Different recoveries of the first and second phases of the M-wave after intermittent maximal voluntary contractions

We investigated the recovery of muscle electrical properties after intermittent intense exercise by examining separately the first and second phases of the muscle compound action potential (M-wave). M-waves and mechanical twitches were obtained using femoral nerve stimulation throughout the 30-min recovery period following 48 successive intermittent 3-s MVCs. The amplitude, duration, and area of the M-wave first and second phases, and the peak twitch force were measured from the knee extensors. The amplitudes of both the first and second M-wave phases were increased immediately after exercise (P < 0.05), but, whereas the first phase remained enlarged for 5 min after exercise, the increase of the second phase only lasted for 10 s. After 30 min of recovery, the amplitude, area, and duration of both the first and second phases were decreased compared to control values (10-20%, P < 0.05). A significant temporal association was found between the changes in the amplitude and duration of the M-wave first phase (maximal cross correlations, 0.9-0.93; time lag, 0 s). A significant, negative temporal relation was found between the amplitude of the M-wave first phase and the peak twitch force during recovery (P < 0.05). The prolonged enlargement of the M-wave first phase during recovery seems primarily related to fatigue-induced changes in membrane properties, whereas the extremely short recovery of the second phase might be related to changes in muscle architectural features. It is concluded that muscle excitability is impaired even after intermittent fatiguing contractions which allow partial clearance of extracellular K(+)

Neural Adaptations to Strength Training

Scientific study of strength training has revealed numerous physiological mechanisms that contribute to: (1) acute fatigue from a single strength training session and (2) chronic adaptation to repetitive and systematic strength training. Therefore, the purpose of this chapter is initially to discuss potential neural mechanisms that influence force production from the perspective of a single repetition. Thereafter, the chapter will highlight scientific evidence for candidate neural mechanisms that acutely limit force production during a single strength training session and long-term adaptations caused by strength training. For some of these potential neural mechanisms, there is strong scientific evidence and for others evidence has emerged in recent years and requires further investigation.peerReviewe