Effects of botulinum toxin type A on non‐injected bi‐articular muscle include a narrower length range of force exertion and increased passive force

INTRODUCTION: The goal of this study was to test the hypothesis that botulinum toxin type A (BTX-A) injection in rat tibialis anterior (TA) muscle affects the mechanics of its bi-articular synergist, both actively and passively. METHODS: Two groups of Wistar rats were tested: control (no BTX-A) and BTX (0.1 U of BTX-A) animals were injected exclusively to the mid-belly of TA. Extensor digitorum longus (EDL) muscle isometric forces were measured after proximal and distal lengthening. RESULTS: Five days after injection, BTX-A administration changed EDL mechanics: (1) active forces decreased (proximal muscle length dependently); (2) length range of active force exertion decreased both proximally and distally; and (3) passive muscle forces increased. CONCLUSIONS: Effects of BTX-A appear to not be limited to decreased active muscle tone, but may cause also a narrower active range of movement and increased passive resistance. Through spread of BTX-A to a bi-articular muscle, such effects are plausible for both joints spanned.