Repeated exercise stress impairs volitional but not magnetically evoked electromechanical delay of the knee flexors

The effects of serial episodes of fatigue and recovery on volitional and magnetically evoked neuromuscular performance of the knee flexors were assessed in 20 female soccer players during: (i) an intervention comprising 4 x 35 s maximal static exercise, and (ii) a control condition. Volitional peak force was impaired progressively (similar to 16% vs. baseline: 235.3 +/- 54.7 to 198.1 +/- 38.5 N) by the fatiguing exercise and recovered to within similar to 97 % of baseline values following 6 min of rest. Evoked peak twitch force was diminished subsequent to the fourth episode of exercise (23.3%: 21.4 +/- 13.8 vs. 16.4 +/- 14.6 N) and remained impaired at this level throughout the recovery. Impairment of volitional electromechanical delay performance following the first episode of exercise (25.5%: 55.3 +/- 11.9 vs. 69.5 +/- 24.5 ms) contrasted with concurrent improvement (10.0%: 24.5 +/- 4.7 vs. 22.1 +/- 5.0 ms) in evoked electromechanical delay (P < 0.05), and this increased disparity between evoked and volitional electromechanical delay remained during subsequent periods of intervention and recovery. The fatiguing exercise provoked substantial impairments to volitional strength and volitional electromechanical delay that showed differential patterns of recovery. However, improved evoked electromechanical delay performance might identify a dormant capability for optimal muscle responses during acute stressful exercise and an improved capacity to maintain dynamic joint stabilty during critical episodes of loading.