The purpose of the study was to investigate the characteristics of extensor and flexor muscles in lean body type boys of early and late developmental stages. Twelve elementary school boys (mean age=10.3 yrs.) and 12 middle school boys (mean age=13.7 yrs.) served as subjects of the study. Basing upon the norm provided by the Japanese Ministry of Health and Welfare, lean and standard body type groups at each of the age levels were formed with equal number of subjects (n = 6). Surface EMG was recorded from major extensors and flexors of the elbow and knee joints during isometric contraction at various effort levels of 25, 50, 75 and 100% of the maximum voluntary contraction (MVC). Parameters examined were MVC, IEMG and the median frequency of the power spectra from FFT (fast fourier transformation) analysis of the EMG data. It was found that the MVC values were similar in both body type groups at the younger age level, however, those of older boys were significantly different. The analysis of EMG data revealed that the lean body type boys at the older age level had significantly lower median frequency than that of standard body type boys. Additionally the IEMG values in extensors of lean body type boys at the older age level were significantly higher than those of standard body type boys. Such differences were not revealed in the younger boys. These results would suggest that the excessive leanness of body in boys at early developmental stage could influence the muscular development in their later years.