Differential behaviour of the medial and lateral heads of gastrocnemius during plantarflexion: the effect of calcaneal inversion and eversion

Abstract

Gastrocnemius medialis (GM) and lateralis (GL) have been considered as plantarflexors of the talocrural joint but also contribute to inversion and eversion of the calcaneus at the subtalar joint. The contractile behaviour has predominately been investigated for GM and findings have been generalised to both gastrocnemii. However, differences in their morphology and, possibly, function have been reported. The purpose of this thesis was to investigate differential contractile behaviour of GM and GL and the effect of inversion/eversion of the calcaneus. The effect of calcaneal inversion/eversion due to standing posture and due to positioning was investigated. A new imaging-based method to assess calcaneal inversion/eversion during standing was developed. Using this method, the validity of the clinically applied method could not be established. Achilles tendon moment arm is unaffected by calcaneal inversion/eversion posture and position. An exploratory investigation into the application of the centre-of-rotation method to determine inversion/eversion Achilles tendon moment arms was also undertaken. Contractile behaviour of GM and GL was unaffected by calcaneal inversion and eversion but fascicle length, pennation angle and tendon length differed between GM and GL. A comparison of GM and GL showed that fascicle behaviour is similar between them but GM tendon elongation and strain exceeded GL tendon elongation and strain, especially at higher contraction levels. The contraction levels at which tendon strain differences occur are dependent on the amount of calcaneal inversion/eversion position. GM and GL differ significantly in their anatomical composition. Adaptations of GM and GL to calcaneal inversion/eversion appear to occur at the tendon level rather than the fascicle level. Given the differential function of GM and GL reported in the literature and the findings of this thesis, it is suggested that GM and GL could be referred to as two separate muscles instead of two heads of the same muscle. It is, furthermore, suggested that GM and GL are not bi-articular but tri-articular due to their actions at the subtalar joint.

The authors' abstract, as published at the source., 2026 · DOI ↗