The purpose of this study was to investigate the changes in muscle thickness of Knee Extensors (KE) and Plantar Flexors (PF) with growth. Subjects were 48 males and divided into the three groups : junior high school students (12-15 years, N=13 ; JHS), high school students (15-18 years, N=15 ; HS) and adults (18-36 years, N=20 ; AD). The muscle thickness (relative to (body weight) 1/3) of KE (medial, center, and lateral) and PF (medial and lateral) was measured by B-mode ultrasound. The medial part of KE for AD was significantly greater than that for JHS, and the lateral part of KE for AD was significantly greater than that of JHS and HS. Although the lateral part of PF did not show significant difference among the three groups, the surface part of PF (medial gastrocnemius muscle) for AD was significantly greater than that for JHS and HS. These results indicated that the effect of growth on the muscle thickness was different among the synergist muscles. Especially, the lateral part of KE (i.e., vastus lateralis muscle) and surface part of PF (i.e., medial gastrocnemius muscle) would predominantly develop after the age of puberty.
