Applying Stretch to Evoke Hyperreflexia in Spasticity Testing: Velocity vs. Acceleration

In neurological disease, muscles often become hyper-resistant to stretch due to hyperreflexia, an exaggerated stretch reflex response that is considered to primarily depend on the muscle’s stretch velocity. However, there still is limited understanding on how different biomechanical triggers applied during clinical tests evoke these reflex responses. We examined the effect of imposing an increasing rotational velocity versus increasing acceleration on evoked plantar flexor muscle responses in children with spastic paresis (SP) and compared the responses to those measured in typically developing (TD) children. A motor-operated ankle manipulator was used to apply different bell-shaped movement profiles, with three levels of maximum velocity (70°/s, 110°/s and 150°/s) and three levels of maximum acceleration (500°/s2, 750°/s2 and 1000°/s2). For each profile and both groups, we evaluated the amount of evoked triceps surae muscle activation. In SP, we evaluated two additional characteristics: the intensity of the response (peak EMG burst) and the time from movement initiation to onset of the EMG burst. As expected, the amount of evoked muscle activation was larger in SP compared to TD (all muscles: p0.33-0.36, p≤0.008), but showed no significant effect for acceleration. However, the EMG burst was evoked faster with higher peak acceleration (all muscles p<0.001) whereas it was delayed in profiles with higher peak velocity (medial gastrocnemius and soleus: p<0.006). We conclude that while exaggerated response intensity (peak EMG burst) seems linked to stretch velocity, higher accelerations seem to evoke faster responses (time to EMG burst onset) in triceps surae muscles in SP. Understanding and controlling for the distinct effects of different biological triggers, including velocity, acceleration but also length and force of the applied movement, will contribute to the development of more precise clinical measurement tools. This is especially important when aiming to understand the role of hyperreflexia during functional movements where the biomechanical inputs are multiple and changing.