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Norikazu Hirose and Chikako Nakahori


To describe cross-sectional age differences in change-of-direction performance (CODp) in female football players and investigate the relationship between CODp and linear-sprint speed, muscle power, and body size.


A sample of 135 well-trained female football players was divided into 8 age groups. Anthropometry (height, body mass, and lean body mass) and athletic performance (10-m sprint speed, 10-m × 5-CODp, and 5-step bounding distance) were compared to determine interage differences using ANOVA. Then, the participants were divided into 3 age groups: 12- to 14-y-olds, 15- to 17-y-olds, and ≥ 18 y-olds. Simple- and multiple-regression analyses were conducted to determine the correlation between CODp and the other measurement variables in each age group.


Age-related differences were found for CODp (F = 10.41, P < .01), sprint speed (F = 3.27, P < .01), and bounding distance (F = 4.20, P < .01). Post hoc analysis revealed that the CODp of 17-y-old players was faster than that of 16-y-old players (P < .01), with no interage differences in sprint speed and bounding distance. Sprint speed and bounding distance were weakly correlated with CODp in 15- to ≥18-y-old players, but only sprint speed was correlated with CODp in 12- to 14-y-old players.


CODp improves from 16 to 17 y of age in female players. Linear-sprint speed, muscle power, and body size were weakly correlated with the age differences in CODp.

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Yuta Sekine and Norikazu Hirose

Purpose: To examine differences in resistance exercise-induced hormonal responses among young athletes according to their maturity levels. Materials and Methods: A total of 12 collegiate and 32 junior high school male athletes were enrolled. The junior high school participants were divided into pre–peak height velocity (PHV) and post-PHV groups, according to their PHV ages. The salivary testosterone, cortisol, and human growth hormone levels were analyzed before (pre), immediately after (post), and at 15 minutes after performing body weight resistance exercise. Results: The testosterone levels were higher in the collegiate than in the junior high school group (P < .01) and increased after 15 minutes of exercise (P < .01). A significant decrease in the cortisol levels postexercise in the junior high school groups (P < .01) and an increase in the human growth hormone levels at 15 minutes after exercise in the post-PHV group were observed (P < .01). In the collegiate and post-PHV groups, the testosterone-to-cortisol ratio increased post and at 15 minutes after exercise (P < .01). The testosterone-to-cortisol ratio values were higher in the collegiate than in the post-PHV (at preexercise and at 15 min after [P < .01]) and pre-PHV groups (at all times [P < .01]). Conclusion: Exercise-induced acute hormonal responses to resistance exercise may depend on individuals’ maturity levels, even in those having the same age.

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Akane Yoshimura, Robert Schleip, and Norikazu Hirose

Context: Several studies have reported that self-massage using a foam roller (FR) increased joint range of motion (ROM) immediately. However, the mechanism of increasing ROM by the FR intervention has not been elucidated. Objective: To clarify the mechanism by investigating properties and morphological changes of muscles targeted by the FR intervention. Design: An interventional study. Setting: An athletic training laboratory. Participants: Ten male college volunteers with no injuries in their lower limbs (mean [SD]: age 23.8 [3.2] y, height 173.2 [4.9] cm, weight 69.5 [8.6] kg). Intervention: The FR intervention on the right plantar flexors for 3 minutes. Main Outcome Measures: Maximum ankle ROM, muscle hardness, and fascicle length of the gastrocnemius muscle at the neutral (0°), maximum dorsiflexion, and maximum plantar flexion positions. All measurements were conducted before (PRE) and after (POST) the FR intervention. Results: Dorsiflexion ROM increased significantly at POST (PRE: 13.6° [8.0°], POST: 16.6° [8.4°]; P < .001), although plantar flexion ROM did not change significantly between PRE and POST (PRE: 40.0° [6.1°], POST: 41.1° [4.9°]). There was no significant difference in muscle hardness and fascicle length between PRE and POST in any of the angles. Conclusions: Dorsiflexion ROM increased significantly by the FR intervention in the present study; however, muscle hardness and fascicle length did not change. FR may affect not only the muscle but also the fascia, tendon, and muscle-tendon unit. The FR protocol of the present study can be applied in clinical situations, because it was found to be effective to increase ROM.

