One method of determining a person’s biological maturity involves evaluating their bone maturity. Differences in physical abilities among athletes during the growth period are thought to be strongly influenced by differences in bone maturity. Thus, it is necessary to understand biological maturity when selecting athletes. Accordingly, this study looks at the biological maturity of young Japanese soccer players in the Japan Professional Soccer League. In total, subjects included 282 male soccer players (mean age, 12.4 ± 0.7 years; mean height, 154.0 ± 9.1 cm) in the J-League Academy, 2007–2012. Results show that the TW2-RUS method (Japan) revealed no significant difference (t = 1.012, df = 277, ns); however, the TW3-RUS method revealed a significant difference (t = -4.075, df = 281, p < 0.05) for chronological age vs skeletal age in t-test. The breakdown of biological maturity according to the TW2-RUS method (Japan) was as follows: mature, 3 persons; early maturing, 45 persons; average, 196 persons; and late maturing, 38 persons. The breakdown of biological maturity according to the TW3-RUS method was as follows: mature, 3 persons; early maturing, 53 persons; average, 132 persons; and late maturing, 94 persons. The chi-square test revealed a significant difference (p < 0.001) between the TW2-RUS method (Japan) and the TW3-RUS method. Our results suggest that it is appropriate to use the TW2-RUS method (Japan) to evaluate skeletal age. With regard to biological maturity, contrary to prior research in Europe and America, a high proportion of our athletes had average biological maturity.

Morphological characteristics are one of the critical factors needed to achieve success in any competition, and these characteristics are dependent on competitions and events. The present study aimed to determine the relationship of height, body mass, muscle mass, fat mass, and the percentage of body fat with athletic performance in male Japanese college sprinters, distance athletes, jumpers, throwers, and decathletes. The subjects included 24 sprinters and hurdlers, 21 distance athletes, 22 jumpers, 21 throwers, and 7 decathletes. The height and body mass of subjects were measured using a standard stadiometer and electrical scale. The muscle mass and fat mass were measured using dual energy X-ray absorptiometry. In the sprinter group and the distance athlete group, there were significant negative correlations between the percentage of fat and IAAF (International Association of Athletics Federations) scores (sprinter: r = -0.456) (distance: r = -0.453). In the decathlete group, there were significant positive correlations between body mass and the IAAF score (r = 0.835) and between muscle mass and the IAAF score (r = 0.797). In the jumper group and the thrower group, there were no significant correlations between body data and IAAF scores. These findings reveal that there were correlations between some body composition indicators and athletic performance in male Japanese college sprinters, distance athletes and decathletes.

The purpose of this study was to clarify the development pattern of batting imagery in youth baseball players. One hundred thirty-eight young baseball players (6–14 years old) were divided into 4 age groups. Tee and toss batting with a stationary tee stand and toss machine were used in the batting trials. In addition, the participants did an imagery task of hitting the optimal point of a ball where they wanted to impact it. To clarify the difference between the image trial and actual batting (tee and toss), the absolute error distance (AED) was calculated by subtracting the impact distance of the image trial from that of the actual batting. Two-way analysis of variance (4 age groups × tee and toss images) revealed that the AED was significantly lower in the 11- to 12-year-old players than in the 6- to 8-year-old players (p < 0.001). The relationships between the tee and toss images showed a significant correlation in all 4 groups (6–8 years old: r = 0.445, 9–10 years old: r = 0.495, 11–12 years old: r = 0.589, and 13–14 years old: r = 0.804; all groups: p < 0.001) and that tee and toss batting imagery appears unchanged as age increases. However, batting imagery seems to develop around 11-12 years old, and at the age group of 13-14 years old players are able to impact the same position on the bat regardless of the batting trials.

N-terminal fragments of titin in urine have been proposed as a noninvasive marker of muscle damage, but there are no data on its time course after 96 h. The aims of this study were to investigate the time course of urinary titin N-terminal fragment (UTF) following eccentric exercise and the subsequent correlations between UTF and other muscle damage indices. Seventeen healthy young men performed 30 maximal isokinetic eccentric exercises at the elbow flexors. Muscle soreness (SOR), range of motion (ROM), maximal voluntary isometric contraction (MVIC), creatine kinase (CK), and UTF were sampled before, immediately after, and 24–144 h after exercise. The changes and correlations were analyzed with one-way ANOVA and Spearman’s rank correlation analysis. All the measured parameters showed significant differences from their pre-exercise values at 24 h or later after exercise (P < 0.05). UTF showed significant correlation with MVIC (r ≤ –0.485) at 24 h and later, with SOR (r ≥ 0.549) at 48 h and later, with ROM (r ≤ –0.485) at 48 h and later, and with CK (r ≥ 0.647) at all time points. UTF reached peak value at 96 h after the exercise and had not recovered completely at 144 h. Because UTF was highly correlated with the other muscle damage indices, especially CK, throughout the measurement period, it may be as useful an indicator of muscle damage as CK obtained from blood.

Repeated sprint training in hypoxia (RSH) is a potential training strategy to improve short-term high-intensity sprint ability and evoke adaptation of lactate metabolism. Therefore, the purpose of the present study was to examine the effects of RSH on Wingate sprint performance and lactate metabolism. Eight university cyclists performed 6 sessions of RSH (3 × 30-sec sprint with 5-min recovery, FiO₂: 14.5%) over 6 consecutive days. Two days before (pre-test) and 7-9 days after (post-test) the training intervention, subjects performed Wingate tests as performance tests. We took blood samples before and 0.5, 1, 2, 3, 4, 5, 7, 10 min after the Wingate test to evaluate physiological adaptations. Mean power outputs were unchanged after the intervention (p = 0.094, d = 0.23). Whereas, the fatigue index was significantly improved (p = 0.025, d = 0.43). According to time course change in power outputs during the Wingate test, power outputs at 26 s, 29 s and 30 s were significantly improved after the intervention (p = 0.029, 0.001 and 0.001; d = 0.54, 0.74 and 0.74, respectively). Area under the curve (AUC) of blood lactate concentration was significantly lowered after the intervention (p = 0.048, d = 0.46). Six sessions of RSH over 6 consecutive days delayed fatigue during the Wingate test. AUC of blood lactate concentration was lowered after the intervention, indicating that glycogen breakdown was reduced (glycogen sparing effect) and/or lactate oxidation was increased during and after the Wingate test when the same work was performed. The effects of glycogen sparing and increased lactate oxidation would provide a competitive advantage to athletes performing multiple sprints.

The purpose of the present study was to examine acute changes in active muscle stiffness with and without the stretch reflex following static stretching. Before and after static stretching for 10 min, active muscle stiffness was measured according to changes in exerted torque and fascicle length during short-range stretch of faster (peak angular velocity of 250 deg·s^-1; without the stretch reflex) and slower (peak angular velocity of 100 deg·s^-1; with the stretch reflex) angular velocities. During the measurement of active muscle stiffness, the electromyographic activities of plantar flexor muscles were recorded and averaged over two different phases: just before (mEMGa) and after (mEMGb) stretch. In addition, the mEMGb/mEMGa ratio was used to evaluate the effects of stretch reflex. After 10 min of stretching, the mEMGb/mEMGa ratio tended to decrease under the 100 deg·s^-1 condition, but not 250 deg·s^-1. Under both conditions, active muscle stiffness did not change after 10 min of static stretching. In conclusion, the prolonged static stretching did not affect active muscle stiffness with or without the stretch reflex, but tended to decrease the stretch reflex. In addition, these results imply that active muscle stiffness measured during contractions was not influenced by the stretch reflex.