Page 17 - Maximizing the efficacy of ankle foot orthoses in children with cerebral palsy

knee flexion and posterior alignment of the ground reaction force with respect to the knee joint rotation center in late stance. Abnormal timing of plantar flexion movement (i.e. early or late heel rise)[16], caused by short calf muscles, plantar flexor weakness or impaired muscle activation[21], can further deteriorate the third rocker function. A reduced push-off prevents a rapid movement of knee and hip into flexion, [16]resulting in inadequate clearance, and insufficient knee extension at the end of the swing phase. The reduced knee extension during this phase leads to mid- or forefoot contact at initial contact, consequently reducing step length. As muscle shortening in spastic CP occurs according to the movement pattern, it seems apparent that gait deviations are related to an impaired muscle length[20]. In turn, this may reduce the (passive) range of motion of the adjacent joints, i.e limited muscle stretch, which has been shown to be an important cause in the development of muscle contractures[12]. Muscle contractures negatively affect the gait deviations[16], leading to a vicious circle in which gait further deteriorates over time.
The underlying impairments often limit walking ability in CP, as gait deviations are associated with increased energy consumption. This especially applies to children walking with excessive knee flexion during stance, as these children are liable to show deterioration of walking in (pre-)puberty[22,23], and their gait patterns are particularly energy consuming[17,24]. In fact, it has been shown that walking energy consumption in CP can be two to three times higher compared to typically developing children[17,24-26]. To minimize the increase in energy consumption, patients often lower their walking speed to maintain walking over longer distances[25,26]. As such, an increased walking energy cost is commonly observed, which reflects poor gait efficiency[25,26]. Although the nature of the association between underlying impairments, gait deviations and the increased energy consumption during walking in CP is not yet unraveled, abnormal knee and ankle kinematics and kinetics during gait are considered key features. First, the increased internal knee extension moment require high muscle forces to maintain posture[27], which could be related to a higher energy consumption during walking[17,28]. Second, reduced ankle range of motion and ankle push-off power generation have been shown associated with a lower walking speed, which subsequently increases walking energy cost[29-31]. To compensate for the reduced ankle push-off, power is often generated in the hip joint around toe off [16,32,33]. This has been shown to be mechanically less efficient[29,34], and is thus likely to further increase the walking energy cost.
I
General introduction
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