Landmark-free statistical shape modelling reveals effects of age and sex on whole muscle morphology among the triceps surae

The shape of skeletal muscle has important influences on muscle function, yet studies of three-dimensional shape variation are rarely performed. Analysis of muscle shape variation using traditional tools is limited by lack of anatomical landmarks, but modern landmark-free methods provide new opportunities to study complex shapes. We used generalized Procrustes surface analysis to characterize shape variation among the triceps surae: medial gastrocnemius (MG), lateral gastrocnemius (LG) and soleus (SOL), digitized using magnetic resonance imaging from 21 younger (8 females, 13 males; 24.6 ± 4.3 years) and 15 older (6 females; 9 males; 70.4 ± 2.4 years) physically active participants. In both gastrocnemii, the first principal component (PC) of shape variance was related to muscle width and thickness. The second PC was related to variation in the MG's insertion and variation in thickness along the LG long axis. In the SOL, the first PC was related to overall muscle thickness and length while the second PC captured variation in lateral margin thickness and curvature of the medial border. Muscle shape differed between young and older adults in MG and LG, while SOL shape differed between males and females. These findings demonstrate statistical shape modelling as a promising tool for disentangling multiple influences on skeletal muscle shape and provide important input for future biomechanical modelling investigations.