Force-sharing between cat soleus and gastrocnemius muscles during walking: Explanations based on electrical activity, properties, and kinematics

Abstract

Studying force sharing between synergistic muscles can be useful for understanding 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 between cat soleus (SO) and gastrocnemius (GA) muscles, and changes in force sharing, as a function of integrated electrical activity (IEMG), contractile and mechanical properties, and kinematics of the muscles for a variety of locomotor conditions. Forces in SO and GA were measured using standard tendon force transducers of the ‘buckle’ type, and EMGs were recorded using bipolar, indwelling fine wire electrodes. Muscle tendon and fiber lengths, as well as the corresponding velocities, were derived from the hindlimb kinematics, anthropometric measurements, and 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 constructed. Force- and IEMG-sharing curves had a loop-like shape. Direction 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 reached the maximum and then decreased faster relative to IEMG of GA. With increasing speeds of locomotion, the width of the force-sharing loops tended 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-sharing loop decreased, and the slope of the IEMG-sharing loop did not change significantly with increasing speeds of locomotion. Length changes and velocities of SO and GA increased with the speed of locomotion and were similar in absolute magnitude for both muscles at a given speed. However, SO tended to work consistently closer than GA to the optimal 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 locomotion increased. The different direction of formation between the force-sharing loops and the IEMG-sharing loops may be explained by the difference in the speed-related contractile parameters (twitch contraction time and twitch half-relaxation time) between SO and GA, and by the steeper ascending limb of the force-length relation of GA compared to SO. The decrease in the width of the force-sharing loop with increasing speeds of locomotion was explained by the steeper decrease in IEMG of SO after achievement of its peak value at high, compared to low speeds 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 conditions of the muscles play an important part in force sharing, despite suggestions to the contrary. The results further imply that SO is not working at its full capacity for any of the speeds of locomotion tested in this study, which is in contrast to suggestions made elsewhere.