Acute passive stretching alters the mechanical but not the electrical properties of calf muscles in humans

Abstract

An acute bout of passive muscle stretching has been shown to diminish maximal force and power output. Two mechanisms have been suggested to explain these findings: a mechanical alteration in the stretched muscle and an impaired neural activation. The aim of the study was to evaluate the stretching-induced changes in the electrical and mechanical properties of the muscle fibres during maximal electrically elicited contractions of the calf muscles. Twelve subjects (age 22±1 years; body mass 75±2 kg; stature 179±2 cm) underwent 6 single twitch electrical stimulations at maximal intensity before and after passive stretching. During contractions, EMG and mechanomyogram (MMG) were simultaneously recorded from the belly of the gastrocnemius muscle, together with the force signal. From the analysis of the 3 signals, after stretching it resulted that: i) the force peak, time-to-peak and the peak rate of force production significantly decreased by 12.6±2.9%, 3.5±1.0% and 13.6±4.9%, respectively; ii) the MMG peak-to-peak also decreased (−3.6±1.3%; p<0.05), iii) the EMG-force and MMG-force delays increased significantly by 8.7±3.1 and 9.7±3.5%, respectively; and iv) no differences were found in EMG parameters. In conclusion, acute passive stretching affected the mechanical but not the electrical properties of the maximally contracting muscles, suggesting an alteration of the musculotendinous stiffness, an impairment of the muscle force generating capacity but not a depression of muscle activation. The study was supported by MURST n. 5059281.