BTX-A has notable effects contradicting some treatment aims in the rat triceps surae compartment, which are not confined to the muscles injected.

Botulinum toxin type-A (BTX-A) is widely used in treating gastrocnemius medial (GM) and lateral (GL) muscles in cerebral palsy to improve joint motion. However, recent animal experiments indicate inferior BTX-A effects beyond the injected muscle. The goal was to test the following hypotheses in a rat model. (1) BTX-A injected into the GM and GL does not spread into the soleus (SOL), and muscles exposed show (2) a wider length range of force exertion (L), (3) reduced passive forces and (4) no intermuscular interaction effects. Confirming them was considered to indicate BTX-A effects in agreement with the treatment aims and confined to the target muscles. Two groups of Wistar rats were tested: Control (no BTX-A injected) and BTX (0.1 units of BTX-A were injected to the GM and GL, each). GM-GL and SOL muscle distal isometric forces were measured after GM-GL lengthening (condition-1, in which SOL length was kept constant) and with added SOL lengthening (condition-2). Five-days post, BTX-A injection caused significant effects: (1) all muscles showed force drops (minimally by 45%) indicating spread into the SOL. (2) L of GM-GL (conditions-1 and 2) and SOL (condition-2) decreased (up to 25%). (3) Passive forces showed no change (condition-1) or opposite-coupled changes for the GM-GL (decrease by 26.6%) and SOL (increase by 25.4%) (condition-2). Intramuscular connective tissues of all muscles increased. (4) Intermuscular interactions affected BTX-A effects (e.g., GM-GL force drop increased by 48.7% in condition-2 vs. 1). Rejected hypotheses indicate complex widespread BTX-A effects contradicting some treatment aims, hence clinical testing.