Difference in medial gastrocnemius mechanical properties between paretic and non-paretic legs in children with unilateral cerebral palsy

Abstract

Children with spastic unilateral cerebral palsy (CP) present very early in childhood alterations in gastrocnemius muscle structural and mechanical properties that can be non-invasively investigated through 2D ultrasonography (US). To gain knowledge about spastic muscle properties during ankle mobilization, the purpose of this study was to assess gastrocnemius medialis (GM) muscle passive stiffness and strain during ankle mobilization in both paretic and non-paretic legs in children with unilateral CP. Thirteen children (6- to 12-year-old) with unilateral spastic CP, participated in two sessions of measurements with a one-week interval. Children were installed prone on an isokinetic dynamometer while their ankle was passively mobilized from 25° plantarflexion to maximal dorsiflexion at 2°/s. EMG activity was monitored to ensure complete relaxation. Displacement of the musculo-tendinous junction (MTJ) was recorded with US during mobilization and was synchonized with passive torque. GM stiffness was then calculated as the slope of the torque/MTJ displacement relationship. GM strain was calculated as the GM excursion divided by GM resting length (measured at neutral position with simple US tape method). Inter-session reliability was assessed with the measurement of coefficient of variation (CV) between both experimental sessions. GM stiffness was higher for the paretic leg than for the non-paretic one (4.2 vs. 3 Nm/cm; P = 0.025). When considered on the same angular range (i.e. −25° plantarflexion to maximal dorsiflexion angle on paretic leg), GM strain was similar between both legs ( P = 0.43). Inter-session reliability of GM stiffness (9.75% ≤ CV ≤ 10.67%), strain (CV ≤ 9.97%) and length (CV ≤ 7.5%) were good. Spastic GM muscle lengthening during ankle mobilization was characterized by an increased stretch resistance when compared to non-paretic muscle despite similar passive GM strain between both legs. This may be due to altered spastic GM growth during childhood, characterized by reduced GM paretic length.