During walking at preferred speeds, young children with cerebral palsy (CP)—in comparison with children with typical development—show differences in timing of trunk and hip muscle activation, marked by excessive muscle activation during almost the entire stride cycle and with increased coactivation between the ipsilateral rectus abdominis and erector spinae muscle pair and the rectus femoris and semitendinosus muscle pair. This conclusion by Prosser and colleagues1 was based on surface electromyogphic recordings of 8 trunk, gluteal, and thigh muscles on both body sides. They hypothesize that the excessive muscle activation may create a functionally rigid trunk, limiting “the child's ability to make fine adjustments to trunk position relative to the lower extremities and the environment and the therapist's ability to grade muscle activity in response to external perturbations.” Similarly, the ability to control the body's center of mass during walking will be hampered. Thus, the authors continue, the physical therapy interventions should focus on the reduction of excessive trunk and hip muscle activation and the improvement of the coordination of trunk movements.
Prosser and colleagues’ study of CP gait is unique in recognizing the importance of evaluating the impaired control of trunk and hip muscle activity during posture, gait, and upright movement in general. The authors emphasize that the literature on CP gait has addressed mainly the kinematics and biomechanics of the lower extremities, which is a general concern in the study of pathological gait.2 Another remarkable feature of Prosser and colleagues’ study is the inclusion of children with CP and children with TD with similar walking experience, ranging from 1 month to 5 years. According to the authors, the best evidence indicates that walking experience is a stronger predictor of walking and balance skill than age in early walkers. Walking experience was estimated by the difference between the child's age on the day of study and the age at onset of walking.
The major concern with the outcomes and the conclusions by Prosser and colleagues is whether the excessive trunk and hip muscle activity observed in the children with CP during walking is the result of their neurological disorder or the low speed at which they prefer to walk. In addition, it is open to further investigation how the excessive trunk and hip muscle activity affects the coordination dynamics and biomechanics of the pelvic, thoracic, and trunk rotations. For example, Wagenaar and Beek3 demonstrated that systematically increasing walking speed from 0.25 to 1.50 m/s results in a change in the coordination of trunk rotation in the transversal plane from an in-phase relationship between pelvic and thoracic rotations (pelvis and thorax move in the same direction) in the lower speed range (0.25–0.75 m/s) to an out-of-phase relationship (counter-rotation between pelvis and thorax) in the higher speed range (1.0–1.5 m/s). That is, at lower walking speeds, there is no counter-rotation in the trunk that coincides with a limited contribution of the transverse pelvic rotation to the lengthening of the stride.
However, from 1.0 m/s onward, there is a significant counter-rotation in the trunk as a result of the increased contribution of the transverse pelvic rotation to the lengthening of the stride (“pelvic step”).
In their study of interlimb coordination during walking, Wagenaar and van Emmerik4 showed these differences in phase relationship between transverse pelvic and thoracic rotations are associated with different coordination patterns (frequency and phase relations) between arm and leg movements. Applying dimensionless analysis, Wagenaar and Beek3 were able to classify disorders in trunk and pelvic rotations in the transversal plane in individuals after a stroke. Although the participants in Wagenaar and Beek's study were adults and Prosser and colleagues’ study included children with ages ranging from 1 to 9 years, these observations create an interesting perspective on the comparison of trunk and hip muscle activity between CP and TD gait in Prosser and colleagues’ study.
Prosser and colleagues state that the majority of the children with CP are unable to walk at speeds similar to those of the children with TD and that children with TD have difficulty walking at a steady state at slow speeds. Therefore, they instructed the participants to walk at their preferred speeds. They report that the average normalized (or dimensionless) walking speed was 0.22 (SD=0.10) for the CP group and 0.42 (SD=0.06) for the TD group. In Wagenaar and Beek's study,3 these normalized walking speeds are close to 0.60 and 1.25 m/s, respectively, which suggests that Prosser and colleagues are comparing trunk and hip muscle activity patterns between the CP and TD groups associated with different trunk and interlimb coordination patterns. The authors indicate that although the walking experience between the CP and TD groups was similar, the TD group most likely had more walking practice, which supports the observed difference in normalized walking speed. In addition, the CP group was older than the TD group (mean [±SD] age: 63.1±23.2 months versus 39.7±19.5 months) and had a larger weight (mean [±SD] weight: 19.6±5.9 kg versus 15.1±3.9 kg). Less walking practice and larger weight may explain the differences in normalized walking speed and trunk and hip muscle activation patterns between the CP and TD groups.
More importantly, the difference in normalized walking speed in itself may have caused a difference in trunk and hip muscle activation patterns independent of the neurological disorder. The same may be true for differences in normalized stride frequency and stride length.3 Therefore, it is important that the authors present the differences in walking speed, stride frequency, stride length, as well as trunk and hip rotations in the various planes, between the CP and TD groups. These data will provide a better understanding of the impact of the observed differences in trunk and hip muscle activation patterns. It also should be noted that the children with CP were allowed to walk with assistive devices, and only 3 children with CP walked without assistive devices. The usage of rolling walkers and bilateral and unilateral forearm crutches will dramatically influence trunk and hip muscle activation patterns.
The excessive, nonreciprocal trunk and hip muscle activation during walking observed by Prosser and colleagues in children with CP compared with children with TD is consistent with the observed excessive muscle activation patterns in the lower extremities in children with CP. Ho and colleagues5 demonstrated that at comfortable walking speeds, children with CP have a smaller normalized stride length, with a slightly higher normalized stride frequency, than children with TD. These differences between children with CP and children with TD coincided with higher values in normalized stiffness estimated by an escapement-driven pendulum and spring system model, approaching the level of significance. It is important to note that in Ho and colleagues’ study, the median absolute and normalized walking speeds were 0.73 m/s and 0.31, respectively, for the children with CP and 1.10 m/s and 0.45, respectively, for the children with TD. These normalized walking speed values are close to the values observed in Prosser and colleagues’ study. After one treatment session applying functional electrical stimulation to the gastrocnemius-soleus muscle complex, Ho et al5 observed an increased normalized impulse without finding any immediate effects on stride length, stride frequency, and stiffness. This finding suggests that the gait of children with CP can be best modeled as a “bouncing ball,” reflecting the usage of the spring-like properties of the lower extremities. However, the gait in children with TD also benefits from the pendulum characteristics of the lower and upper extremities, which requires no counter-rotation between the pelvis and thorax at speeds lower than 1.0 m/s and a counter-rotation in the trunk for 1.0 m/s onward.4 From a dynamics systems perspective, it can be hypothesized that the difference in timing of trunk and hip muscle activity with excessive coactivation patterns in CP gait is an adaptation in muscle dynamics to allow for small increases in stride length, stride frequency, and walking speed. From this perspective, it can be hypothesized that the application of functional electrical stimulation for a number of weeks in children with CP improves not only impulse, but also stiffness, stride length, stride frequency, and walking speed.
A full understanding of changes in trunk and hip muscle activation patterns during walking in children with CP in comparison with children with TD requires using normalized walking speed as a reference and detailed understanding of the coordination dynamics and biomechanics of gait. Without these requirements, it is difficult to provide a valid comparison between CP and TD gait, allowing for a theoretical understanding of the observed changes in muscle dynamics in children with CP. Children with CP walk slower than children with TD, which may allow for the emergence of different trunk and interlimb coordination patterns and, therefore, different muscle activation patterns.
- © 2010 American Physical Therapy Association
Issue highlights
