Synergistic yet different: Rethinking the gastrocnemii as two functionally distinct muscles

The human medial and lateral gastrocnemius muscles play important roles in gait, standing balance, and numerous sports activities. Because they share a name and primary functions, they are often considered to form a single muscle with two heads, leading to the assumption that they behave identically. In this perspective article, we summarise and discuss evidence of meaningful differences in their anatomy, neural control, biomechanical behaviour, and functional roles, notably through re-analysis of previously published data. Anatomically, the two non-fused gastrocnemius muscles transfer forces to the foot through partially independent distal sub-tendons, and exhibit differences in volume and architecture, with the magnitude of these differences varying greatly across individuals. In contrast to other anatomically defined synergist muscles, they can be independently controlled, suggesting that they receive partially independent neural drive. These differences lead to marked differences in their biomechanical behaviour, including force production during gait. Altogether, these findings provide evidence, albeit indirect, for distinct secondary functions, particularly in generating torque in the frontal plane. These insights highlight the importance of treating the medial and lateral gastrocnemius as distinct muscles; this should benefit experimental biomechanical and modelling studies.