Cerebral palsy (CP) is the most common cause of movement disability in childhood, with an incidence of 1.5–2.5 per 1000 live born children. It is a non-progressive disorder that covers a number of neurological conditions, resulting in an abnormal development of movement and postural control. From the perspective of the International Classification of Functioning, Disability and Health (ICF), CP patients present with impairments in body function such as spasticity, low muscle strength, and selective motor control. These impairments may limit the performance of activities and participation in daily life. Improving and optimizing activities and participation are important treatment goals for therapeutic interventions.
A recent review has shown that low muscle strength, and not spasticity, causes the greatest limitations in motor function in children with CP, and this has shifted the focus from spasticity management towards strength training for these children. To be successful, strength training must be individualized, and should involve a progressive increase in intensity, thereby stimulating strength gains that are greater than those associated with normal growth and development (i.e. "overload"). This is known as Progressive Resistance Exercise (PRE), and for this type of exercise any method can be used to bear, overcome or resist force, such as body weight, free weights or machines.
Until recently, PRE was thought to be inappropriate, or potentially dangerous for children with CP because of the unfounded assumptions that such training would increase spasticity. However, this concern is not supported by the results of recent studies which have shown that PRE strength training programs can improve lower limb muscle strength in patients with CP without increasing spasticity [8–11]. These results have been summarised in recent reviews [12–14], in which it was further concluded that PRE strength training can increase muscle strength, but that the effects are probably over-estimated because of the low methodological quality of these studies, and that future studies should furthermore develop more functional training programs aiming at a maximal carry-over into everyday activities.
Recently, new randomized clinical trials (RCT's) have been carried out to evaluate the effect of this type of functional strength training[10, 15, 16] in children with CP. Conflicting results were found on isometric muscle strength[15, 16], gross motor function[10, 15, 16] and walking ability[10, 16]. This might be due to the slightly different evaluation methods, but more probably to the differences in relevant training characteristics, such as type of training (e.g. home or school based), intensity (e.g. load based on body mass or repetition maximum), progression (e.g. none, weekly, individually based) and duration of the training (e.g. 5 or 12 weeks). Unfortunately, this information was not always provided, which hampers correct interpretations. For an adequate interpretation of the effectiveness of the intervention, standardisation and reporting on all relevant aspects of the training is therefore of utmost importance.
For PRE strength training, the key principle should always be standardized: the timely progression in strength intensity based on the child's individual level of strength, to ensure the principle of progressive overload[17, 18]. This is best assessed by the repetition maximum (RM), which is the maximum number of repetitions that can be performed correctly under a given load. The heaviest load with which an exercise can be performed for 1 complete repetition with correct performance is the 1 RM. According to current guidelines, training should start with a dynamic warm-up period, initially 1 or 2 sets of 8–15 repetitions with a light to moderate load (about 30–60% 1 RM) to learn the right technique, and then progress to 3–5 sets of 8–15 repetitions. Loads can safely be progressed to 70–85% of 1 RM. A training frequency of at least 2 non-consecutive days per week is further recommended. In addition, for children the training should be fun, and group-training is thought to increase both the fun and the individual motivation to progress.
It is therefore thought that individualized, but group-given, school-based, functional PRE strength training, with sufficient frequency, intensity and progression, increases the effectiveness of the training, although these aspects need further investigation. The purpose of this study was to evaluate, in an RCT, the effectiveness of such functional PRE lower limb strengthening program in a group of children with CP. A protocol was therefore developed to train lower limb muscle strength, based on the current guidelines for PRE strength training in healthy adults and children[17, 19], the recent literature on strength training for CP [12–14], and the expertise of experienced paediatric physical therapists in the Netherlands. In this protocol, the frequency, duration, weekly intensity and progression on the basis of RM testing of the training, and also the type and technique of each of the exercises were standardized. We hypothesize that children who will follow this structured functional PRE strength training program will increase in muscle strength, which accordingly will lead to functional improvement in gross motor function and walking ability, but with no negative effect increasing spasticity or decreasing range of motion, compared to children receiving usual care. This paper describes the study design and all relevant elements of the functional PRE strength training protocol.