In vivo measurement-based estimations of the moment arm in the human tibialis anterior muscle-tendon unit

The aim of this study was to estimate the moment arm of human tibialis anterior (TA) muscle–tendon unit at rest and during isometric dorsiflexion maximum voluntary contraction (MVC) from in vivo sagittal-plane magnetic resonance (MR) and ultrasound scans. Two methods were employed, both of them based on the assumption that the ankle joint complex and TA muscle–tendon unit operate in the sagittal plane. Using method A, moment arms were obtained from MR scans of the foot by measuring the perpendicular distance between a moving centre of rotation in the talo-crural joint and the TA tendon action line. Using method B, moment arms were calculated from the ratio of TA tendon displacement, which was estimated from a planimetric muscle model using pennation angles and muscle thickness measured by ultrasonography, to the tibial rotation around the talus, which was measured from the foot MR scans. Using either of the two methods at rest, the estimated TA moment arm decreased from 4.5 to 2.9 cm in the transition from dorsiflexion to plantarflexion. Using method A, moment arms during MVC were larger by 0.9–1.5 cm (33–44%, P<0.01) than the respective resting estimations. In contrast, no difference (P>0.05) was found between the resting and MVC moment arm estimations of method B. Limitations in the oversimplified musculoskeletal model used raise questions for the validity of both method estimations.