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Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, Colorado 80309-0354
Fourteen young
subjects (7 men and 7 women) performed a fatiguing isometric
contraction with the elbow flexor muscles at 20% of maximal voluntary
contraction (MVC) force on three occasions. Endurance time for
session 3 [1,718 ± 1,189 (SD) s] was longer than for
session 1 (1,225 ± 683 s) and session
2 (1,410 ± 977 s). Five men and four women increased
endurance time between session 1 and 3 by 60 ± 28% (responders), whereas two men and three women did not (
3 ± 11%; nonresponders). The MVC force was similar for the responders
and nonresponders, both before and after the fatiguing contraction.
Fatiguing contractions were characterized by an increase in the
electromyogram (EMG) amplitude and number of bursts during the
fatiguing contractions. The responders achieved a similar level of EMG
at exhaustion but a reduced rate of increase in the EMG across
sessions. The rate of increase in EMG across sessions declined for the
nonresponders, but it remained greater than that of the responders. The
increase in burst rate during the contractions declined across sessions
with a negative relation between burst rate and endurance time
(r = 0.42). Normalized force fluctuations increased
during the fatiguing contractions, and there was a positive relation
(r = 0.60) between the force fluctuations and burst
rate. Changes in mean arterial pressure and heart rate during the
fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who
increased the endurance time of a submaximal contraction across three
sessions did so by altering the level and pattern of muscle activation.
electromyography; mean arterial pressure; elbow flexor muscles; fluctuations in muscle force
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