A developmental index of muscle strength and assessment of quadriceps function in children with spina bifida.

This study was designed to develop a new index of muscle strength for children that was independent of body weight thus accommodated their developmental changes and to assess the quadriceps strength influencing ambulatory status in children with spina bifida. Maximum voluntary strength in isometric knee extension was measured from 60 children with spina bifida and 92 normal children. The measured strength was described in terms of torque. A muscle strength index (MSI) was defined as a unitless measure independent of the subject's body weight (i) by normalizing the residual of the maximum torque from the developmental regression line for normal children by body weight and, furthermore, (ii) by dividing the normalized residuals by their standard deviation among normal children. Children with spina bifida demonstrated a tendency to increase in maximum quadriceps torque with body weight; however, spina bifida children with a body weight above 30 kg showed a progressive decrease in MSI below a normal limit of muscle weakness. The MSI was closely related to both neurologic level and ambulatory ability compared with the maximum torque. These findings suggest MSI provides useful and detailed information for assessing and predicting ambulatory ability in children with spina bifida.