The Association Between Shear Wave Elastography-Derived Muscle Stiffness and Muscle Force/Activation in Foot and Ankle Muscles: A Systematic Review

Background/Objectives: Shear wave elastography (SWE) quantifies muscle stiffness in vivo, yet its relationship with neuromuscular force and activation in ankle and foot muscles has not been systematically explored. This systematic review aimed to synthesize available evidence on the association between SWE-derived stiffness and neuromuscular force and activation outcomes in ankle and foot muscles. Methods: PubMed, Scopus, and CINAHL were searched through November 2025 for studies reporting SWE-derived stiffness alongside force or activation outcomes in at least one foot or ankle muscle. Methodological quality was assessed with the Downs and Black checklist, and overall certainty of evidence was evaluated using the GRADE framework. This review is registered with PROSPERO (CRD420261348340). Results: Twenty studies (637 participants) were included; the triceps surae was most frequently examined (16 studies). Fourteen studies reported at least one significant association between SWE stiffness and a force or activation outcome. Resting passive stiffness showed weak or absent correlations with maximal voluntary contraction torque but was consistently associated with rate of torque development at elongated muscle lengths. Active SWE stiffness tracked contraction intensity within individuals and correlated with concurrently measured joint moment. The stiffness–force relationship was modulated by joint angle, contraction state, and muscle examined. Methodological quality was fair in 17 studies and poor in three. Conclusions: SWE-derived stiffness is associated with neuromuscular outcomes in a context-dependent manner as follows: passive stiffness reflects explosive force capacity rather than maximal strength, while active stiffness tracks within-subject contraction intensity rather than serving as a between-subject surrogate for absolute force.