Variations of isometric elbow force during voluntary contraction after botulinum toxin therapy

Large force variability is a potentially unique signature of spastic muscle that can be estimated using analysis of the force trace during voluntary contractions. Underlying mechanisms of force variation remain largely unknown. However, recently reported findings indicates changes in motor unit discharges in the muscle. Botulinum toxin (BT) deactivates portion of the active contractile fibers in spastic muscle by means of cholinergic neuromuscular blockade. Accordingly, we sought to assess force variability during isometric contractions of BT injected muscles in chronic stroke survivors. We assess force variability by estimating the coefficient of variation and peak frequency of recorded force signals for 12 weeks after BT injections of the biceps brachii. We found higher force variation immediately after the injection and this is followed by a lowering of the values at the end of 12 weeks. The results provide insight into the force production capacity of the BT injected muscle.