Effects of consecutive days of exercise and recovery on muscle mechanical function.

Purpose: To investigate the effects of three consecutive days of prolonged exercise on muscle mechanical function, 12 volunteers (.VO(2peak) = 44.8 +/- 2.0 mL.kg(-1).min(-1), mean +/- SE) cycled at approximately 60% .VO(2peak) until fatigue.

Methods: Quadriceps muscle function was assessed before and after exercise on day 1 (E1) and day 3 (E3) and during three consecutive days of recovery (R1, R2, R3), using both voluntary and electrically induced contractions at various stimulation frequencies.

Results: Exercise on E1 and E3 resulted in a 40% (120 +/- 12 vs 72 +/- 10 N) and 35% (117 +/- 14 vs 78 +/- 8 N) deficit (P < 0.05) in force at 10 Hz, respectively, which remained depressed (P < 0.05) by 32-34% during R1-R3. At 100 Hz, force, although not altered by exercise at E1 or E3, was decreased (P < 0.05) by 12-16% during recovery. The maximal rate of relaxation (-dF/dtmax) at 10 Hz was reduced (P < 0.05) by 38% on E1, by 32% on E3, and remained depressed by 38% through R3. At 100 Hz, -dF/dtmax was only depressed (P < 0.05) during recovery. Maximal rate of force development (+dF/dtmax) at 10 Hz was reduced (P < 0.05) by exercise, but not in recovery. Maximal voluntary contraction force was depressed (P < 0.05) with exercise at both E1 and E3 and remained depressed (P < 0.05) throughout recovery. The reduction (P < 0.05) in motor unit activation assessed with the interpolated twitch technique, observed during recovery, suggests that part of the incomplete recovery (weakness) is central in origin.

Conclusions: These results demonstrate that three consecutive days of prolonged exercise result in a weakness that persists for at least 3 d, compromising force during both voluntary and induced contractions.