Muscle activity and time to task failure differ with load compliance and target force for elbow flexor muscles

The primary purpose of this study was to determine the influence of load compliance on time to failure during sustained isometric contractions performed with the elbow flexor muscles at four submaximal target forces. Subjects pulled against a rigid restraint during the force task and maintained a constant elbow angle, while supporting an equivalent inertial load during the position task. Each task was sustained for as long as possible. Twenty-one healthy adults (23 ± 6 yr; 11 men) participated in the study. The maximal voluntary contraction (MVC) force was similar ( P = 0.95) before the subjects performed the force and position tasks at each of the four target forces: 20, 30, 45, and 60% of MVC force. The time to task failure was longer for the force tasks (576 ± 80 and 325 ± 70 s) than for the position tasks (299 ± 77 and 168 ± 35 s) at target forces of 20 and 30% ( P < 0.001), but was similar for the force tasks (178 ± 35 and 86 ± 14 s) and the position tasks (132 ± 29 and 87 ± 14 s) at target forces of 45 and 60% ( P > 0.19). The briefer times to failure for the position task at the lower forces were accompanied by greater rates of increase in elbow flexor muscle activity, mean arterial pressure, heart rate, and rating of perceived exertion. There was no difference in the estimates of external mechanical work at any target force. The dominant mechanisms limiting time to failure of sustained isometric contractions with the elbow flexor muscles appear to change at target forces between 30 and 45% MVC, with load compliance being a significant factor at lower forces only.

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Men are more fatigable than strength-matched women when performing intermittent submaximal contractions

Journal of Applied Physiology ◽

10.1152/japplphysiol.01342.2003 ◽

2004 ◽

Vol 96 (6) ◽

pp. 2125-2132 ◽

Cited By ~ 97

Author(s):

Sandra K. Hunter ◽

Ashley Critchlow ◽

In-Sik Shin ◽

Roger M. Enoka

Keyword(s):

Perceived Exertion ◽

Voluntary Contraction ◽

Elbow Flexor ◽

Rating Of Perceived Exertion ◽

Men And Women ◽

Task Failure ◽

Rate Of Increase ◽

Absolute Strength ◽

Flexor Muscles ◽

Elbow Flexor Muscles

The purpose of this study was to compare the time to task failure for a series of intermittent submaximal contractions performed with the elbow flexor muscles by men and women who were matched for strength ( n = 20, 18–34 yr). The fatigue task comprised isometric contractions at 50% of maximal voluntary contraction (MVC) torque (6-s contraction, 4-s rest). The MVC torque was similar for the men and women [64.8 ± 9.2 (SD) vs. 62.2 ± 7.9 N·m; P > 0.05]. However, the time to task failure was longer for the women (1,408 ± 1,133 vs. 513 ± 194 s; P < 0.05), despite the similar torque levels. The mean arterial pressure, heart rate, and rating of perceived exertion started and ended at similar values for the men and women, but the rate of increase was less for the women. The rate of increase in the average of the rectified electromyogram (AEMG; % peak MVC) for the elbow flexor muscles was less for the women: the AEMG was greater for the men compared with the women at task failure (72 ± 28 vs. 50 ± 21%; P < 0.05), despite similar AEMG values at the start of the fatiguing contraction (32 ± 9 vs. 36 ± 13%). These results indicate that for intermittent contractions performed with the elbow flexor muscles 1) the sex difference in time to task failure was not explained by the absolute strength of the men and women, but involved another mechanism that is present during perfused conditions, and 2) men required a more rapid increase in descending drive to maintain a similar torque.

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Influence of neural adjustments and muscle oxygenation on task failure during sustained isometric contractions with elbow flexor muscles

Experimental Physiology ◽

10.1113/expphysiol.2011.064303 ◽

2012 ◽

Vol 97 (8) ◽

pp. 918-929 ◽

Cited By ~ 17

Author(s):

Cédric Booghs ◽

Stéphane Baudry ◽

Roger Enoka ◽

Jacques Duchateau

Keyword(s):

Elbow Flexor ◽

Muscle Oxygenation ◽

Isometric Contractions ◽

Task Failure ◽

Flexor Muscles ◽

Elbow Flexor Muscles

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Fatigability of the elbow flexor muscles for a sustained submaximal contraction is similar in men and women matched for strength

Journal of Applied Physiology ◽

10.1152/japplphysiol.00893.2003 ◽

2004 ◽

Vol 96 (1) ◽

pp. 195-202 ◽

Cited By ~ 91

Author(s):

Sandra K. Hunter ◽

Ashley Critchlow ◽

In-Sik Shin ◽

Roger M. Enoka

Keyword(s):

Perceived Exertion ◽

Maximum Voluntary Contraction ◽

Elbow Flexor ◽

Rating Of Perceived Exertion ◽

Emg Activity ◽

Men And Women ◽

Task Failure ◽

Rate Of Increase ◽

Flexor Muscles ◽

Elbow Flexor Muscles

The purpose of this study was to compare the time to task failure for a submaximal fatiguing contraction sustained with the elbow flexor muscles by men and women who were matched for strength ( n = 20, 18-35 yr). The maximal torque exerted at the wrist was similar for the men and women [64.5 ± 8.7 (SD) vs. 64.5 ± 8.3 N·m; P > 0.05], which meant that the average torque exerted during the fatiguing contraction [20% of maximum voluntary contraction (MVC)] was similar for the two sexes. The time to task failure was similar for these strength-matched men and women (819 ± 306 vs. 864 ± 391 s; P > 0.05). The mean arterial pressure was similar at the beginning of the contraction for men (97 ± 12 mmHg) and women (96 ± 15 mmHg; P > 0.05) and at task failure (134 ± 18 vs. 126 ± 26 mmHg; P > 0.05, respectively). Furthermore, the increases in heart rate, torque fluctuations, and rating of perceived exertion during the fatiguing contraction were similar for the two sexes. However, the electromyogram (EMG) activity differed for the men and women: the rate of increase in the average of the rectified EMG (% peak MVC) for all the elbow flexor muscles was less for the women compared with the men ( P < 0.05). Furthermore, the bursts of EMG activity for the elbow flexor muscles increased toward exhaustion for all subjects but at a greater rate for the women compared with the men ( P < 0.05). The results indicate that strength-matched men and women experienced similar levels of muscle fatigue and cardiovascular adjustments during a sustained low-force isometric contraction, despite differences in the EMG activity for the two groups of subjects.

