MOTOR UNIT DISCHARGE DIFFERS WITH INTENSITY AND TYPE OF ISOMETRIC CONTRACTION PERFORMED WITH THE ELBOW FLEXOR MUSCLES

The rate of change in the fluctuations in motor output differs during the performance of fatiguing contractions that involve different types of loads. The purpose of this study was to examine the contribution of frequency modulation of motor unit discharge to the fluctuations in the motor output during sustained contractions with the force and position tasks. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task). The force and position tasks were performed in random order at a target force equal to 3.6 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of an isolated motor unit from the biceps brachii. Each subject maintained the two tasks for an identical duration (161 ± 93 s) at a mean target force of 22.4 ± 13.6% MVC. As expected, the rate of increase in the fluctuations in motor output (force task: SD for detrended force; position task: SD for vertical acceleration) was greater for the position task than the force task ( P < 0.001). The amplitude of the coefficient of variation (CV) and the power spectra for motor unit discharge were similar between tasks ( P > 0.1) and did not change with time ( P > 0.1), and could not explain the different rates of increase in motor output fluctuations for the two tasks. Nonetheless, frequency modulation of motor unit discharge differed during the two tasks and predicted ( P < 0.001) both the CV for discharge rate (force task: 1–3, 12–13, and 14–15 Hz; position task: 0–1, and 1–2 Hz) and the fluctuations in motor output (force task: 5–6, 9–10, 12–13, and 14–15 Hz; position task: 6–7, 14–15, 17–19, 20–21, and 23–24 Hz). Frequency modulation of motor unit discharge rate differed for the force and position tasks and influenced the ability to sustain steady contractions.

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Downhill running impairs peripheral but not central neuromuscular indices in elbow flexor muscles

Sports Medicine and Health Science ◽

10.1016/j.smhs.2021.03.001 ◽

2021 ◽

Author(s):

Xin Ye ◽

Robert J. Benton ◽

William M. Miller ◽

Sunggun Jeon ◽

Jun Seob Song

Keyword(s):

Elbow Flexor ◽

Downhill Running ◽

Flexor Muscles ◽

Elbow Flexor Muscles

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Low-frequency common modulation of soleus motor unit discharge is enhanced during postural control in humans

Experimental Brain Research ◽

10.1007/s00221-006-0575-7 ◽

2006 ◽

Vol 175 (4) ◽

pp. 584-595 ◽

Cited By ~ 41

Author(s):

G. Mochizuki ◽

J. G. Semmler ◽

T. D. Ivanova ◽

S. J. Garland

Keyword(s):

Postural Control ◽

Motor Unit ◽

Low Frequency ◽

Unit Discharge ◽

Motor Unit Discharge

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Suppression of single motor unit discharge by transcranial magnetic stimulation in health, multiple sclerosis and stroke

Electroencephalography and Clinical Neurophysiology ◽

10.1016/0013-4694(93)91160-3 ◽

1993 ◽

Vol 87 (2) ◽

pp. S69

Author(s):

S.J. Boniface ◽

K.R. Mills ◽

M. Schubert ◽

R. Dengler ◽

K. Wohlfarth

Keyword(s):

Multiple Sclerosis ◽

Transcranial Magnetic Stimulation ◽

Motor Unit ◽

Magnetic Stimulation ◽

Unit Discharge ◽

Single Motor Unit ◽

Single Motor ◽

Motor Unit Discharge

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Viscosity of the elbow flexor muscles during maximal eccentric and concentric actions

European Journal of Applied Physiology and Occupational Physiology ◽

10.1007/bf00262825 ◽

1996 ◽

Vol 73 (1-2) ◽

pp. 157-162 ◽

Cited By ~ 8

Author(s):

A. Martin ◽

B. Morlon ◽

M. Pousson ◽

J. Van Hoecke

Keyword(s):

Elbow Flexor ◽

Flexor Muscles ◽

Elbow Flexor Muscles

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Blockade of 5-HT2 receptors suppresses rate of torque development and motor unit discharge rate during rapid contractions

Journal of Neurophysiology ◽

10.1152/jn.00470.2021 ◽

2021 ◽

Author(s):

Benjamin Ian Goodlich ◽

Sean A Horan ◽

Justin J Kavanagh

Keyword(s):

Motor Unit ◽

Unit Activity ◽

Repeated Measures ◽

Discharge Rate ◽

Voluntary Contraction ◽

Unit Discharge ◽

Motor Unit Activity ◽

Rate Of Torque Development ◽

Motor Unit Discharge ◽

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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Older Adults Use a Unique Strategy to Lift Inertial Loads With the Elbow Flexor Muscles

Journal of Neurophysiology ◽

10.1152/jn.2000.83.4.2030 ◽

2000 ◽

Vol 83 (4) ◽

pp. 2030-2039 ◽

Cited By ~ 83

Author(s):

Andrew E. Graves ◽

Kurt W. Kornatz ◽

Roger M. Enoka

Keyword(s):

Young Adults ◽

Elbow Joint ◽

Muscle Activation ◽

Angular Displacement ◽

Elbow Flexor ◽

Lengthening Contractions ◽

Old Adults ◽

Flexor Muscles ◽

Elbow Flexor Muscles ◽

Target Force

The purpose of this study was to determine the effect of age on the ability to exert steady forces and to perform steady flexion movements with the muscles that cross the elbow joint. An isometric task required subjects to exert a steady force to match a target force that was displayed on a monitor. An anisometric task required subjects to raise and lower inertial loads so that the angular displacement around the elbow joint matched a template displayed on a monitor. Steadiness was measured as the coefficient of variation of force and as the normalized standard deviation of wrist acceleration. For the isometric task, steadiness as a function of target force decreased similarly for old adults and young adults. For the anisometric task, steadiness increased as a function of the inertial load and there were significant differences caused by age. Old adults were less steady than young adults during both shortening and lengthening contractions with the lightest loads. Furthermore, old adults were least steady when performing lengthening contractions. These behaviors appear to be associated with the patterns of muscle activation. These results suggest that different neural strategies are used to control isometric and anisometric contractions performed with the elbow flexor muscles and that these strategies do not change in parallel with advancing age.

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