Achilles tendon moment arm length is smaller in children with cerebral palsy than in typically developing children.

When studying muscle and whole-body function in children with cerebral palsy (CP), knowledge about both internal and external moment arms is essential since they determine the mechanical advantage of a muscle over an external force. Here we asked if Achilles tendon moment arm (MA) length is different in children with CP and age-matched typically developing (TD) children, and if MA can be predicted from anthropometric measurements. Sixteen children with CP (age: 10y 7m±3y, 7 hemiplegia, 12 diplegia, GMFCS level: I (11) and II (8)) and twenty TD children (age: 10y 6m±3y) participated in this case-control study. MA was calculated at 20° plantarflexion by differentiating calcaneus displacement with respect to ankle angle. Seven anthropometric variables were measured and related to MA. We found normalized MA to be 15% (∼7mm) smaller in children with CP compared to TD children (p=0.003). MA could be predicted by all anthropometric measurements with tibia length explaining 79% and 72% of variance in children with CP and TD children, respectively. Our findings have important implications for clinical decision making since MA influences the mechanical advantage about the ankle, which contributes to movement function and is manipulated surgically.