Experimental evidence supporting isometric functioning of the extrinsic toe flexors during gait.

Background: The extrinsic toe flexors, flexor hallucis longus and flexor digitorum longus, play an important role in stabilizing the longitudinal arch and supporting high forefoot loads during the stance phase of gait. It was hypothesized that these muscles function isometrically during stance, a strategy thought to provide efficient energy transfer across adjoining body segments, but one for which there is little direct experimental evidence in vivo or in situ.

Methods: Eight lower extremity cadavers were loaded into a robotic apparatus that simulates the kinematics and extrinsic muscle activity of the foot and distal tibia during the stance phase of gait. Instantaneous tendon excursions and forces of the extrinsic toe flexors, as well as plantar pressure distributions during stance, were measured under two muscle control strategies: (1) force feedback control, where tendon forces were matched to forces predicted from normal electromyographic patterns and (2) isometric displacement control, where the representative myotendinous junction was held in a constant location.

Results: Tendon excursions of the flexor hallucis longus (7.18 (1.75)mm) and flexor digitorum longus (6.32 (1.74)mm) under force feedback control were small relative to optimal muscle fiber length (13.6% and 14.2%, respectively). Instantaneous tendon forces and plantar pressure variables were not different (P=0.112-0.912) between the two different muscle control strategies for either muscle.

Interpretation: These findings suggest that the extrinsic toe flexors function isometrically during the stance phase of gait in vivo.