A cross-sectional study of the size and strength of the lower leg muscles during growth.

The influences of age and sex on the cross-sectional area (CSA) and isometric strength of the ankle dorsiflexors and plantarflexors (PF) were investigated in four age groups of 121 boys and 121 girls aged: 7-9, 10-12, 13-15, and 16-18 years. A single anatomical cross-section was determined at 30% of the distance from the articular cleft between the femur and tibiacondyles by using an ultrasonic apparatus. In both sexes, the increase in age was associated with significant increases in the CSA and strength (ST) of these opposing muscle groups. The sex differences became apparent in the 13-15 year group for CSA and in the 16-18 year group for ST but the differences reduced considerably when CSA and ST were expressed per unit of the second power of the lower leg length (CSA.LL-2) and the product of CSA and the lower leg length (ST.CSA-1.LL-1), respectively. However, CSA.LL-2 of both muscles had a tendency to be increased at and over the age of 10-12 years, and was the highest at 16-18 years, and ST.CSA-1.LL-1 of PF showed higher values in the older boys than in the younger. Thus, it appeared that, at least in the reciprocal muscle groups of the ankle joint, the sex differences in muscle CSA and ST during growth could be accounted for by differences in LL and muscle mass, respectively. However, other factors must also be involved to explain completely the age differences in these variables.