A specific kind of obesity that occurs in young women who appear thin but who have relatively greater percentage of body fat than is normal for their weight is called “masked obesity.” The definition of masked obesity is nutritionally and anthropometrically clear: body mass index (BMI) <25 kg/m2 and percentage of body fat (%BF) ≥30%. However, this definition allowed obesity to be underestimated. Thus, we determined that when mathematically analyzing the relationship between BMI and %BF, the equation %BF=aBMIb could be applied, where a (i.e., %BF/BMIb), converted by logarithmic function, is defined as an index for masked obesity (MOI). The MOI equation (a) was thus %BF/BMI2.12. We measured the height, weight, and %BF of 8,068 young women from all over Japan. The percentage of subjects with masked obesity above the defined criteria (BMI <25 kg/m2 and %BF ≥30%) was 7.4%. The mean MOI was 0.0444 in the masked obesity group and we determined additional cases of masked obesity if the MOI was greater than 0.0444. The percentage of additional masked obesity cases within the standard scale was 10.4%, and thus, the percentage of masked obesity was 17.8% in the young Japanese women tested.

The service is the most powerful and potential shot in tennis. To reach global elite status, Japanese players, being generally inferior in height, must overcome the limitations in service performance. If their height is insufficient, they must improve their service performance speed while maintaining the spin rate. Therefore, in this study, we aimed to examine the relationship between service performance according to the speed and spin rates and physical factors among national-level male and female junior tennis players. Twenty-eight elite tennis players participated in this study. Service tests were completed using the TrackMan tennis radar device. A total of 12 services per player were collected. Physical strength tests were conducted according to the Japan Tennis Association methods. Based on a previous study, 16 measurement variables related to service performance were selected. Among the male players, both the 1st and 2nd services showed significant correlations of service speed with physique and physical strength. On the other hand, no correlation was observed between service spin and physique or physical strength. Among female players, there was a significant correlation between service speed and physique, but not physical strength. In contrast to male players, there were significant correlations between service spin and some parameters of physical strength. As described above, the results were different for males and females. It is recommended that male players should focus intensively on muscular strength and power during training, whereas female players should focus on acquiring the skills needed to increase the spin rate, with strength as their secondary focus. Additionally, the increased drive from the legs can be converted to increased service speed. Therefore, male and female players should be coached on service from different perspectives.

In tennis, service requires a variety of complicated movements. Given the importance of taking the initiative to obtain points in a tennis match, it is crucial to make full use of speed and spin rate of service. Generally, a service that requires a higher spin rate would slow down, and a service that has increased speed would have a decreased spin rate. For players who are disadvantaged in height, although controlling spin rate is essential, slowing down service speed should be avoided. For these players, the challenge of service is to improve the speed without decreasing the spin rate. Players must also be trained to build physical strength required for this skill. It is not uncommon to work on physical training without a racket; however, few studies have reported on the effects of cultivated physical strength on on-court tennis performance. Therefore, this study aimed to propose physical measurements that could be used as indices to improve service performance in 58 elite Japanese junior male players. To test service performance, we used TrackMan tennis radar device to assess speed, spin rate, impact height, and impact depth. To test physical strength, we measured 5- and 20-m sprint, broad jump, medicine ball throw (forward, backward). We used a significant multiple regression equation to predict the first service speed obtained from the broad jump and the Medicine ball throw (backward). Additionally, a strong correlation was obtained between the predicted and measured values. In addition to physical strength, we suggest that the depth of the impact point (taking the hitting point forward toward the net) is important for improving the first service speed. However, we were not able to identify the physical strength test items that improve service spin rate. Other item should be examined in the future to determine the physical strength associated with spin rate. This result could help connect physical training and service performance.