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Prolonged Vibration of the Biceps Brachii Tendon Reduces Time to Failure When Maintaining Arm Position With a Submaximal Load

Journal of Neurophysiology ◽

10.1152/jn.00807.2005 ◽

2006 ◽

Vol 95 (2) ◽

pp. 1185-1193 ◽

Cited By ~ 17

Author(s):

Carol J. Mottram ◽

Katrina S. Maluf ◽

Jennifer L. Stephenson ◽

Melissa K. Anderson ◽

Roger M. Enoka

Keyword(s):

Biceps Brachii ◽

Elbow Flexor ◽

Time To Failure ◽

Tonic Vibration Reflex ◽

Task Failure ◽

Limb Position ◽

Flexor Muscles ◽

Elbow Flexor Muscles ◽

Biceps Brachii Tendon ◽

The Difference

Vibration reduces the amplitudes of the tendon jerk response and the Hoffmann and stretch reflexes in the muscle exposed to the vibration, yet does not alter the time to task failure when the task involves exerting a submaximal force against a rigid restraint. Because the amplitude of the stretch reflex is greater when a limb acts against a compliant load than a rigid restraint, the purpose was to determine the influence of prolonged tendon vibration on the time to failure when maintaining limb position with the elbow flexor muscles. Twenty-five healthy men performed the fatiguing contraction by maintaining elbow angle at 1.57 rad until failure while supporting a load equal to 20% of maximal voluntary contraction (MVC) force. The fatiguing contraction was performed on 3 separate days with different levels of vibration applied to the biceps brachii tendon: no vibration, subthreshold for a tonic vibration reflex (TVR), and suprathreshold for a TVR. MVC force before the fatiguing contraction was similar across the three sessions (mean of 3 sessions: 313 ± 54 N, P = 0.83). Despite a similar decline in MVC force after the fatiguing contraction across conditions (–18.0 ± 8.0%, P > 0.05), the time to task failure was 3.7 ± 1.4 min for the suprathreshold TVR condition, 4.3 ± 2.1 min for the subthreshold TVR condition, and 5.0 ± 2.2 min for the no-vibration condition ( P < 0 0.001). The average EMG of the elbow flexor muscles was similar ( P = 0.22) during the fatiguing contractions. However, the fluctuations in limb acceleration at task onset were greater for the suprathreshold TVR condition ( P < 0.01), but were not different between the subthreshold TVR and no-vibration conditions ( P ≥ 0.22). Furthermore, the difference in the SD of limb acceleration between the no-vibration and vibration conditions was correlated with the difference in time to failure for the no-vibration and subthreshold TVR conditions ( P = 0.03; r2 = 0.22), but not for the no-vibration and suprathreshold TVR conditions ( P = 0.90; r2 = 0.001). These findings indicate that prolonged vibration reduced the time to failure of a sustained contraction when subjects maintained limb position, suggesting that peripheral inputs to the motor neuron pool play a significant role in sustaining a contraction during tasks that require active control of limb position.

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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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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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Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions

Journal of Applied Physiology ◽

10.1152/jappl.2001.91.6.2686 ◽

2001 ◽

Vol 91 (6) ◽

pp. 2686-2694 ◽

Cited By ~ 194

Author(s):

Sandra K. Hunter ◽

Roger M. Enoka

Keyword(s):

Muscle Activation ◽

Pressor Response ◽

Voluntary Contraction ◽

Elbow Flexor ◽

Endurance Time ◽

Muscle Activation Patterns ◽

Flexor Muscles ◽

Elbow Flexor Muscles ◽

Target Force ◽

The Absolute

Women are capable of longer endurance times compared with men for contractions performed at low to moderate intensities. The purpose of the study was 1) to determine the relation between the absolute target force and endurance time for a submaximal isometric contraction and 2) to compare the pressor response and muscle activation patterns of men [26.3 ± 1.1 (SE) yr] and women (27.5 ± 2.3 yr) during a fatiguing contraction performed with the elbow flexor muscles. Maximal voluntary contraction (MVC) force was greater for men (393 ± 23 vs. 177 ± 7 N), which meant that the average target force (20% of MVC) was greater for men (79.7 ± 6.5 vs. 36.7 ± 2.0 N). The endurance time for the fatiguing contractions was 118% longer for women (1,806 ± 239 vs. 829 ± 94 s). The average of the rectified electromyogram (%MVC) for the elbow flexor muscles at exhaustion was similar for men (31 ± 2%) and women (30 ± 2%). In contrast, the heart rate and mean arterial pressure (MAP) were less at exhaustion for women (94 ± 6 vs. 111 ± 7 beats/min and 121 ± 5 vs. 150 ± 6 mmHg, respectively). The target force and change in MAP during the fatiguing contraction were exponentially related to endurance time ( r 2 = 0.68 and r 2 = 0.64, respectively), whereas the change in MAP was linearly related to target force ( r 2 = 0.51). The difference in fatigability of men and women when performing a submaximal contraction was related to the absolute contraction intensity and was limited by mechanisms that were distal to the activation of muscle.

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