Influence of mechanical properties on the relation between EMG activity and torque.

An attempt was made to determine the average stiffness and viscosity of the muscle groups responsible, in human subjects, for the flexion and extension of the forearm, when these movements are produced intentionally. To this end, the subjects were instructed to produce sinusoidal movements by tracking acoustic and visual signals. 1. By recording simultaneously the sinusoidal angular variations and the EMG activity of both triceps and biceps it was possible to determine the phase angle between EMG and inertial torque at different frequencies (between 0.2 and 5 Hz). 2. The known phase relations between EMG and torque were then subtracted from this plot obtaining, therefore, the phase characteristics of the mechanical system (forearm-lever complex). 3. The average resonant frequency for different amounts of intentional contraction of the muscles responsible for the motion was 0.8 Hz, leading to an estimate of the average stiffness of 2.57 kgw.m/rad. 4. The slope of the phase in proximity of the resonant frequency lends an estimate for the viscosity of 0.85 kgw.m.sec/rad. 5. The contribution to the motion by elastic forces and the active contraction due to intentional motor commands was also considered.