Functional role of muscle reflexes for force generation in the decerebratewalking cat

1. To quantify the importance of reflexes due to muscle length changes in g enerating force during walking, we studied high decerebrate cats that walke d on a treadmill. One leg was denervated except for the triceps surae and a few other selected muscles. The triceps surae muscles are ankle extensor m uscles that attach to the Achilles' tendon which was cut and connected to a muscle puller. In some steps the EMG activity triggered the puller to move the muscle through the pattern of length changes that are normally produce d by ankle movements in intact cats walking over ground (simulated walking) . In other steps the muscles were held isometrically. The EMG and force pro duced during the two types of steps were compared. On average about 50% mor e EMG was generated during the E2 part of the simulated stance phase in the triceps surae muscles, but not in other muscles studied. 2. Force was increased significantly over the entire stance phase by about 20%, when muscle stretches simulating walking were applied. However, during much of the stance phase the triceps surae muscles are shortening and so w ould produce less force. The effect of shortening was assessed in control e xperiments in which these muscles were stimulated at a constant frequency, either isometrically or during simulated walking movements. 3. By combining data from the walking and control experiments, we estimate that about 35% of the force produced in the cat ankle extensors during stan ce is produced by reflexes due to muscle length changes. Other sensory inpu ts may also contribute to force production, but the total reflex contributi on will vary under different conditions of speed, length, loading, task dif ficulty, etc. Since a substantial percentage of the force in the stance pha se of walking is normally produced by muscle reflexes,, this force can be c ontinuously adjusted up or down, if the muscles receive extra stretch or un loading during a particular step cycle.