Mechanical equilibrium of the ankle joint: Implications for Achilles tendon injury

A longer Achilles tendon (AT) moment arm (MA) relative to the foot length requires less muscle force for a given plantarflexion moment, thereby lowering mechanical load on the tendon. We examined whether the AT and foot MA lengths in healthy (N = 28) differ from height and body mass matched individuals with AT tendinopathy (N = 28) and rupture (N = 29). We hypothesized that a longer AT MA has a protective effect on tendon loading with the patient groups having smaller MA compared to asymptomatic individuals.

MAs were assessed using 2-D image-based method. AT MA was defined as the shortest distance from the centre line of action of the AT to the line between malleoli. Ratio of ankle lever was calculated by dividing AT by Foot MAs.

AT MA was longer in healthy than either the tendinopathy (mean difference 3.86 mm, p = 0.02) or rupture group (5.41 mm, p < 0.001). Foot MA was not statistically different between the groups (p = 0.06). Ratio of ankle lever differed between groups with the healthy group showing higher ratio compared to both the tendinopathy group (0.04, p = 0.001), and the rupture group (0.06, p < 0.001). AT and Foot MAs correlated with body mass and height, whereas the ratio of ankle lever did not.

A longer AT relative to foot MA length may improve the tendon's safety factor by reducing mechanical stress. This ratio may serve as an accessible intrinsic risk factor for AT injuries, even when assessed only using simple 2-D anatomical measures.