Measuring isometric muscle strength in children with CP is associated with uncertainties not least because of their deficits in performing voluntary movements [23]. Despite difficulties involved with the measurements, strength does not seem to be a determining factor in whether the children with CP need support during standing; only minor differences were found in lower limb muscle strength between children standing with or without support. The need for support, as well as a more apparent crouched standing posture in the same individuals, might lead us to believe that difficulties during standing originate from muscle weakness. But contrary to this assumption, the children in this study, were equally strong in their hip and ankle muscles, and the children who required support were somewhat stronger in the knee extensors. The strong knee muscles, however, might be a consequence of the more flexed knees during standing observed in the children SwS. On the other hand, these children stood not only with increased knee flexion, but with a more forward leaning trunk compared to the children in SwoS. As an effect of the flexed body position, the projection of the ground reaction force can be assumed as shifted anteriorly, reducing the internal knee extension joint moment and therefore decreasing the required effort of the knee muscles. Accordingly, the crouched posture during standing in this group of children may not have required much stronger knee muscles than in those standing independently. Furthermore, calf muscle weakness causing instability in the ankle joint may contribute to the flexed knee position during standing. In our study group, the plantarflexors were equally strong in both groups and there were no differences in the amount of calf muscle surgery between the groups. Secondary musculoskeletal problems with decreased joint ROM is frequently observed in children with CP [1, 24]. In our study, hip, knee and ankle contractures were equally pronounced in both groups, whereas knee flexion contractures were more common in the SwS-group. The knee contractures were not believed to negatively influence the children’s ability to produce maximal knee extensor force, as the measurements were performed with a 90° flexed knee.
In a previous study we reported that children with need for support in standing, stood with less knee extension than was passively available in a non-weight bearing position [13]. This finding could be confirmed in the present study where the children stood with more flexed knees than necessary with respect to their passive ROMs. Not utilizing the full possible knee extension range may indicate difficulties in producing antigravity reactions during standing as a consequence of perceptual problems [11]. Furthermore, impaired proprioception in the lower limbs has been reported to be associated with postural instability in bilateral CP and could have contributed to children’s difficulties to extend their legs against gravity [12].
In bilateral CP, an uneven weight bearing is frequently observed during standing, giving an impression of asymmetric muscle strength. We found that the children’s more-WB limb was more extended both in the children in SwS or SwoS. We hypothesized that the support limb, i.e. the more WB limb was the strongest, but this could not be verified despite an asymmetrical loading on the limbs while standing. This is in accordance with Wiley et al. who found both limbs equally strong in children with bilateral CP [3].
Even though there are known uncertainties regarding strength measurements in CP, the strength values obtained in this study correspond to previous findings [25]. Dallmeijer et al. employed a method similar to ours and showed equivalent muscle strength values in the knee extensors, dorsiflexors, and plantarflexors in 25 adolescents with bilateral CP, GMFCS level II-III [25]. In accordance with earlier studies comparing muscle strength between GMFCS levels II and III, the children in our groups, SwS and SwoS, were equally strong in the hip flexors and knee extensors when measured in 90° knee flexion [2, 5]. However, when measured in 30° knee flexion, Thompson et al. reported children in GMFCS level III to be weaker in the knee extensors compared with children in level II [5]. This may be explained by the more severe motor disorder in GMFCS level III with difficulties in performing voluntary movements with an almost extended knee. Contrary to the findings of Eek et al. who reported stronger dorsiflexors and plantarflexors in children in GMFCS level II compared to III [2], we found these muscle groups to be equally strong in both groups. The testing positions used in our study were chosen in order to, to the best of our knowledge, reduce the impact of spasticity, tight muscles, and co-contraction [14]. For example the testing of the dorsiflexors in a sitting position, with a slightly flexed knee, avoiding constraint from tight muscles could have enhanced the children’s ability to produce a maximal voluntary contraction. The possibility to observe the foot during testing may also have supported the ability to selectively perform the dorsiflexion as well as to compensate for probable difficulties in proprioception [14, 26]. On the other hand in our study, there were three children who could not undertake strength measurements of the calf muscles due to poor selective motor control distally, in the plantarflexors. All three had a passive range of motion in the ankle to at least neutral position, hence it was not reduced mobility in the ankle that prevented children to perform the movement. Worth noting is that two of these children had the ability to stand without support and therefore, could be expected to have a milder motor disorder compared to children who required support for standing.
To examine whether the children’s ability to produce strength was influenced by various seated conditions, the measurements were conducted in a stable sitting position on a chair and in a more demanding sitting position on a stool. Contrary to Ferrari et al. we could not find any differences between the two conditions and speculate that children in our study were allowed to remain within their safe base of support at both measurements. In the above mentioned study, children’s limits of stability were provoked during a seated reaching task, thus our method might not have been challenging enough for the sensory-motor system to elicit possible perceptual impairments [9, 10].
A limitation of our study was the choice to use the first trial from each seated condition for the analyses. In order to compare muscle strength between groups, it has been recommended to use the highest value alternative an average of two trials for statistical analyses [27, 28]. However, most of the children had trouble to maintain attention during the testing, and it was demanding to accomplish three strength measurements per muscle group in the two seated conditions. Calculations on the maximum strength values from each muscle group were carried out with the same outcome and therefore, the choice to use data from the first trial does not seem to have affected the results. When using an HHD it is difficult to separate out forces caused by strength from spasticity or non-neural components as contractures. Since spasticity in the calf muscle could be expected to be easily elicited during the measurements as well as some children had contractures in the ankle, plantar flexor data from our study must be interpreted with caution. Our results though, were remarkably similar to the values from the Australian study, conducted on adolescents with CP [25].