FORCE-SHARING BETWEEN CAT SOLEUS AND GASTROCNEMIUS-MUSCLES DURING WALKING - EXPLANATIONS BASED ON ELECTRICAL-ACTIVITY, PROPERTIES, AND KINEMATICS

Studying force sharing between synergistic muscles can be useful for u nderstanding the Functional significance of musculoskeletal redundancy and the mechanisms underlying the control of synergistic muscles. The purpose of this study was to quantify and explain force sharing betwe en cat soleus (SO) and gastrocnemius (GA) muscles, and changes in forc e sharing, as a function of integrated electrical activity (IEMG), con tractile and mechanical properties, and kinematics of the muscles for a variety of locomotor conditions. Forces in SO and GA were measured u sing standard tendon force transducers of the 'buckle' type, and EMGs were recorded using bipolar, indwelling fine wire electrodes. Muscle t endon and fiber lengths, as well as the corresponding velocities, were derived from the hindlimb kinematics, anthropometric measurements, an d a muscle model. In order to describe force- and IEMG-sharing between SO and GA, SO force vs GA force and SO IEMG vs GA IEMG plots were con structed. Force- and IEMG-sharing curves had a loop-like shape. Direct ion of formation of the loop was typically counterclockwise for forces and clockwise for IEMG; that is, forces of GA reached the maximum and then decreased faster relative to forces of SO, and IEMG of SO reache d the maximum and then decreased faster relative to IEMG of GA. With i ncreasing speeds of locomotion, the width of the force-sharing loops t ended to decrease, and the width of the IEMG-sharing loops increased. Peak forces in GA muscle and peak IEMGs in SO and GA muscles tended to increase with increasing speeds of locomotion, whereas peak SO forces remained nearly constant for all activities. Because of these changes in the peak forces and IEMGs of SO and GA, the slope of the force-sha ring loop decreased, and the slope of the IEMG-sharing loop did not ch ange significantly with increasing speeds of locomotion. Length change s and velocities of SO and GA increased with the speed of locomotion a nd were similar in absolute magnitude for both muscles at a given spee d. However, SO tended to work consistently closer than GA to the optim al length for all activities. The normalized velocities of elongation and shortening of SO fibers were consistently larger than those of GA, and the differences in these velocities increased as the speed of loc omotion increased, The different direction of formation between the fo rce-sharing loops and the IEMG-sharing loops may be explained by the d ifference in the speed-related contractile parameters (twitch contract ion time and twitch half-relaxation time) between SO and GA, and by th e steeper ascending limb of the force-length relation of GA compared t o SO. The decrease in the width of the force-sharing loop with increas ing speeds of locomotion was explained by the steeper decrease in IEMG of SO after achievement of its peak value at high, compared to low sp eeds of locomotion, and also by the faster increase in the normalized fiber shortening velocity of SO compared to GA, with increasing speeds of locomotion. The results of this study suggest that contractile con ditions of the muscles play an important part in force sharing, despit e suggestions to the contrary. The results further imply that SO is no t working at its full capacity for any of the speeds of locomotion tes ted in this study, which is in contrast to suggestions made elsewhere.