The primary aim of this study was to determine whether the distribution of force between the three heads of the triceps surae
differs between people with Achilles tendinopathy and controls. We also aimed to determine the effect of this force distribution on subtendon strain.
Data were collected for 21 participants with Achilles tendinopathy and 21, case-wise paired, asymptomatic controls. Ultrasonography was used to measure muscle volume, fascicle length, pennation angle and subtendon length at rest. Muscle activation was estimated using surface electromyography during maximal and submaximal isometric plantarflexion tasks. The product of normalized activation, physiological cross-sectional area (PCSA), and the cosine of the pennation angle was considered as an index of individual muscle force. Displacement of the distal myotendinous junction of each muscle was measured during the submaximal contractions.
The contribution of the gastrocnemius
lateralis (GL) to the overall triceps surae PCSA and activation was 8.5% (p=0.047, d=0.75) and 24.7% lower (main effect group p=0.009, d=0.67) in people with Achilles tendinopathy than in the controls, respectively. Consequently, GL contributed ~28% less (main effect group p=0.025, d=0.62) of the triceps surae force in people with Achilles tendinopathy. The contribution of gastrocnemius medialis
was not different between groups. Subtendon strain was not different between groups (p=0.835).
These results provide evidence for a difference in force-sharing strategy within the triceps surae in people with Achilles tendinopathy compared to the controls. Whether this altered strategy is a cause or a consequence of Achilles tendinopathy should be explored further.