An examination of motor unit firing rates during steady torque of maximal efforts with either an explosive or slower rate of torque development

Serotonin (5-HT) is a neuromodulator that is critical for regulating the excitability of spinal motoneurons and the generation of muscle torque. However, the role of 5-HT in modulating human motor unit activity during rapid contractions has yet to be assessed. Nine healthy participants (23.7 ± 2.2 yr) ingested 8 mg of the competitive 5-HT2 antagonist cyproheptadine in a double-blinded, placebo-controlled, repeated-measures experiment. Rapid dorsiflexion contractions were performed at 30%, 50% and 70% of maximal voluntary contraction (MVC), where motor unit activity was assessed by high-density surface electromyographic decomposition. A second protocol was performed where a sustained, fatigue-inducing dorsiflexion contraction was completed prior to undertaking the same 30%, 50% and 70% MVC rapid contractions and motor unit analysis. Motor unit discharge rate (p < 0.001) and rate of torque development (RTD; p = 0.019) for the unfatigued muscle were both significantly lower for the cyproheptadine condition. Following the fatigue inducing contraction, cyproheptadine reduced motor unit discharge rate (p < 0.001) and RTD (p = 0.024), where the effects of cyproheptadine on motor unit discharge rate and RTD increased with increasing contraction intensity. Overall, these results support the viewpoint that serotonergic effects in the central nervous system occur fast enough to regulate motor unit discharge rate during rapid powerful contractions.

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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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Stroke increases ischemia-related decreases in motor unit discharge rates

Journal of Neurophysiology ◽

10.1152/jn.00923.2017 ◽

2018 ◽

Vol 120 (6) ◽

pp. 3246-3256 ◽

Cited By ~ 2

Author(s):

Spencer A. Murphy ◽

Francesco Negro ◽

Dario Farina ◽

Tanya Onushko ◽

Matthew Durand ◽

...

Keyword(s):

Motor Unit ◽

Time Constant ◽

Motor Units ◽

Unit Discharge ◽

Firing Rates ◽

Sensory Pathways ◽

Discharge Rates ◽

Motor Unit Firing ◽

Motor Unit Discharge ◽

Density Surface

Following stroke, hyperexcitable sensory pathways, such as the group III/IV afferents that are sensitive to ischemia, may inhibit paretic motor neurons during exercise. We quantified the effects of whole leg ischemia on paretic vastus lateralis motor unit firing rates during submaximal isometric contractions. Ten chronic stroke survivors (>1 yr poststroke) and 10 controls participated. During conditions of whole leg occlusion, the discharge timings of motor units were identified from decomposition of high-density surface electromyography signals during repeated submaximal knee extensor contractions. Quadriceps resting twitch responses and near-infrared spectroscopy measurements of oxygen saturation as an indirect measure of blood flow were made. There was a greater decrease in paretic motor unit discharge rates during the occlusion compared with the controls (average decrease for stroke and controls, 12.3 ± 10.0% and 0.1 ± 12.4%, respectively; P < 0.001). The motor unit recruitment thresholds did not change with the occlusion (stroke: without occlusion, 11.68 ± 5.83%MVC vs. with occlusion, 11.11 ± 5.26%MVC; control: 11.87 ± 5.63 vs. 11.28 ± 5.29%MVC). Resting twitch amplitudes declined similarly for both groups in response to whole leg occlusion (stroke: 29.16 ± 6.88 vs. 25.75 ± 6.78 Nm; control: 38.80 ± 13.23 vs 30.14 ± 9.64 Nm). Controls had a greater exponential decline (lower time constant) in oxygen saturation compared with the stroke group (stroke time constant, 22.90 ± 10.26 min vs. control time constant, 5.46 ± 4.09 min; P < 0.001). Ischemia of the muscle resulted in greater neural inhibition of paretic motor units compared with controls and may contribute to deficient muscle activation poststroke. NEW & NOTEWORTHY Hyperexcitable inhibitory sensory pathways sensitive to ischemia may play a role in deficient motor unit activation post stroke. Using high-density surface electromyography recordings to detect motor unit firing instances, we show that ischemia of the exercising muscle results in greater inhibition of paretic motor unit firing rates compared with controls. These findings are impactful to neurophysiologists and clinicians because they implicate a novel mechanism of force-generating impairment poststroke that likely exacerbates baseline weakness.

