Frequency Modulation of Motor Unit Discharge Has Task-Dependent Effects on Fluctuations in Motor Output

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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Motor-Unit Activity Differs With Load Type During a Fatiguing Contraction

Journal of Neurophysiology ◽

10.1152/jn.00837.2004 ◽

2005 ◽

Vol 93 (3) ◽

pp. 1381-1392 ◽

Cited By ~ 114

Author(s):

Carol J. Mottram ◽

Jennifer M. Jakobi ◽

John G. Semmler ◽

Roger M. Enoka

Keyword(s):

Mean Arterial Pressure ◽

Arterial Pressure ◽

Motor Unit ◽

Perceived Exertion ◽

Biceps Brachii ◽

Discharge Rate ◽

Motor Output ◽

Rating Of Perceived Exertion ◽

Position Task ◽

Target Force

Despite a similar rate of change in average electromyographic (EMG) activity, previous studies have observed different rates of change in mean arterial pressure, heart rate, perceived exertion, and fluctuations in motor output during the performance of fatiguing contractions that involved different types of loads. To obtain a more direct measure of the motor output from the spinal cord, the purpose of this study was to compare the discharge characteristics of the same motor unit in biceps brachii during the performance of two types of fatiguing contractions. 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) for a prescribed duration. The force and position tasks were performed in random order at a target force equal to 3.5 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of the isolated motor unit. Each subject maintained the two tasks for an identical duration (161 ± 96 s) at a mean target force of 22.2 ± 13.4% MVC (range: 3–49% MVC). The dependent variables included the discharge characteristics of the same motor unit in biceps brachii, fluctuations in motor output (force or acceleration), mean arterial pressure, heart rate, and rating of perceived exertion. Despite similar increases in the amplitude of the averaged EMG (% MVC) for the elbow flexor muscles during both tasks ( P = 0.60), the rates of increase in mean arterial pressure ( P < 0.001), rating of perceived exertion ( P = 0.023), and fluctuations in motor output ( P = 0.003) were greater during the position task compared with the force task. Consistent with these differences, mean discharge rate declined at a greater rate during the position task ( P = 0.03), and the coefficient of variation for discharge rate increased only during the position task ( P = 0.02). Furthermore, more motor units were recruited during the position task compared with the force task ( P = 0.01). These findings indicate that despite a comparable net muscle torque, the rate of increase in the motor output from the spinal cord was greater during the position task.

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Practice reduces motor unit discharge variability in a hand muscle and improves manual dexterity in old adults

Journal of Applied Physiology ◽

10.1152/japplphysiol.01149.2004 ◽

2005 ◽

Vol 98 (6) ◽

pp. 2072-2080 ◽

Cited By ~ 144

Author(s):

Kurt W. Kornatz ◽

Evangelos A. Christou ◽

Roger M. Enoka

Keyword(s):

Motor Unit ◽

Index Finger ◽

Discharge Rate ◽

Manual Dexterity ◽

Motor Output ◽

Heavy Load ◽

Unit Discharge ◽

Light Load ◽

Maximal Load ◽

Motor Unit Discharge

A steadiness-improving intervention was used to determine the contribution of variability in motor unit discharge rate to the fluctuations in index finger acceleration and manual dexterity in older adults. Ten healthy and sedentary old adults (age 72.9 ± 5.8 yr; 5 men) participated in the study involving abduction of the left index finger. Single motor unit activity was recorded in the first dorsal interosseus muscle before, after 2 wk of light-load training (10% maximal load), and after 4 wk of heavy-load training (70% maximal load). As expected, the light-load training was effective in reducing the fluctuations in index finger acceleration during slow shortening (0.25 ± 0.12 to 0.13 ± 0.08 m/s2) and lengthening contractions (0.29 ± 0.10 to 0.14 ± 0.06 m/s2). Along with the decline in the magnitude of the fluctuations, there was a parallel decrease in the coefficient of variation for discharge rate during both contraction types (33.8 ± 6.8 to 25.0 ± 5.9%). The heavy-load training did not further improve either the fluctuations in acceleration or discharge rate variability. Furthermore, the manual dexterity of the left hand improved significantly with training (Purdue pegboard test: 11 ± 3 to 14 ± 1 pegs). Bivariate correlations indicated that the reduction in fluctuations in motor output during shortening ( r2 = 0.24) and lengthening ( r2 = 0.14) contractions and improvement in manual dexterity ( r2 = 0.26) was directly associated with a decline in motor unit discharge rate variability. There was a strong association between the fluctuations in motor output and manual dexterity ( r2 = 0.56). These results indicate that practice of a simple finger task was accompanied by a reduction in the discharge rate variability of motor units, a decrease in the fluctuations in motor output of a hand muscle, and an improvement in the manual dexterity of older adults.

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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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