Functional role of muscle reflexes for force generation in the decerebrate walking cat

To quantify the importance of reflexes due to muscle length changes in generating force during walking, we studied high decerebrate cats that walked 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 muscles 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 produced by ankle movements in intact cats walking over ground (simulated walking). In other steps the muscles were held isometrically. The EMG and force produced during the two types of steps were compared. On average about 50 % more EMG was generated during the E2 part of the simulated stance phase in the triceps surae muscles, but not in other muscles studied. 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 would produce less force. The effect of shortening was assessed in control experiments in which these muscles were stimulated at a constant frequency, either isometrically or during simulated walking movements. By combining data from the walking and control experiments, we estimate that about 35 % of the force produced in the cat ankle extensors during stance is produced by reflexes due to muscle length changes. Other sensory inputs may also contribute to force production, but the total reflex contribution will vary under different conditions of speed, length, loading, task difficulty, etc. Since a substantial percentage of the force in the stance phase of walking is normally produced by muscle reflexes, this force can be continuously adjusted up or down, if the muscles receive extra stretch or unloading during a particular step cycle.