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. Author manuscript; available in PMC: 2019 Sep 9.
Published in final edited form as: Sci Am. 1978 Dec;239(6):148–163. doi: 10.1038/scientificamerican1278-148

Figure 4.

Figure 4

STIFFNESS OF AN ISOLATED MUSCLE (the change in the force developed by the muscle when it is stretched, divided by its change in length) increases as the muscle is stretched. Recent results indicate that in the body the reflexes maintain the stiffness of an actively contracting muscle at an almost constant level. J. A. Hoffer and S. Andreassen Ref. [2] of the University of Alberta measured the increase in the stiffness of the cat soleus muscle (a plantar-flexor muscle in the ankle) as a function of force. When the pathways leading from the receptors back to the central nervous system were severed, the stiffness rose in proportion to the force exerted by the muscle (broken black line). With those pathways intact, however, the muscle stiffness was higher. (A closed-loop negative-feedback control system is always stiffer than an open-loop one.) After rising with low forces the stiffness reached a plateau at moderate and high forces (broken color curve). These findings suggest that spring, which has constant stiffness, is a suitable representation of the muscles involved in running, particularly in moderate-to-large range of forces.