Comparison between the directly measured achilles tendon force and the tendon force calculated from the ankle joint moment during vertical jumps.

UNLABELLED: The purpose of this study was to compare the relationship between the achilles tendon force and the tendon force estimated from the ankle joint moment (EATF) during vertical jump. A healthy male subject performed the following jumps on the force platform: (a) maximal vertical jump from a squat position without counter-movement; (b) maximal vertical jump from an erect standing position with a preliminary counter-movement; (c) repetitive submaximal hopping on the spot with preferred frequency. The achilles tendon force was measured directly with an implanted tendon transducer in all jumping conditions. In addition the joint moment obtained from the film and ground reaction force was used to estimate the tendon force. Measurements revealed that the achilles tendon sustained high loads during the vertical jumps. The peak values of the real achilles tendon forces were 2234 N, 1896 N, and 3787 N in the squatting, the counter-movement, and the hopping jumps respectively. The achilles tendon forces were about twice the ground reaction force. The estimated achilles tendon force showed almost the same impulses with actual force, especially during hopping. However, estimated force overestimated the actual force during the early plantarflexion phase of jumping and the maximal error of the estimation was relatively large. RELEVANCE: The direct measurement of the achilles tendon forces during human movement may open ways to better understanding of the mechanisms of muscle function. Its clinical importance can be seen in its possibility to reveal the true loading characteristics of the muscle-tendon complex in almost any movement situation, including those which are expected to cause injury. The findings can be used for estimation of the true tendon forces from the method of inverse dynamics.