Revisiting hyper-resistance to muscle stretch in cerebral palsy: muscle hypo-extensibility is more of an issue than hyperreflexia

Hyper-resistance to passive muscle stretch is a common, debilitating symptom of spastic paresis. Although straightforward to assess, hyper-resistance is caused by a complex interaction of altered tissue properties, stretch hyperreflexia, and involuntary background muscle activation. Identifying the contribution of each underlying component causing hyper-resistance is of great significance for designing treatments. The aim of this study was to investigate the components contributing to ankle plantarflexors' hyper-resistance in spastic cerebral palsy. We compared ankle biomechanical and reflex variables during ankle plantarflexor stretches at various velocities in 15 individuals with mild spastic cerebral palsy (GMFCS I, age range: 9-22 yr, 10 males) versus 15 age- and sex-matched typically developing controls. In addition, we evaluated associations between biomechanical and reflex variables. The cerebral palsy group had a median 9° lower maximum passive dorsiflexion range of motion at slow stretch velocity (<i>P</i> = 0.001), a 9° lower stretch reflex threshold (<i>P</i> < 0.01) with higher stretch reflex response magnitude (<i>P</i> ≤ 0.001) for both soleus and medial gastrocnemius muscles, and higher peak torques at fast stretch velocities (<i>P</i> < 0.01). When normalized to the maximum passive range of motion, stretch reflex thresholds were not different between groups. Although hyperreflexia directly contributed to hyper-resistance, normalized stretch reflexes did not occur earlier in the stretch in individuals with cerebral palsy compared with typically developing controls, suggesting a direct influence of muscle hypo-extensibility on hyperreflexia. Treatments for hypo-extensibility are urgently needed, more so than treatments to reduce hyperreflexia. [Clinical trial prospectively registered in the International Standard Randomized Controlled Trial registry (No. 69044459)]<b>NEW & NOTEWORTHY</b> The difference in stretch reflex threshold angles as a measure of hyperreflexia between mild spastic cerebral palsy and typically developing groups was lost when normalized to the passive range of motion. Yet, in the cerebral palsy group, low stretch reflex thresholds correlated with high peak torques during stretches at fast velocities.