Force-generating capacity of the toe flexor muscles and dynamic function of the foot arch in upright standing.

The muscle and tendon complex of the foot helps to support the foot arch and generates the muscle force of the foot. The present study investigated the force-generating capacity of the toe flexor muscles and the dynamic function of the foot arch when standing upright, and the relationships between these indices. The maximum toe flexor force and foot arch height in the sitting and standing positions were studied in the left and right feet of 224 healthy young individuals. To measure the maximum isometric force of the toe flexor muscles, the subjects exerted maximum force on a toe grip dynamometer. Measurements were repeated three times with at least a 1-min rest period between bouts, and the maximum value among the measurements for each foot was used for further analysis. The absolute value of the toe flexor strength was normalised by body mass. The foot arch height was measured the distance between the tuberosity of the navicular bone and the floor, and normalised by height. The relative foot arch height difference between the sitting and standing positions was evaluated as the foot arch dynamics. The maximum isometric toe flexor strength was 42% higher in the standing position than in the sitting position. There was no relationship between the relative toe flexor strength and the relative foot arch height in either the sitting or standing positions; however, the relative increase in toe flexor strength from sitting to standing (the force amplification factor) was related to the foot arch dynamics, and the flexible foot arch showed a greater increase in the toe flexor strength from sitting to standing compared with the strength in the stiff foot arch. The results of this study suggest that the force-generating capacity of the toe flexor muscles is augmented by bodyweight bearing in upright standing. Additionally, the force amplification mechanism is mechanically regulated by the dynamic function of the foot arch in conjunction with the stretching of the muscle-tendon complex of the foot.