Introduction: One reason athletes train their trunk muscles is that the body's trunk stability has been shown to prevent injury. However, the relationship between body trunk muscle thickness, particularly that of deep muscles, and athletic performance remains to be clarified. Purpose: We aimed to explore the relationship between 100-m sprint performance and the sizes of the trunk stabilizing muscles, the psoas major muscle (PM), transversus abdominis (TA), and multifidus muscle (MM), in collegiate sprinters. Methods: Fourteen male sprinters belonging to a university athletics club participated in this study. The thicknesses of the TA and MM were measured using an ultrasonic diagnostic apparatus (ProSound C3; Aloka, Tokyo, Japan). The cross-sectional area of the PM was assessed by a magnetic resonance imaging apparatus (Vantage Elan; Toshiba Medical Systems, Tokyo, Japan). The relationship between these anthropometric parameters and the 100-m sprint time was analyzed by Spearman's correlation coefficient, multi- regression analysis, and the change-point regression model. Results: The sizes (mean ± SD) of the muscles were: PM, 43.074 ± 7.35 cm²; TA, 4.36 ± 0.72 mm; and MM, 3.99 ± 0.48 cm. The mean 100-m sprint time was 11.00 ± 0.48 s. Spearman's correlation analysis revealed that the 100-m sprint time had a significant moderate negative correlation with TA (ρ = −0.691, p < 0.01) and a low negative but not significant correlation with MM (ρ = −0.327, p = 0.28), whereas PM did not show a significant or in-negligible correlation. The change-point regression model found the change-points in the 100-m sprint time and the thickness of the TA and MM at 4.70 mm (95% CI: 4.00–5.43 mm) and 3.84 cm (95% CI: 3.28–4.31 cm), respectively. The sprint time decreased with an increase in the thickness of the muscles up to the change-points, whereas it did not change even if the muscles became thicker than the change-points. The change-points were consistently observed when the thickness of the muscles was normalized by body mass. Conclusion: Sprint performance for 100-m was found to be associated with TA and MM thickness in a biphasic manner. As muscle thickness increased, the sprint time decreased, followed by a plateau phase.

In this study, we aim to clarify the influence based on bone resorption markers at onset of stress fracture. Also, we will clarify the state of the bone resorption markers of female long distance runners who have a history of stress fracture and also ones who routinely practices running long distances. Participants comprised 19 female long distance athletes. The survey period was 2011–2014, and we measured u-NTX as a bone resorption marker at least twice a year, taking the mean±SD of the periodic measured values without stress fracture as the mean value. Measurements were collected sample when stress fractures developed. 132 u-NTX measurements were taken from 19 participants. As a result, the average was 41.03±12.31 nmolBCE/mmolCRE (Q1: 33.15, Q2: 40.55, Q3: 47.95). In six of the 19 participants, u-NTX could be measured following a stress fracture. The mean value of u-NTX for those participants was 40.16±9.10 nmolBCE/mmolCRE, increasing to 64.08±16.07 nmolBCE/mmol CRE with the stress fracture (p<0.01). The findings showed that, in adult female long distance runners, u-NTX values when there was no stress fracture were within the standard value for mean premenopausal women, but increased when the athletes suffered from a stress fracture.

Studies have not adequately addressed the influence of fatigue, which is considered a major risk factor for hamstring injuries. Therefore, this study aimed to clarify how a muscle fatigue condition affects the success of hamstring injury prevention programs in sprinters. The study subjects were 613 collegiate male sprinters. They employed submaximal/maximal running for a large number of runs and supramaximal running for a small number of runs in daily training. The hamstring injury prevention program had become the most effective strategy in the past 24 seasons of track and field for preventing hamstring injuries. The number of sprinters who experienced hamstring injuries in three periods over the 24 seasons was recorded. The incidents of hamstring injuries during supramaximal running per athlete-seasons were 137.9, 60.6, and 6.7 for Periods I, II, and III, respectively, showing a significant decline (p < 0.01). Furthermore, the incidents of hamstring injuries during submaximal and maximal running per season showed no significant change. The results of this study indicate that by inducing muscle fatigue, a small number of runs makes hamstring injury prevention programs effective.

