽

Cited By ~ 23

Author(s):

I. Phanachet ◽

T. Whittle ◽

K. Wanigaratne ◽

G. M. Murray

Keyword(s):

Firing Rate ◽

Correlation Coefficients ◽

Motor Units ◽

Lateral Pterygoid Muscle ◽

Jaw Movements ◽

Firing Rates ◽

Single Motor ◽

Single Motor Units ◽

Fine Control ◽

Contralateral Movement

The precise function of the inferior head of the human lateral pterygoid muscle (IHLP) is unclear. The aim of this study was to clarify the normal function of the IHLP. The hypothesis was that an important function of the IHLP is the generation and fine control of horizontal (i.e., anteroposterior and mediolateral) jaw movements. The activities of 50 single motor units (SMUs) were recorded from IHLP (14 subjects) during two- or three-step contralateral movement ( n = 36) and/or protrusion ( n = 33). Most recording sites were identified by computer tomography. There was a statistically significant overall increase in firing rate as the magnitude of jaw displacement increased between the holding phases (range of increments: 0.3–1.6 mm). The firing rates during the dynamic phases for each unit were significantly greater than those during the previous holding phases but less than those during the subsequent holding phases. For the contralateral step task at the intermediate rate, the cross-correlation coefficients between jaw displacement in the mediolateral axis and the mean firing rate of each unit ranged from r = 0.29 to 0.77; mean ± SD; r = 0.49 ± 0.13 (protrusive step task: r = 0.12–0.74, r = 0.44 ± 0.14 for correlation with anterior–posterior axis). The correlation coefficients at the fast rate during the contralateral step task and the protrusive step task were significantly higher than those at the slow rate. The firing rate change of the SMUs per unit displacement between holding phases was significantly greater for the lower-threshold than for the higher-threshold units during contralateral movement and protrusion. After dividing IHLP into four regions, the SMUs recorded in the superior part exhibited significantly greater mean firing rate changes per unit displacement during protrusion than for the SMUs recorded in the inferior part. Significantly fewer units were related to the protrusive task in the superior–medial part. These data support previously proposed notions of functional heterogeneity within IHLP. The present findings provide further evidence for an involvement of the IHLP in the generation and fine control of horizontal jaw movements.

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