Variation in the human Achilles tendon moment arm during walking

Abstract

The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.

Keywords: Gait; musculoskeletal model; plantarflexors; triceps surae; ultrasound.

Fig. 1

We used ultrasound imaging and quantitative motion capture analysis during walking to measure the AT moment arm from the superficial, midline, and deep edges of the tendon. We defined the ankle “joint center” during walking as the instantaneous transmalleolar midpoint.

Fig 2

A: Group mean (standard error) AT moment arm over an average gait cycle and group mean EMG profiles of the lateral gastrocnemius (LG) and soleus (SOL) muscles. Asterisks (*) indicate significantly different from preceding 20% gait cycle (p<0.001). B: Group mean AT moment arm versus ankle joint angle. Heel strikes (HS) and toe-offs (TO) were identified from each subject's curves using a 10% threshold of the vertical ground reaction force. The events shown here represent the group average instants of HS and TO. Asterisk (*) indicates a significant increase in AT moment arm during push-off compared to heel strike (p<0.001).

Fig 3

Comparison of AT moment arm values versus plantarflexion angle estimated during walking to previous literature across a variety of techniques and conditions. Center of Rotation (CoR): 1-3; Model Prediction: 4; Tendon Excursion (TE): 5,6,8,10; Hybrid technique: 6,7. MVC: maximum voluntary contraction.