Time Course for the Acute Effects of Passive Stretching on Isometric Strength and Neuromuscular Function

Abstract

PURPOSE: To examine the time course for the acute effects of eight min of passive stretching on isometric peak torque (PT), percent voluntary activation (%VA), electromyography (EMG), and mechanomygraphy (MMG) of the plantarflexor muscles. METHODS: Fourteen (mean age ± SD = 22 ± 3 yr) subjects performed sixteen consecutive 30-s passive stretches (8 min of time under stretch) on an active isokinetic dynamometer. PT, %VA, EMG, and MMG amplitude were assessed before, immediately after, and at 10, 20, and 30 min post-stretching. Torque (Nm) values were sampled from the dynamometer, while normalized EMG and MMG amplitude values were recorded from the soleus and medial gastrocnemius muscles. %VA was assessed by tibial nerve stimulation and the twitch interpolation technique. RESULTS: There was a 9% decrease (P<0.05) in PT immediately after the stretching. PT remained non-significantly (P>0.05) depressed at 10 (5%), 20 (3%), and 30 (6%) min post-stretching. Normalized MMG amplitude was unchanged immediately after the stretching, but progressively increased at 10 (20%, P>0.05), 20 (51%, P<0.05), and 30 min (40%, P<0.05) post-stretching. %VA and normalized EMG amplitude were unaltered (P>0.05). CONCLUSIONS: Eight min of passive stretching reduced plantarflexor strength immediately after the stretching, however, most of the force deficit had recovered after 10 min. Interestingly, the stretching-induced force deficit was not accompanied by decreases in muscle activation (%VA or EMG amplitude) or changes in MMG amplitude. However, the delayed increase in MMG amplitude may have reflected increases in active musculotendinous compliance, which may have been masked by inflammation or swelling shortly after the stretching protocol. Therefore, these findings suggested that decreases in plantarflexor strength after only 8 min of passive stretching may be attributed to increases in musculotendinous compliance, rather than decreases in muscle activation.