Surface mechanomyogram reflects the changes in the mechanical properties of muscle at fatigue

The contractile properties of muscle are usually investigated by analysing the force signal recorded during electrically elicited contractions. The electrically stimulated muscle shows surface oscillations that can be detected by an accelerometer; the acceleration signal is termed the surface mechanomyogram (MMG). In the study described here we compared, in the human tibialis anterior muscle, changes in the MMG and force signal characteristics before, and immediately after fatigue, as well as during 6 min of recovery, when changes in the contractile properties of muscle occur. Fatigue was induced by sustained electrical stimulation. The final aim was to evaluate the reliability of the MMG as a tool to follow the changes in the mechanical properties of muscle caused by fatigue. Because of fatigue, the parameters of the force peak, the peak rate of force production and the peak of the acceleration of force production (d2F/dt2) decreased, while the contraction time and the half-relaxation time (1/2-RT) increased. The MMG peak-to-peak (p-p) also decreased. The attenuation rate of the force oscillation amplitude and MMG p-p at increasing stimulation frequency was greater after fatigue. With the exception of 1/2-RT, all of the force and MMG parameters were restored within 2 min of recovery. A high correlation was found between MMG and d2F/dt2 in un-fatigued muscle and during recovery. In conclusion, the MMG reflects specific aspects of muscle mechanics and can be used to follow the changes in the contractile properties of muscle caused by localised muscle fatigue.