Objective: This study examined the prevalence of and factors affecting normal weight obesity (NWO) among women aged under 40 years old. NWO was having a normal BMI but a high body-fat percentage (BFP). Materials and Methods: We recruited 399 participants aged 18-39 during a health checkup and surveyed them about lifestyle patterns, height, weight, body composition, and girth and visceral fat area. Participants were divided into three. Those with a BMI of <25 and a BFP of <30 were classified as non-obese, a BMI of <25 and a BFP of ≥30 as NWO, a BMI of ≥25 and a BFP of ≥30 as obese. The analysis was conducted on NWO (97) and non-obese (262) groups. Results: NWO prevalence ranged from 20% to 30% across the three age groups. NWO participants had lower values for segmental lean body mass, particularly in lower limb, and higher values for abdominal girth and visceral fat area. Among participants aged 30-39, those with NWO had higher values for triglycerides and LDL cholesterol and lower values for HDL cholesterol. Regarding lifestyle factors, among those aged 30-39, NWO was associated with past weight fluctuation, respectively. In the latter age group, a gain of more than 10 kg since the age of 20 was associated with a 13-fold increase in the odds for NWO. Conclusions: Results suggest that for women under 40 years, detecting NWO early and guidance on effective lifestyle and exercise are necessary.

(1) Background: Although innovations and improvements in towing systems have been available, tow-training method has not been considered favored in the training context. Tow-training may enable high stride frequency if hamstring injuries do not occur. The purpose of this study was to prevent hamstring injuries during supramaximal running and to optimize tow-training. (2) Methods: We investigated the relationship between the number of hamstring injuries that occurred during supramaximal running and the contents of the prevention programs that have been implemented, i.e., 4 years of the baseline programs and 12 years of the intervention. (3) Results: The incidence of hamstring injuries per 1000 sprinters was 57.5 for baseline and 6.7 for intervention. A significant difference was observed in the incidence of hamstring injury between the different combinations of prevention programs (p < 0.01). (4) Conclusions: Tow-training was optimized by (1) preventing hamstring injuries by combination of strength, agility, and flexibility training programs and (2) advising the sprinters to press the leg onto the ground as fast as possible to increase stride frequency and to prevent stride lengthening.

Objective: The purpose of this study was to investigate the effect of different contraction types on muscle structures, functions and muscle hardness. Materials: 6 healthy males (age: 22.8±1.5 years, height: 176.1±4.0 cm, weight: 72.3±11.0 kg) participated in this study.Methods: Subjects performed concentric contraction (CC) exercise of elbow flexor with one arm, and eccentric contraction (EC) exercise of elbow flexor with the other arm with the same total work. Muscle hardness of the biceps brachii measured by strain ratio, elbow joint angle, and maximal voluntary contraction (MVC) torque of elbow flexors were measured before (PRE), 1 hour (1H), 3 hours (3H) and 24 hours (24H) after the exercise protocol. Muscle soreness was measured at 1H, 3H, and 24H time points.Results: A significant interactions (time×type) was found in MVC torque (p<0.01), elbow joint angle (p<0.01), and muscle thickness (p=0.013). Strain ratio, MVC torque significantly decreased at 1H, 3H, and 24H compared to PRE after the EC exercise (p<0.05). Main effect in time was detected in Strain ratio (p=0.046) and upper arm circumference (p=0.044). Main effect in contraction type was detected in muscle soreness (p<0.01) with the greater level after the EC exercise. Elbow joint angle significantly increased at 1H, 3H, and 24H (p<0.05) compared to PRE after the EC exercise, and at 1H, 3H compared to PRE after the CC exercise (p<0.05). Upper arm circumference significantly increased at 1H compared to PRE after the CC exercise (p<0.05). Conclusions: The disruption of muscle function is greater and longer lasting after the eccentric contraction exercise than the concentric contraction exercise, and the muscle hardness only increased after the eccentric exercise when the exercises are performed with the same total work. The mechanisms of functional disruption following exercises and its effect on the muscle hardness may different according to the type of muscle contraction.