With an incidence of 2–3 per 1000 living births, Cerebral Palsy (CP) is the most frequent cause of motor disorders in childhood in Western countries
. Spastic motor disorders are most common in children with CP, with symptoms of spasticity, muscle weakness and decreased selective motor control
, often causing limitations in mobility
, which may lead to a restricted participation in everyday life
Although more than half of all children with bilateral spastic CP (SCP) walk independently with or without an assistive device
, most experience mobility-related problems, such as reduced gait speed and/or an increased walking energy cost
[6–12]. These problems are often caused by gait deviations
[13–16], which can be corrected by prescribing ankle-foot orthoses (AFOs). An AFO imposes a mechanical constraint on the ankle, either to compensate for loss of function
[17–19] or to counteract an excess of function
[20, 21]. An AFO therefore acts by applying control to the ankle and foot and, dependent on its design, it can indirectly stabilise the knee and hip joints
. As such, AFOs aim to improve, i.e. normalise joint kinetics, joint kinematics and spatio-temporal parameters
[17, 23–26]. Improvements in joint kinetics and kinematics have been shown to be closely coupled to an improved walking energy cost, which leads to benefits in walking ability; an effect also noted in the context of orthotic interventions
[23, 25–27]. This applies especially to children who walk with excessive knee flexion in midstance, since this walking pattern is particularly energy consuming
[9, 10] and these children are liable to show deterioration in walking ability in (pre-) puberty
A variety of AFO types are available, depending on the specific gait deviations of the child. For children who walk with excessive knee flexion, orthoses with a ventral shell, also known as Floor Reaction Orthoses (FROs), are commonly prescribed
. Although FROs are widely used in SCP, evidence supporting their effectiveness is so far lacking. The decision-making process leading to FRO prescription is still based on expert opinion and experience (i.e. a trial-and-error approach), resulting in differences in treatment paradigms with respect to both the indication and the mechanical construction of FROs
[30, 31]. This is reflected in current literature, as studies have shown that wearing an FRO can be effective in decreasing walking energy cost, but may also have no effect
 or even be adverse in some children in terms of walking energy cost or gait speed
This variation in FRO effectiveness might be partly explained by the match of the mechanical properties of the orthosis to a patient’s specific gait deviations. Research in adults with neurological disorders has shown that walking energy cost with a typical spring-like AFO could be optimised by choosing the correct AFO ankle stiffness
, suggesting that there may be an optimal match between a patient’s characteristics and the mechanical properties of an AFO. A similar principal might also apply to FROs.
A conventional FRO is a rigid type of AFO, and includes a ventral shell and a rigid footplate. The biomechanical mechanism of an FRO is to create a knee-extensor moment during midstance and terminal stance, by shifting of the ground reaction force forward
. Although an FRO might be effective in this respect, ankle push-off power is obstructed by an impeded plantar flexion in terminal stance and preswing. To enhance push-off power, a more spring-like FRO could potentially be beneficial, since it could store energy at the beginning of the stance phase that is released and returned in preswing. Achieving a sufficiently high stiffness to counteract knee flexion while including the potential benefit of spring-like properties in terms of walking energy cost may result in an optimal FRO stiffness based on the least compromise between these two goals.
Designing and evaluating the efficacy of such an optimal FRO requires an evaluation of the effects of different degrees of FRO ankle stiffness on various aspects of gait, i.e. function, mobility and participation. This implies a need for a set of outcome measures that covers all domains of the International Classification of Functioning, Disability and Health (ICF)
. Evaluating the effects of an intervention on more than one of the ICF domains will provide insights into mutual relations, thereby aiming to identify possible working mechanisms
, which will contribute to improved FRO treatment.
FRO treatment could be further improved by identifying those children who could benefit from FROs
. Rogozinski et al.
 explored clinical examination parameters that might explain the efficacy of FROs in CP children walking with excessive knee flexion. They found a strong, negative correlation between knee and hip flexion contractures and peak knee extension, achieved during walking with an FRO. Other studies have shown that child characteristics and environmental factors predict the response to rehabilitation interventions, such as Botulinum toxin A injections
[36–38] and surgery
[39–41]. Specific patient characteristics might also be relevant predictive factors for FRO efficacy.
In summary, evidence supporting the efficacy of FROs in children with SCP walking with excessive knee flexion remains inconclusive. Understanding of both the underlying working mechanisms and the factors predictive of treatment success is still lacking. Therefore, this project has two main goals:
1. To study the effect of an FRO optimised for ankle stiffness on walking energy cost in children with SCP walking with excessive knee flexion, compared to walking with shoes alone.
2. To identify the possible working mechanisms of an FRO, and the predictors for success of FRO treatment in children with SCP, walking with excessive knee flexion.