Fig. 3

Muscle mechanics during cat walking. Force, electromyographical (EMG) signal, muscle length and fascicle/fibre length (a) for the medial gastrocnemius of a cat during a step cycle. The downward and upward arrows indicate paw contact and paw liftoff at the beginning and end of the stance phase, respectively. b Difference between muscle length changes and fascicle/fibre length changes. Note specifically that at initial paw contact, fascicle/fibre lengths decrease while the muscle is stretched, while just after paw liftoff, the opposite is correct: the muscle shortens while the fascicles/fibres are elongating. Hoffer et al. (1989) [7] were the pioneers in measuring fascicle and muscle lengths in freely moving animals simultaneously, demonstrating the importance of muscle elasticity and reinforcing the notion that muscle fascicle/fibre length did not only depend on muscle length exclusively, but also depended crucially on muscle force. [Reprinted with permission from Elsevier Science Publishers, in Progress in Brain Research, Hoffer et al. 1989 [7]]