The role of the ankle plantar flexors in normal walking.

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Abstract
Gait studies were performed on 5 normal adult subjects before and after tibial-nerve block to assess the role of the ankle plantar-flexor muscles during walking. After the block all subjects were unable to transfer weight to the forward part of the foot on the blocked side, stance-phase ankle dorsiflexion and knee flexion were increased, and the duration of single-limb stance was shortened on the side of the block. Quadriceps-muscle phasic activity was prolonged to compensate for loss of the normal contribution of the ankle plantar-flexor muscles to knee stability, and there was an average reduction of step length on the opposite side of 16 cm as well as a 23% average reduction in walking velocity. In the absence of the normal plantar-flexor phasic activity, the horizontal and vertical velocities increased during part of the gait cycle, indicating an exaggerated fall of the center of mass. The excessive work output that was demonstrated after tibial-nerve block in each subject is explained by the exaggerated fall of the center of mass, necessitating excessive energy expenditure to lift the center of mass through the sound limb. The role of the ankle plantar-flexors during gait is to contribute to knee stability, provide ankle stability, restrain the forward rotation of the tibia on the talus during stance phase, and conserve energy by minimizing vertical oscillation of the whole-body center of mass. To accomplish these functions, the plantar flexors first provide active resistance to forward rotation of the tibia by lengthening (eccentric muscle reaction) while providing increasing resistance that eventually checks further forward rotation of the tibia. They then progressively shorten (concentric muscle reaction), reducing ankle dorsiflexion, adjusting limb length, and restraining the drop of the whole-body center of mass. They do not propel the body forward although, paradoxically, maximum step length is not possible without the essential stabilizing effect of the plantar flexors. Identification of the individual components of the calcaneal limp provides the necessary foundation for clinical recognition of this disorder. The single most important clinical observation is that ankle dorsiflexion progressively increases during single-limb stance until opposite foot-strike occurs, and the ipsilateral heel does not leave the floor until weight is transferred to the opposite foot. Measurement of the significant gait variables affected by weakness of the triceps surae indicates that the disturbance of walking is profound, despite all appearances to the contrary. Measurements must be made before and after treatment if orthotic and surgical corrective measures are to be assessed accurately.

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