The Role of Muscle Stiffness in Meeting the Changing Postural and Locomotor Requirements for Force Development by the Ankle Extensors

Abstract The relationship between the length and stiffness (force/length change) of active muscle and the rates at which motor units are activated by distributed stimulation of divided ventral rootlets supplying the muscle has been explored in cat soleus and lateral gastrocnemius. At stimulus rates equivalent to those used in locomotion (15—55 p/s) peak muscle stiffness is at muscle lengths also used in locomotion. At lower rates of stimulation equivalent to those developed in the tonic stretch reflex of the decerebrate cat, peak muscle stiffness occurs at larger muscle lengths but these lengths are within the range of those used for investigation of the reflex. These experimental results were then compared to the sets of theoretically derived results concerning: a) the amount of force needed from the ankle extensors to counteract a load applied to the hindlimb along a vertical axis when ankle and metatarsophalangeal angles are changed relative to the mechanical axis; and b) the amount of force developed by ankle extensors during slow walking. In all cases (theoretical and experimental) the results reveal a close correlation between required force, muscle length and muscle stiffness. The time lag between a change in motor unit discharge and tension development was also calculated to reveal that in many situations (notably gallop) only muscle stiffness can provide a load compensation, whereas the delays are too long to allow a reflexly induced load compensation.