EMG-force model of the elbows antagonistic muscle pair. The effect of joint position, gravity and recruitment.

A piecewise linear model of the elbow antagonistic muscle pair under isometric conditions was developed. The model consists of a linear myoelectric signal-joint force relationship with a mild slope for the force range of 0-30%, and a second, linear segment of steeper slope for the range of 30%-100%. The slope of the agonist, whether biceps or triceps, is fixed for most of the force range at about 1.16 regardless of elbow angle, whereas the slope of the antagonist varies with elbow angle. The antagonist muscle varies its response in such a way that it regulates the net torque about the joint, compensating for the effect of the gravity vector on the joint geometry and limb mass to stabilize the joint. The myoelectric profile of the medial gastrocnemius muscle of the cat during recruitment at synchronous firing rate of 51 pps (maximal tetanic rate) was obtained and compared with the above model. The force-EMG relationship for recruitment was fitted with a second-order polynomial similar but not equivalent to that of the EMG-joint force model. The minor differences were attributed to the effect of the antagonist, proprioception, ligament status and joint geometry.