Normalizing lower-extremity strength data for children without disability using allometric scaling.

OBJECTIVES To evaluate existing approaches for normalizing lower-extremity strength data and to develop normalization equations using allometric scaling in children without disabilities. DESIGN Cross-sectional study evaluating traditional mass normalization and allometry as methods of adjusting lower-extremity muscle torques for the influence of body mass. SETTING Motion analysis laboratory. PARTICIPANTS Thirty-nine children without disability (age range, 4-17y). INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Maximum torque generated during hip abduction and adduction, knee extension and flexion, and ankle dorsiflexion and plantarflexion. RESULTS Linear regressions of torque/mass(1.0) versus body mass and age produced slopes that differed significantly from zero (P<.001) for all muscle groups except the ankle plantarflexors versus body mass (P=.28). Regressions for torque/body mass index also produced slopes that differed significantly from zero (P<.001). Regressions of torque/(mass x height) produced slopes that differed from zero in some cases but not others. Allometric scaling exponents (exponent b) differed significantly from the theoretical value of 1.0 for all muscle groups except the ankle plantarflexors (1.32; 95% confidence interval, 0.98-1.67). Linear regressions performed using torque/mass(b) produced slopes that did not differ significantly from zero for all muscle groups (P>/=.10). Regressions performed using torque/mass(1.6) for the hip and knee and torque/mass(1.4) for the ankle also produced slopes that did not differ significantly from zero. CONCLUSIONS Traditional mass normalization does not effectively adjust for the influence of body mass. Allometric scaling performed using torque/mass(1.6) for the hip and knee or torque/mass(1.4) for the ankle provides more appropriate normalization.