Mechanical properties of the plantarflexor musculotendinous unit during passive dorsiflexion in children with cerebral palsy compared with typically developing children.

AIM To examine the passive length-tension relations in the myotendinous components of the plantarflexor muscles of children with and without cerebral palsy (CP) under conditions excluding reflex muscle contraction. METHOD A cross-sectional, non-interventional study was conducted in a hospital outpatient clinic. Passive torque-angle characteristics of the ankle were quantified from full plantarflexion to full available dorsiflexion in 26 independently ambulant children with CP (11 females, 15 males; mean age: 6 y 11 mo, range 4 y 7 mo-9 y 7 mo) and 26 age-matched typically developing children (18 females, 8 males; mean age 7 y 2 mo, range 4 y 1 mo-10 y 4 mo). In the children with CP, the affected (hemiplegia; n=21) or more affected (diplegia; n=5) leg was tested; in typically developing children, the leg tested was randomly selected. Gross Motor Function Classification System levels were I (n=15) and II (n=11). Care was taken to eliminate active or reflex muscle contribution to the movement, confirmed by the absence of electromyographic activity. RESULTS There were small but significant differences between the two groups for maximum ankle dorsiflexion (p=0.003), but large and significant differences in the torques required to produce the same displacement (p<0.001). Further, the hysteresis of the average loading cycle in the children with CP was over three times that of the typically developing children (p<0.001). INTERPRETATION We believe that the plantarflexor muscles of children with CP are stiffer and intrinsically more resistant to stretch, even though they retain near normal excursion. This increased stiffness is a non-neurally-mediated feature demonstrated by these children. The extent to which it influences function and predisposes the children to development of soft tissue contracture is unknown.