The effect of submaximal isometric contraction on fatigue and motor unit recruitment in human elbow flexor muscles

The purpose of this study was to determine the effect of eccentric muscle damage on recruitment threshold force and repetitive discharge properties of low-threshold motor units. Ten subjects performed four tasks involving isometric contraction of elbow flexors while electromyographic (EMG) data were recorded from human biceps brachii and brachialis muscles. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; 3) motor unit recruitment threshold task; and 4) minimum motor unit discharge rate task. These tasks were performed on three separate days before, immediately after, and 24 h after eccentric exercise of elbow flexor muscles. MVC force declined (42%) immediately after exercise and remained depressed (29%) 24 h later, indicative of muscle damage. Mean motor unit recruitment threshold for biceps brachii was 8.4 ± 4.2% MVC, ( n = 34) before eccentric exercise, and was reduced by 41% (5.0 ± 3.0% MVC, n = 34) immediately after and by 39% (5.2 ± 2.5% MVC, n = 34) 24 h after exercise. No significant changes in motor unit recruitment threshold were observed in the brachialis muscle. However, for the minimum tonic discharge rate task, motor units in both muscles discharged 11% faster (10.8 ± 2.0 vs. 9.7 ± 1.7 Hz) immediately after ( n = 29) exercise compared with that before ( n = 32). The minimum discharge rate variability was greater in brachialis muscle immediately after exercise (13.8 ± 3.1%) compared with that before (11.9 ± 3.1%) and 24 h after exercise (11.7 ± 2.4%). No significant changes in minimum discharge rate variability were observed in the biceps brachii motor units after exercise. These results indicate that muscle damage from eccentric exercise alters motor unit recruitment thresholds for ≥24 h, but the effect is not the same in the different elbow flexor muscles.

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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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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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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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Determining Consistency of Elbow Joint Threshold Angle in Elbow Flexor Muscles With Spastic Hypertonia

Physical Therapy ◽

10.1093/ptj/76.6.586 ◽

1996 ◽

Vol 76 (6) ◽

pp. 586-600 ◽

Cited By ~ 15

Author(s):

Steven L Wolf ◽

Richard L Segal ◽

Pamela A Catlin ◽

Julie Tschorn ◽

Tina Raleigh ◽

...

Keyword(s):

Elbow Joint ◽

Elbow Flexor ◽

Flexor Muscles ◽

Elbow Flexor Muscles

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Task Differences With the Same Load Torque Alter the Endurance Time of Submaximal Fatiguing Contractions in Humans

Journal of Neurophysiology ◽

10.1152/jn.00232.2002 ◽

2002 ◽

Vol 88 (6) ◽

pp. 3087-3096 ◽

Cited By ~ 118

Author(s):

Sandra K. Hunter ◽

Daphne L. Ryan ◽

Justus D. Ortega ◽

Roger M. Enoka

Keyword(s):

Mean Arterial Pressure ◽

Arterial Pressure ◽

Elbow Flexor ◽

Emg Activity ◽

Load Torque ◽

Endurance Time ◽

Position Task ◽

Flexor Muscles ◽

Elbow Flexor Muscles ◽

Fatiguing Contractions

Endurance time, muscle activation, and mean arterial pressure were measured during two types of submaximal fatiguing contractions that required each subject to exert the same net muscle torque in the two tasks. Sixteen men and women performed isometric contractions at 15% of the maximum voluntary contraction (MVC) force with the elbow flexor muscles, either by maintaining a constant force while pushing against a force transducer (force task) or by supporting an equivalent inertial load while maintaining a constant elbow angle (position task). The endurance time for the force task (1402 ± 728 s) was twice as long as that for the position task (702 ± 582 s, P < 0.05), despite a similar reduction in the load torque at exhaustion for each contraction. The rate of increase in average electromyographic activity (EMG, % peak MVC value) for the elbow flexor muscles was similar for the two tasks. However, the average EMG was greater at exhaustion for the force task (22.4 ± 1.2%) compared with the position task (14.9 ± 1.0%, P < 0.05). In contrast, the rates of increase in the mean arterial pressure, the rating of perceived exertion, anterior deltoid EMG, and fluctuations in motor output (force or acceleration) were greater for the position task compared with the force task ( P < 0.05). Furthermore, the rate of bursts in EMG activity, which corresponded to the transient recruitment of motor units, was greater for the brachialis muscle during the position task. These results indicate that the briefer endurance time for the position task was associated with greater levels of excitatory and inhibitory input to the motor neurons compared with the force task.

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