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 ante rior (TA) muscle-tendon unit at rest and during isometric dorsiflexion maxi mum voluntary contraction (MVC) from in vivo sagittal-plane magnetic resona nce (MR) and ultrasound scans. Two methods were employed, both of them base d 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 fr om MR scans of the foot by measuring the perpendicular distance between a m oving centre of rotation in the talo-crural joint and the TA tendon action line. Using method B, moment arms mere calculated from the ratio of TA tend on displacement, which was estimated from a planimetric muscle model using pennation angles and muscle thickness measured by ultrasonography, to the t ibial 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 decre ased from similar to 4.5 to similar to 2.9 cm in the transition from dorsif lexion to plantarflexion. Using method A, moment arms during MVC were large r 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 MV C moment arm estimations of method B. Limitations in the oversimplified mus culoskeletal model used raise questions for the validity of both method est imations. (C) 2000 Elsevier Science Ltd. All rights reserved.