Changes in the tibialis anterior tendon moment arm from rest to maximum isometric dorsiflexion : in vivo observations in man

Objective: In the present study, we examined the hypothesis that the tibialis anterior tendon moment arm increases during maximum isometric dorsiflexion as compared with rest. Background: In musculoskeletal modelling applications, moment arms from passive muscles at rest are assumed representative of those measured during isometric muscle contraction. The validity of this assumption is questionable in musculotendon actuators enclosed by retinacular systems as in tibialis anterior. Design and methods: Sagittal-plane magnetic resonance images of the right ankle were taken in six subjects at rest and during maximum isometric dorsiflexion at six ankle angles between dorsiflexion and plantarflexion having the body placed in the supine position and the knee flexed at 90°. Instant centres of rotation in the tibio-talar joint, tibialis anterior tendon action lines and moment arms were identified in the sagittal plane at ankle angles of −15°, 0°, +15° and +30° at rest and during maximum isometric dorsiflexion. Results: At any given ankle angle, the tibialis anterior tendon moment arm during maximum isometric dorsiflexion increased by 0.9–1.5 cm (P < 0.01) compared with rest. This was attributed to a displacement of both tibialis anterior tendon action line by 0.8–1.2 cm (P < 0.01) and all instant centres of rotation by 0.3–0.4 cm (P < 0.01) distally in relation to their corresponding resting positions. Conclusions and implications: The assumption that the tibialis anterior tendon moment arm does not change from rest to maximum isometric dorsiflexion is invalid. Erroneous tendon forces, muscle stresses and joint moments by as much as 30% would be calculated using resting tibialis anterior tendon moment arms in the moment equilibrium equation around the ankle joint during maximum isometric dorsiflexion