In vivo measurement-based estimations of the human Achilles tendon moment arm

The aim of the present study was to estimate and compare in vivo measuremen t-based Achilles tendon moment arm lengths at rest and during isometric pla ntarflexion maximum voluntary contraction (MVC) using the centre-of-rotatio n (COR) and the tendon-excursion (TE) methods. Both methods were based on m orphometric analysis of sagittal-plane magnetic resonance images of the foo t. Using the COR method, moment arms were obtained at ankle angles from 15 degrees of dorsiflexion to 30 degrees of plantarflexion in steps of 15 degr ees, digitizing the perpendicular distance from a moving centre of rotation in the tibio-talar joint to the Achilles tendon action line. The TE method was based on measurement of calcaneal displacement along the tibial axis d uring 15 degrees rotations of the ankle joint, from 30 degrees of dorsiflex ion to 45 degrees of plantarflexion. The two methods gave similar estimatio ns at rest varying from 4.3 to 5.6 cm. Using the COR method, the Achilles t endon moment arm during MVC was larger by 1-1.5 cm (22-27%, P < 0.01) than the respective resting value. In contrast, no difference (P > 0.05) was fou nd between the resting and MVC moment arm estimations of the TE method. The disagreement in moment arms during MVC may be attributed to differences in the assumptions made between the two methods. The TE method has more limit ations than the COR method and its estimations during MVC should be treated with caution. Resting Achilles tendon moment arm estimations of the COR method should be multiplied by 1.22-1.27 when maximal isometric plantarflexion joint moments , musculotendon forces and stresses are predicted using modelling.