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Articles in PresS, published online ahead of print September 13, 2002
J Appl Physiol, 10.1152/jap.00635.2002
Submitted on July 12, 2002
Accepted on September 10, 2002
1 Department of Kinesiology and Applied Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: Roger.Enoka{at}colorado.edu.
Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximum voluntary contraction (MVC) force on three occasions. Endurance time for session 3 (mean ± SD: 1718 ± 1189 s) was longer than session 1 (1225 ± 683 s) and session 2 (1410 ± 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%, non-responders). The MVC force was similar for the responders and non-responders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the 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 non-responders, 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 non-responders 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.
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