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Keitaro Kubo, Takanori Teshima, Norikazu Hirose, and Naoya Tsunoda

The purpose of this study was to compare the morphological and mechanical properties of the human patellar tendon among elementary school children (prepubertal), junior high school students (pubertal), and adults. Twenty-one elementary school children, 18 junior high school students, and 22 adults participated in this study. The maximal strain, stiffness, Young’s modulus, hysteresis, and cross-sectional area of the patellar tendon were measured using ultrasonography. No significant difference was observed in the relative length (to thigh length) or cross-sectional area (to body mass2/3) of the patellar tendon among the three groups. Stiffness and Young’s modulus were significantly lower in elementary school children than in the other groups, while no significant differences were observed between junior high school students and adults. No significant differences were observed in maximal strain or hysteresis among the three groups. These results suggest that the material property (Young’s modulus) of the patellar tendons of elementary school children was lower than that of the other groups, whereas that of junior high school students was already similar to that of adults. In addition, no significant differences were observed in the extensibility (maximal strain) or viscosity (hysteresis) of the patellar tendon among the three groups.

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Toshiaki Soga, Taspol Keerasomboon, Kei Akiyama, and Norikazu Hirose

Context: This study aimed to examine the differences in electromyographic (EMG) activity of the biceps femoris long head (BFlh) and semitendinosus (ST) muscles, break-point angle (BPA), and the angle at peak BFlh EMG activity between bilateral and unilateral Nordic hamstring exercise (NHE) on a sloped platform. Design: This study was designed as a case-control study. Methods: Fourteen men participated in the study. The participants initially performed maximum voluntary isometric contraction (MVIC) on the prone leg curl to normalize the peak hamstring EMG amplitude as the %MVIC. Then, participants were randomized to perform the following 3 variations of NHE: bilateral (N40) or unilateral (N40U) NHE with a platform angle of 40°, and unilateral NHE with a platform angle of 50° (N50U). The EMG activities of the BFlh and ST and the knee flexion angle during the NHE variations were recorded to calculate the EMG activity of the BFlh and ST in terms of the %MVIC, the angle at peak BFlh EMG, and BPA. Results: The BFlh %MVIC was significantly higher in N40U (P < .05) and N50U (P < .05) than in N40. A significant difference in BFlh %MVIC and ST %MVIC was observed between N40U (P < .05) and N50U (P < .05). The mean values of BPA and the angle at peak BFlh EMG were <30° for all NHE variations. Conclusions: In the late swing phase of high-speed running, BFlh showed higher EMG activity; thus, unilateral NHE may be a specific hamstring exercise for hamstring injury prevention.

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Toshiaki Soga, Hiromi Saito, Kei Akiyama, and Norikazu Hirose

This crossover trial aimed to investigate whether additional loading of unilateral Nordic hamstring exercise on a sloped platform would increase the biceps femoris long head electromyographic activity. Participants were randomly allocated to unilateral Nordic hamstring exercise under three conditions: bodyweight only (BW) or BW with an added weighted ball of 3 kg (BW + 3 kg) or 6 kg (BW + 6 kg), respectively. The biceps femoris long head electromyographic activity was significantly higher for BW + 6 kg (p < .001) than for BW and BW + 3 kg (p < .01). Therefore, adding a load to unilateral Nordic hamstring exercise on a sloped platform might be effective for rehabilitation and prevention of hamstring strain injury recurrence.

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Takeshi Koyama, Akira Rikukawa, Yasuharu Nagano, Shogo Sasaki, Hiroshi Ichikawa, and Norikazu Hirose

Purpose: To evaluate the effect of the number of high-acceleration movements on muscle damage and the rating of perceived exertion (RPE) in basketball games. Methods: Twenty-one male collegiate basketball players (mean age, 20.0 [1.0] y) were included. A triaxial accelerometer was used to measure acceleration in basketball-simulated scrimmages. To detect higher physical load during the actual game, the resultant acceleration was calculated, and 3 thresholds were set: >4G, >6G, and >8G resultant accelerations. The number of the extracted movements was calculated at each acceleration threshold. Plasma creatine kinase (CK) levels (marker of muscle damage) were estimated before and 24 hours after the match, and the session-RPE load was calculated within 30 minutes after the match. Pearson product-moment correlations with 95% confidence intervals were used to determine the relationships between the number of high-acceleration movements and plasma CK and session-RPE load. Results: Significant correlations were observed between the number of high-acceleration movements >8G and CK level (r = .74; 95% confidence interval, 0.44–0.89; P < .0001). Furthermore, the correlation coefficient between acceleration and CK increased with increased acceleration threshold (>4G: r = .65; >6G: r = .69). Contrastingly, the correlation coefficient between acceleration and the session-RPE load decreased with increased acceleration threshold (>4G: r = .72; >6G: r = .52; >8G: r = .43). Conclusions: The session-RPE reflects the total amount of movement, while the high-acceleration movement reflects the momentary large impact load or intensity, and they evaluated different factors. Basketball coaching and conditioning professionals recommended combining acceleration and session-RPE when monitoring the load of athletes.