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 el ectrically stimulated muscle shows surface oscillations that can be detecte d by an accelerometer; the acceleration signal is termed the surface mechan omyogram (MMG). In the study described here we compared, in the human tibia lis anterior muscle, changes in the MMG and force signal characteristics be fore, and immediately after fatigue, as well as during 6 min of recovery, w hen changes in the contractile properties of muscle occur. Fatigue was indu ced 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 pr operties 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 acce leration of force production (d(2)F/dt(2)) 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 amplit ude and MMG p-p at increasing stimulation frequency was greater after fatig ue. 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 d(2)F/dt(2) in un-fatigued muscle and during recovery. In conclusion, the MMG reflects specific aspects of muscle mechanics and can be used to fo llow the changes in the contractile properties of muscle caused by localise d muscle fatigue.