The purpose of this study was to determine the differences in the growth patterns of the external oblique (EO), internal oblique (IO), and transversus abdominis (TrA) muscles in adolescence baseball players. The subjects were 17 elementary school students (ES), 56 junior high school students (JS), and 21 high school students (HS) who played baseball. The EO, IO, and TrA muscle thicknesses were imaged using an ultrasound imaging device and analyzed separately on the dominant and non-dominant sides. Curve regression estimation of the relationship between EO, IO, and TrA and chronological age was used to read inflection points. An analysis of covariance was also performed, using body weight as a covariate for comparison with each of the EO, IO, and TrA grades. The inflection point readings showed that TrA was 11.26 years old (p<0.01, R2=0.45) for the dominant side and 10.10 years old (p<0.01, R2=0.46) for the non-dominant side, earlier than the inflection points of EO and IO. Analysis of covariance showed no significant difference in EO between any of the grades. IO showed significantly higher values for muscle thickness as the grade increased, both on the dominant and non-dominant side. TrA was higher for JS among ES and JS and higher for HS among ES and HS on both the dominant and non-dominant sides. EO may growth with increasing body size, while IO is interpreted to increase with increasing age due to the movement of baseball. TrA may grow earlier than EO and IO.

The purpose of this study was to determine the developmental pattern of swing speed in youth baseball players between 6 and 14 years old. A total of 1213 baseball players performed toss batting. Swing speed was measured with a specialized accelerometer. In addition, 618 of the 1213 players were measured for total fat free mass by using Inbody770. In study1, Regression analysis was performed on the relationship between chronological age or height and swing speed, and the extreme values were calculated. Then, an allometric equation based on height was also used to estimate the relative growth of swing speed and total fat free mass and to compare the difference of relative growth patterns between swing speed and total fat free mass in study2. Swing speed was strongly related to both chronological age and height, with cubic regression for the relationship with chronological age and linear for the relationship with height. The regression equation obtained for chronological age was solved and found that developmental rate was maximal at approximately 10.25 years old. Allometric analysis showed that the developmental pattern of swing speed differed from that of fat free mass, with a temporary decline in developmental rate from 153.6 cm to 160.9 cm. Based on the results of this study, further research in youth baseball players would be expected to identify the factors causing sluggish development of swing speed as well as to develop an exercise program for improving batting ability.