Variations of Tendon Tap Force Threshold needed to Evoke Surface Electromyogram Responses after Botulinum Toxin Injection in Chronic Stroke Survivors*

Botulinum toxin (BT) is a safe and effective neuromuscular blocking agent that is clinically utilized to reduce spasticity after stroke. It is often injected repeatedly at a minimum of 12-week intervals. BT targets the neuromuscular junction and chemically denervates muscle fibers from their corresponding spinal motoneurons (MN). We explored the effect of BT on the amplitude of the smallest tendon tap force (i.e. force threshold) required to elicit a detectable biceps brachii surface electromyogram (sEMG) reflex response. We hypothesized that after BT injection, the force threshold would increase due to a decrease in available efferent activation. Two chronic stroke survivors were recruited. Data were collected before and up to 18 weeks after BT injection. For each subject, sEMG responses were analyzed using high-density sEMG (HDsEMG) recordings, and the threshold tapping forces were identified and mapped for all channels. Unexpectedly, median threshold forces (MTF) decreased post-BT (B01: 30%, B02: 50%). However, after the initial decrease, MTF then increased progressively compared to pre-BT and peaked around 12 weeks (B01: ~4 folds, B02: 50%). This is likely because post-BT, fewer available muscle fibers would require larger tapping forces to evoke detectable sEMG responses. In the last recording session (> 12 weeks), MTF did not return to pre-BT levels, indicating that successive botulinum toxin injections may still be effective if spaced much further apart in time.