Background:Children with spastic cerebral palsy gradually lose muscle extensibility but the interplay between the muscular and neurological components of the condition is unclear especially in the pathophysiology of equinovalgus gait. Aim:This study aimed to quantify the muscular and neurological disorders in young children with unilateral cerebral palsy, and to investigate the role of the peroneus longus (PL) in equinovalgus gait. Design, setting and population:This was an observational study with prospective assessments of 31 children (median age: 2.9 years, range: 2-6) from outpatient clinic in a tertiary teaching hospital. Methods:Clinical measures of plantar flexor extensibility (XV1), stretch response (XV3), and active ankle dorsiflexion angle (XA) were obtained as well as walking velocity and electromyography of tibialis anterior (TA), gastrocnemius medialis (GM) and PL during walking. Results:We found reduced extensibility of the triceps surae on the paretic side (effect size r = 0.73, p < 0.001 for soleus and r = 0.68, p < 0.001 for gastrocnemius) and a correlation between reduced triceps surae extensibility and earlier stretch response (rho = 0.5, p = 0.004). During the swing phase, there was major co-contraction between TA and GM/PL, and significantly larger activation of PL compared to GM (r = 0.46, p = 0.011). Both GM and PL activation decreased with age. Conclusions:Our results suggest gradual deterioration of the muscular disorder and a link between the muscular and neurological disorders, although plantar flexor co-contraction improved with age. The PL was more activated than the GM and may be considered an intervention target to treat equinovalgus gait.