MECHANICAL POWER AND WORK OF CAT SOLEUS, GASTROCNEMIUS AND PLANTARIS MUSCLES DURING LOCOMOTION - POSSIBLE FUNCTIONAL-SIGNIFICANCE OF MUSCLEDESIGN AND FORCE PATTERNS

Electrical activity, forces, power and work of the soleus (SO), the ga strocnemius (GA) and the plantaris (PL) muscles were measured during l ocomotion in the cat in order to study the functional role of these an kle extensor muscles. Forces and electrical activity (EMG) of the thre e muscles were measured using home-made force transducers and bipolar, indwelling wire electrodes, respectively, for walking and trotting at speeds of 0.4 to 1.8 m s(-1) on a motor-driven treadmill. Video recor ds and a geometrical model of the cat hindlimb were used for calculati ng the rates of change in lengths of the SO, GA and PL muscles. The in stantaneous maximum possible force that can be produced by a muscle at a given fibre length and the rate of change in fibre length (termed c ontractile abilities) were estimated for each muscle throughout the st ep cycle. Fibre lengths of the SO, GA and PL were calculated using a p lanar, geometrical muscle model, measured muscle forces and kinematics , and morphological measurements from the animal after it had been kil led. Mechanical power and work of SO, GA and PL were calculated for 14 4 step cycles. The contribution of the positive work done by the ankle extensor muscles of one hindlimb to the increase of the total mechani cal energy of the body (estimated from values in the literature) incre ased from 4-11% at speeds of locomotion of 0.4 and 0.8 ms(-1) to 7-16 % at speeds of 1.2 ms(-1) and above. The relative contributions of the negative and positive work to the total negative and positive work do ne by the three ankle extensor muscles increased for GA, decreased for SO and remained about the same for FL, with increasing speeds of loco motion. At speeds of 0.4-0.8 m s(-1) the positive work normalized to m uscle mass was 7.5-11.0 J kg(-1), 1.9-3.0 J kg(-1) and 5.3-8.4 J kg(-1 ) for SO, GA and PL, respectively. At speeds of 1.2-1.8 ms(-1), the co rresponding values were 9.8-16.7 J kg(-1), 6.0-10.7 J kg(-1) and 13.4- 25.0 J kg(-1). Peak forces of GA and PL increased and peak forces of S O did not change substantially with increasing speeds of locomotion. T he time of decrease of force and the time of decrease of power after p eak values had been achieved were much shorter for SO than the corresp onding times for GA and PL at fast speeds of locomotion. The faster de crease in the force and power of SO compared with GA and PL was caused by the fast decrease of the contractile abilities and the activation of SO. The results of this study suggest that the ankle extensor muscl es play a significant role in the generation of mechanical energy for locomotion.