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Investigating the Effect of Persistent Inward Currents on Motor Unit Firing Rates and Beta-Band Coherence in a Model of the First Dorsal Interosseous Muscle*

2019 41st Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC) ◽

10.1109/embc.2019.8857534 ◽

2019 ◽

Author(s):

Sageanne Senneff ◽

Lara McManus ◽

Madeleine M. Lowery

Keyword(s):

Motor Unit ◽

First Dorsal Interosseous ◽

Beta Band ◽

Persistent Inward Currents ◽

Firing Rates ◽

Motor Unit Firing ◽

First Dorsal Interosseous Muscle ◽

Inward Currents ◽

Dorsal Interosseous Muscle

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Sex-related differences in motor unit firing rates and action potential amplitudes of the first dorsal interosseous during high-, but not low-intensity contractions

Experimental Brain Research ◽

10.1007/s00221-020-05759-1 ◽

2020 ◽

Vol 238 (5) ◽

pp. 1133-1144

Author(s):

Mandy E. Parra ◽

Adam J. Sterczala ◽

Jonathan D. Miller ◽

Michael A. Trevino ◽

Hannah L. Dimmick ◽

...

Keyword(s):

Action Potential ◽

Motor Unit ◽

First Dorsal Interosseous ◽

Firing Rates ◽

Low Intensity ◽

Motor Unit Firing

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Abnormal motor unit firing rates in chronic inflammatory demyelinating polyneuropathy

Journal of the Neurological Sciences ◽

10.1016/j.jns.2020.116859 ◽

2020 ◽

Vol 414 ◽

pp. 116859

Author(s):

Kevin J. Gilmore ◽

Eric A. Kirk ◽

Timothy J. Doherty ◽

Kurt Kimpinski ◽

Charles L. Rice

Keyword(s):

Motor Unit ◽

Chronic Inflammatory Demyelinating Polyneuropathy ◽

Demyelinating Polyneuropathy ◽

Firing Rates ◽

Motor Unit Firing ◽

Inflammatory Demyelinating Polyneuropathy ◽

Abnormal Motor

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The effects of vibration-induced altered stretch reflex sensitivity on maximal motor unit firing properties

Journal of Neurophysiology ◽

10.1152/jn.00326.2018 ◽

2019 ◽

Vol 121 (6) ◽

pp. 2215-2221 ◽

Cited By ~ 3

Author(s):

Alejandra Barrera-Curiel ◽

Ryan J. Colquhoun ◽

Jesus A. Hernandez-Sarabia ◽

Jason M. DeFreitas

Keyword(s):

Motor Unit ◽

Muscle Spindle ◽

Stretch Reflex ◽

Muscle Spindles ◽

Motor Unit Action Potential ◽

Firing Rates ◽

Motor Unit Firing ◽

Firing Properties ◽

Spindle Function ◽

Voluntary Strength

It is well known that muscle spindles have a monosynaptic, excitatory connection with α-motoneurons. However, the influence of muscle spindles on human motor unit behavior during maximal efforts remains untested. It has also been shown that muscle spindle function, as assessed by peripheral reflexes, can be systematically manipulated with muscle vibration. Therefore, the purpose of this study was to analyze the effects of brief and prolonged vibration on maximal motor unit firing properties. A crossover design was used, in which each of the 24 participants performed one to three maximal knee extensions under three separate conditions: 1) control, 2) brief vibration that was applied during the contraction, and 3) after prolonged vibration that was applied for ~20 min before the contraction. Multichannel EMG was recorded from the vastus lateralis during each contraction and was decomposed into its constituent motor unit action potential trains. Surprisingly, an approximate 9% reduction in maximal voluntary strength was observed not only after prolonged vibration but also during brief vibration. In addition, both vibration conditions had a large, significant effect on firing rates (a decrease in the rates) and a small to moderate, nonsignificant effect on recruitment thresholds (a small increase in the thresholds). Therefore, vibration had a detrimental influence on both maximal voluntary strength and motor unit firing properties, which we propose is due to altered function of the stretch reflex pathway. NEW & NOTEWORTHY We used vibration to alter muscle spindle function and examined the vibration’s influence on maximal motor unit properties. We discovered that vibration had a detrimental influence on motor unit behavior and motor output by decreasing motor unit firing rates, increasing recruitment thresholds, which led to decreased maximal strength. We believe that understanding the role of muscle spindles during maximal contractions provides a deeper insight into motor control and sensorimotor integration.

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