The purpose of this study was to examine the relationships between running performance and the cross–sectional area of the psoas major, peak oxygen uptake, and running economy in male junior long–distance runners. The subjects were 37 male junior athletes who achieved good records in interscholastic athletic competition during 5 years (2011–2015). Their seasonal best times in a 5,000 m race (5,000m–SB) were 14:04.11 ± 0:07.25 (13:53.64–14:16.15). In a multiple regression analysis, 5,000m–SB was statistical significantly correlated with the cross–sectional area of psoas major (16.0 ± 1.7 cm²) measured on magnetic resonance imaging and peak oxygen uptake (4.25 ± 0.36 l min^-1 [76.9 ± 5.8 ml min^-1 kg^-1]) during a lactate curve test comprising five stages. However, 5,000m–SB was not related to oxygen uptake at the work–load which was less than the load at the lactate threshold estimated by using the lactate curve test results. These results suggest that a high volume of the psoas major, the largest hip–flexor muscle, and peak oxygen uptake are among the important factors for junior long–distance runner performance.

Previous research has demonstrated that the concentric muscle strength of knee extension exerted at slow to medium angular velocities is increased by teeth clenching. This study evaluated the isokinetic eccentric strength of knee extensors at 4 angular velocities (60, 120, 180 and 240 degrees per second) with and without teeth clenching on the Cybex 6000 Extremity Testing and Rehabilitation System. Twelve healthy adult male (26.8±1.8 years) volunteered for the study. Peak torque per body weight and average power per body weight were analyzed. The results demonstrated that peak torque per body weight accompanied by teeth clenching was significantly increased at 60, 120, 180 and 240 deg/s by 9.5%, 8.2%, 8.2% and 9.8%, respectively (p<0.01) . Similarly average power per body weight during teeth clenching was significantly higher at 60, 120, 180 and 240 deg/s by 9.1%, 9.5%, 9.4% and 7.7%, respectively (p<0.05) . These findings suggest that teeth clenching is a factor that leads to augmented eccentric strength of knee extensors. Its effect appears to be independent of angular velocity.