Lower-Extremity Strength Profiles in Spastic Cerebral Palsy,
by M. Wiley and D. Damiano, Developmental Medicine and Child Neurology, 1998;40:100–107.
The purpose of this study was to investigate lower extremity muscle weakness in children with spastic cerebral palsy (CP). The authors questioned whether this weakness was specific to certain lower extremity muscle groups or if it affects the entire limb. They proposed that muscle strength would be significantly lower in both lower extremities of children with spastic diplegia and in the involved extremity of children with hemiplegia as compared to the muscle strength in the lower extremities of children without developmental deficits. Furthermore, they suspected that ratios of muscle strength across lower extremity joints of children with CP would not equal those of children developing typically.
Data were collected and analyzed on 45 children aged five to 12 years. Fourteen of these children had spastic diplegia, 15 had hemiplegia, and 16 age-matched peers had no known neurological or orthopedic problems (comparison group). Participants met three inclusion criteria: they 1) had not undergone surgery within the last year, 2) were ambulatory, and 3) were able to follow instructions. Six of the children with diplegia ambulated using posterior walkers while the rest of the subjects ambulated independently.
Testing was conducted by one of two therapists either at the child’s home or at the Motion Analysis Lab at Kluge Children’s Rehabilitation Center. The subjects’ weight, handedness, surgical history, and more involved side were recorded. For subjects with hemiplegia, the noninvolved side was considered dominant. Parents of the children with diplegia reported if one limb was less involved than the other. For the comparison group, the dominant limb was determined by handedness. This criterion was also used for those children with diplegia when it was not known which side was less involved. The lower extremity muscles tested included iliopsoas, rectus femoris, gluteus maximus, hip abductors and adductors, hamstrings, quadriceps at 90-degree knee flexion and at 30-degree knee flexion, gastrocnemius, soleus, and tibialis anterior with knee extended and knee flexed. Specifics regarding the testing positions and any deviations that were used are detailed in the report. First, the children were shown the handheld dynamometer and were told what was expected of them. The testing began with the subject sitting so they could further their understanding of the proper testing procedure. Two trials were conducted for each muscle with the tester using the break test against the subject’s maximum voluntary contraction. The force of the tester was applied through the dynamometer and perpendicular to the long axis of the segment being tested. If the subject produced a force greater than the dynamometer was capable of measuring, the force value was recorded at 265 newtons, the force limit of the device.
Mean values of strength were calculated and reported separately for each subject’s lower extremity. Repeated measures analysis of variance (ANOVA) were used to assess the significance of differences found between values of the different diagnostic groups. For multiple comparisons, post hoc analyses were completed using a Tukey test. For subjects with diplegia, all muscles but rectus femoris on both the dominant and nondominant sides were significantly weaker than those of the comparison group. On the more involved side of those with hemiplegia, all muscles were significantly weaker than the comparison group.
Among those with hemiplegia, the plantarflexors and dorsiflexors of the nondominant limb, in both test positions, were significantly weaker than those on the dominant side. The following significant differences were found between the dominant hemiplegic side and that of the comparison group: gluteus maximus, iliopsoas, and tibialis anterior with knee extended were weaker, yet soleus with knee flexed was stronger. No significant differences were found between the dominant and nondominant sides within either the group with diplegia or the comparison group.
One-way ANOVA and Tukey post hoc analyses were used to investigate differences in ratios between agonist and antagonist muscle groups between groups and limbs. The ratios were determined by dividing the normalized maximum force value for a muscle by the normalized maximum force value for the antagonist muscle. Hip flexion/extension and plantarflexion/dorsiflexion ratios were found to be significantly higher for the involved hemiplegic side than for the comparison group.
Using factorial ANOVA procedures, strength values of ankle musculature normalized by body weight were compared among 13 subjects who had previous heel cord-lengthening surgery and the 32 subjects who had not. The analysis showed that normalized dorsiflexion strength was slightly higher (p < 0.65) and normalized plantarflexion strength was lower (p < 0.31) in the children who had undergone surgery. No significant differences were found when they compared the strength ratios of those who had prior surgery to those who did not.
The authors report that hip extensors and ankle plantarflexors and dorsiflexors were the weakest muscles of the subjects with CP. They conclude that, because two of the three weakest muscle groups are distal, this study supports the findings of other recent studies, which found distal involvement to be greater than proximal. However, gluteus maximus was the weakest muscle in all subjects with diplegia. In looking at children with hemiplegia, the authors found that the majority of muscles tested on the uninvolved side tended to be weaker than both lower extremities of the comparison group. They offer possible explanations for this, but stress that within this population, the uninvolved limb may also be affected by the condition. Therefore, the uninvolved limb should not be used as the basis for comparing the involved side. Instead, comparisons should be made with the dominant limb of age- or size-matched peers without developmental problems. Lastly, the need for additional research into the implications of surgery on force production is addressed.
Limitations and Implications
By conducting a literature review from a historical perspective, the authors were able to illustrate the logical need for additional research to analyze muscle weakness within an extremity in children with CP. The procedure was fairly well described and the results were presented clearly both in text and by the use of tables and graphs. Within the discussion, the authors provided realistic explanations to support the research findings and, where appropriate, disclosed the need for additional research to accept or reject these explanations.
In the discussion, the authors acknowledge that the number of subjects was limited to children with CP who are ambulatory. However, almost half of the children with diplegia, six of 15, used a posterior walker. Perhaps a separate group should have been formed to determine if differences existed between those who used a device to ambulate and those who did not. It is noted that the subjects of this research were relatively less involved and probably somewhat stronger than many other children with CP. Therefore, additional research should be conducted to confirm suspicions that this discrepancy of strength would be even greater in children with more severe CP.
In addition, it is stated that during the test trials, “strong verbal encouragement” was used to produce the child’s maximal effort. No indication was made within the report whether this motivation was controlled among subjects. If it was not the same for each child and during each trial, this could affect the reliability of the study.
The report did not specify the amount of training either tester had in the proper use of the dynamometer. Nor did the report indicate how subjects were assigned to a tester. This could have an impact on both the validity and the reliability of the data.
Clinically, physical therapists help patients maximize function and independence. Muscle weakness is one factor that may limit a child’s ability to function. Wiley and Damiano have shown that lower extremity muscle weakness is prevalent among children with CP. Thus, when treating children with CP, muscle strength needs to be assessed; and where appropriate, strengthening should be incorporated into the plan of care. Furthermore, attention should be given to the strength ratios across various joints within this population. Muscle shortening occurs in children with CP; and this may be related to an imbalance in muscle forces across a particular joint. Working to prevent these imbalances may, in turn, reduce the occurrence of contractures. Additional research needs to be conducted to determine what causes weakness in this population. From this knowledge, therapeutic intervention strategies can be developed to further address the problem and ultimately facilitate the patient’s ability to function.