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 tibial is anterior tendon moment arm increases during maximum isometric dorsiflexi on as compared with rest. Background. In musculoskeletal modelling applications, moment arms from pas sive muscles at rest are assumed representative of those measured during is ometric 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 a nkle were taken in six subjects at rest and during maximum isometric dorsif lexion at six ankle angles between dorsiflexion and plantarflexion having t he body placed in the supine position and the knee flexed at 90 degrees. In stant centres of rotation in the tibio-talar joint, tibialis anterior tendo n action lines and moment arms were identified in the sagittal plane at ank le angles of -15 degrees, 0 degrees, +15 degrees and +30 degrees at rest an d 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) com pared with rest. This was attributed to a displacement of both tibialis ant erior tendon action line by 0.8-1.2 cm (P < 0.01) and all instant centres o f rotation by 0.3-0.4 cm (P < 0.01) distally in relation to their correspon ding resting positions. Conclusions and implications. The assumption that the tibialis anterior ten don moment arm does not change from rest to maximum isometric dorsiflexion is invalid. Erroneous tendon forces, muscle stresses and joint moments by a s much as 30% would be calculated using resting tibialis anterior tendon mo ment arms in the moment equilibrium equation around the ankle joint during maximum isometric dorsiflexion.