scholarly journals Identification of single motor units in skeletal muscle under low force isometric voluntary contractions using ultrafast ultrasound

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Robin Rohlén ◽  
Erik Stålberg ◽  
Christer Grönlund
Keyword(s):  
Exercise Physiology ◽  
Ultrasound Image ◽  
Neuromuscular Diseases ◽  
Motor Units ◽  
Rehabilitation Medicine ◽  
Non Invasive ◽  
Single Motor Units ◽  
The Central Nervous System ◽  
Spatio Temporal ◽  
Voluntary Contractions

AbstractThe central nervous system (CNS) controls skeletal muscles by the recruitment of motor units (MUs). Understanding MU function is critical in the diagnosis of neuromuscular diseases, exercise physiology and sports, and rehabilitation medicine. Recording and analyzing the MUs’ electrical depolarization is the basis for state-of-the-art methods. Ultrafast ultrasound is a method that has the potential to study MUs because of the electrical depolarizations and consequent mechanical twitches. In this study, we evaluate if single MUs and their mechanical twitches can be identified using ultrafast ultrasound imaging of voluntary contractions. We compared decomposed spatio-temporal components of ultrasound image sequences against the gold standard needle electromyography. We found that 31% of the MUs could be successfully located and their firing pattern extracted. This method allows new non-invasive opportunities to study mechanical properties of MUs and the CNS control in neuromuscular physiology.

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

2014 ◽  
Vol 111 (7) ◽  
pp. 1499-1506 ◽  
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.

scholarly journals Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor

2000 ◽  
Vol 528 (1) ◽  
pp. 227-234 ◽  
Author(s):  
Vaughan G. Macefield ◽  
Andrew J. Fuglevand ◽  
John N. Howell ◽  
Brenda Bigland‐Ritchie
Keyword(s):  
Motor Units ◽  
Single Motor ◽  
Single Motor Units ◽  
Voluntary Contractions ◽  
Discharge Behaviour

Synchronization of single motor units during voluntary contractions in the upper and lower extremities

2001 ◽  
Vol 112 (7) ◽  
pp. 1243-1249 ◽  
Author(s):  
Myung-Shin Kim ◽  
Yoshihisa Masakado ◽  
Yutaka Tomita ◽  
Naoichi Chino ◽  
Young Sook Pae ◽  
...  
Keyword(s):  
Lower Extremities ◽  
Motor Units ◽  
Single Motor ◽  
Single Motor Units ◽  
Upper And Lower Extremities ◽  
Voluntary Contractions

scholarly journals Time course of human motoneuron recovery after sustained low-level voluntary activity

2016 ◽  
Vol 115 (2) ◽  
pp. 803-812 ◽  
Author(s):  
Martin E. Héroux ◽  
Annie A. Butler ◽  
Simon C. Gandevia ◽  
Janet L. Taylor ◽  
Jane E. Butler
Keyword(s):  
Time Course ◽  
Discharge Rate ◽  
Rest Period ◽  
Motor Units ◽  
Triceps Brachii ◽  
Voluntary Activity ◽  
Constant Frequency ◽  
Sustained Activity ◽  
Single Motor Units ◽  
Voluntary Contractions

Motoneurons often fire repetitively and for long periods. In sustained voluntary contractions the excitability of motoneurons declines. We provide the first detailed description of the time course of human motoneuron recovery after sustained activity at a constant discharge rate. We recorded the discharge of single motor units (MUs, n = 30) with intramuscular wire electrodes inserted in triceps brachii during weak isometric contractions. Subjects ( n = 15) discharged single MUs at a constant frequency (∼10 Hz) with visual feedback for prolonged durations (3–7 min) until rectified surface electromyogram (sEMG) of triceps brachii increased by ∼100%. After a rest of 1–2, 15, 30, 60, 120, or 240 s, subjects briefly resumed the contraction with the target MU at the same discharge rate. Each MU was tested with three to four rest periods. The magnitude of sEMG was increased when contractions were resumed, and the target motoneuron discharged at the test frequency following rest intervals of 2–60 s ( P = 0.001–0.038). The increased sEMG indicates that greater excitatory drive was needed to discharge the motoneuron at the test rate. The increase in EMG recovered exponentially with a time constant of 28 s but did not return to baseline even after a rest period of ∼240 s. Thus the decline in motoneuron excitability from a weak contraction takes several minutes to recover fully.

Maximum shortening speed of motor units of various types in cat lumbrical muscles

1993 ◽  
Vol 69 (2) ◽  
pp. 442-448 ◽  
Author(s):  
J. Petit ◽  
M. Chua ◽  
C. C. Hunt
Keyword(s):  
Motor Unit ◽  
Inverse Correlation ◽  
Motor Units ◽  
Maximum Speed ◽  
Single Motor Units ◽  
Maximal Speed ◽  
Contraction Times ◽  
Isometric Twitch ◽  
Lumbrical Muscles ◽  
Speed Of Shortening

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.

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.

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