ACUTE MORPHOLOGICAL AND FUNCTIONAL MODIFICATIONS OF THE GASTROCNEMIUS MUSCLE-TENDON UNIT INDUCED BY PASSIVE STRETCHING

Aim. The aim of this study was to evaluate the changes in the gastrocnemius medialis (GM) muscle-tendon unit (MTU) stiffness induced by passive static stretching and to correlate them with electromyogram (EMG) and mechanomyogram (MMG) signals. Methods. Maximum voluntary isometric force (MVC) of the triceps surae complex was measured at 0° (corresponding to right angle at the ankle joint); the myo-tendinous junction (MTJ) displacement and passive torque (T) of the GM were measured at different dorsiflexion angles (0°, 10° and 20°) and supramaximal tetanic stimulation was applied (50Hz for 2s) at each angle. EMG and MMG were recorded during MVC and stimulation at each angle. All the measurements were repeated after a bout of passive stretching (6 elongations of 45s each, 15s rest in between). Stiffness was calculated for MTU, muscle and tendon components. From EMG signal, root mean square (RMS) and mean frequency (MF) were calculated. From MMG signal, peak-to-peak (p-p) and RMS were identified. Peak torque (pT) was assessed during tetanic evoked contraction. Results. After stretching, MVC, pT and T significantly decreased (P<0.05), as well as MTU and tendon stiffness (P<0.05). EMG parameters significantly decreased during MVC (P<0.05); MMG p-p and RMS were reduced and increased, respectively, in both MVC and tetanic contractions (P<0.05). Strong inverse correlations (R ranging from -0.55 to -0.81) were found between MTU and tendon stiffness and MMG RMS and MMG RMS, before and after stretching in both voluntary and tetanic contractions (P<0.001). Conclusion. Passive stretching decreased MVC under voluntary and electrically-evoked conditions, decreasing the global MTU stiffness, with a main effect on tendon. MMG amplitude reflected the stiffness at MTU and tendon level, and could be used as an indirect index to monitor its changes under different physiological conditions (fatigue, training, muscle temperature manipulation). References. Esposito F, Limonta E, Cè E (2011). Time course of stretching-induced changes in mechanomyogram and force characteristics. Journal of Electromyography and Kinesiology, Oct;21(5):795-802. Morse C. I., Degens H., Seynnes O. R., Maganaris C. N., Jones D. A. (2008).The acute effect of stretching on the passive stiffness of the human gastrocnemius muscle tendon unit. Journal of Physiology 586.1: 97–106. Kato E, Vieillevoye S, Costantino B, Guissard N, Duchateau J (2011). Acute effect of muscle stretching on the steadiness of sustained submaximal contractions of the plantar flexor muscles. Journal of Applied Physiology 110: 407–415.