A STATIC SHOULDER MODEL-BASED ON A TIME-DEPENDENT CRITERION FOR LOAD SHARING BETWEEN SYNERGISTIC MUSCLES

A static shoulder model including a novel muscular synergy principle f or computing load sharing between the shoulder muscles is presented. T his principle is feasible especially when analysing endurance-type act ivities. According to the principle, time elapsed from the start of th e activity decreases the allowable muscle stress levels on the basis o f the stress-endurance time curves of individual muscles. At low load levels the sum of the squared forces is minimized. At higher load leve ls the increase in the stress levels of individual muscles is countera cted in order to prevent fatigue. In the model the direction of the co ntact force in the glenohumeral joint is constrained. The stiffness of the shoulder can also be constrained to produce more muscular co-cont raction in tasks with high precision demands.