The effect of the Achilles tendon moment arm on knee joint contact force

Abstract

Musculoskeletal (MSK) models are used extensively in the study of human movement. The validity of a given MSK model is affected by the architectural features of the system. The Achilles tendon moment arm is one important architectural characteristic of lower extremity MSK models as it transforms ankle plantar flexion moment present during many functional weight-bearing activities to Achilles tendon force. Because the gastrocnemius muscle, which crosses both the knee and ankle joints, inserts to the Achilles tendon, the magnitude of the Achilles tendon moment arm affects knee-joint contact forces. However, current estimates of Achilles tendon moment arm vary widely and are apparently influenced by factors including ankle joint position, measurement technique, and muscle contraction level. For example, some estimates of Achilles tendon moment are a constant value while others vary as a function of ankle angle. Imaging techniques (such as magnetic resonance imaging (MRI)) of the Achilles tendon moment arm tend to be larger than those using ultrasound (US) technology. Finally, estimates of the Achilles tendon moment arm while plantarflexor muscles are contracted tend to be larger than those measured during a state of rest. However, the effect of these varied estimates on knee joint contact and Achilles tendon forces during walking and running is unclear. The purpose of this study is to compare Achilles tendon force and knee-joint contact forces during walking and running using a musculoskeletal model and five different estimates of Achilles tendon moment arm (MRI/contracted, MRI/rest, US/contracted, US/rest, constant value). Three dimensional gait and ground reaction force data from 10 participants will be used as inputs to a knee-joint musculoskeletal model from which tibiofemoral joint, patellofemoral joint, and Achilles peak force and force*time impulse will be derived using 5 separate Achilles tendon moment arm estimates. Comparisons of the effect of Achilles tendon moment arm will be made descriptively.