Acute effects of passive stretching on the electromechanical delay and evoked twitch properties.

The purpose of this study was to investigate the acute effects of passive stretching on the electromechanical delay (EMD), peak twitch force (PTF), rate of force development (RFD), and compound muscle action potential (M-wave) amplitude during evoked twitches of the plantar flexor muscles. 16 men (mean age +/- SD = 21.1 +/- 1.7 years; body mass = 75.9 +/- 11.4 kg; height = 176.5 +/- 8.6 cm) participated in this study. A single, square-wave, supramaximal transcutaneous electrical stimulus was delivered to the tibial nerve before and after passive stretching. The stretching protocol consisted of nine repetitions of passive assisted stretching designed to stretch the calf muscles. Each repetition was held for 135 s separated by 5-10 s of rest. Dependent-samples t tests (pre- vs. post-stretching) were used to analyze the EMD, PTF, RFD, and M-wave amplitude data. There were significant changes (P < or = 0.05) from pre- to post-stretching for EMD (mean +/- SE = 4.84 +/- 0.31 and 6.22 +/- 0.34 ms), PTF (17.2 +/- 1.3 and 15.6 +/- 1.5), and RFD (320.5 +/- 24.5 and 279.8 +/- 28.2), however, the M-wave amplitude did not change (P > 0.05). These findings suggested that passively stretching the calf muscles affected the mechanical aspects of force production from the onset of the electrically evoked twitch to the peak twitch force. These results may help to explain the mechanisms underlying the stretching-induced force deficit that have been reported as either "mechanical" or "electrical" in origin.