Some researches have observed that explosive-strength training or plyometric training enhance running economy in highly trained long distance runners. Plyometric training may increase muscle stiffness and reduce time of stretch-shortening cycle and improve running economy in long distance running. Plyometric training generally improves explosive power, which is frequently assessed on the basis of the reactive leg strength index (RLS index). The purpose of this study was to investigate the relationships between the RLS index and the average running velocity during 5000m race (Av@5000m) for long distance runners, and get useful information to help improve traditional methods in Japan, where most long distance runners do not use explosive-strength training. Well-trained college-level distance runners and healthy recreational distance runners were categorized into 4 major group; CM: 18 college-level male runners (age = 21.1 ± 1.9years, height = 171.3 ± 5.1cm, mass = 58.2 ± 4.2kg mean ± SD), CF: 9 college-level female distance runners (19.2 ± 1.0years, 158.3 ± 4.3cm, 47.7 ± 5.1kg), RM: 13 recreational male runners (46.2 ± 8.3years, 168.2 ± 7.7cm, 61.1 ± 7.1kg), RF: 12 recreational female runners (39.4 ± 5.6years, 157.6 ± 4.6cm, 47.9 ± 4.1kg). The RLS index was assessed though a counter-movement-jump test using FiTRO jumper (Slovakia). The subjects were instructed to jump for maximal height and minimum ground contact time. The RLS index was calculated using the jump height-ground contact time ratio (Jh/Ct). The relationship between the RLS index and Av@5000m was analyzed for the four groups through ANOVA. In the event of a significant F ratio, a multiple comparison procedure (Tukey test) was employed. Av@5000m for CM, CF, RM and RF was 320.5 ± 12.2, 281.7 ± 5.2, 232.2 ± 23.4 and 206.2 ± 6.4 m/min, respectively. Ct was 0.161 ± 0.009, 0.179 ± 0.011, 0.170 ± 0.015 and 0.175 ± 0.013 seconds, respectively. Jh was 32.1 ± 3.4, 26.8 ± 2.9, 23.7 ± 2.3 and 21.4 ± 2.4 cm, respectively. The RLS index was 200.3 ± 25.8, 150.5 ± 17.6, 122.6 ± 16.9 and 121.1 ± 24.3, respectively. Av@5000m was CM > CF > RM > RF, the RLS index was CM > CF, RM, RF, and Jh was CM > CF, RM, RF. No significant difference could be observed in Ct between the four groups. Av@5000m and Jh correlate strongly for CM (r = 0.686, p < 0.01) and RM (r = 0.765, p < 0.01), and correlate less strongly for CF (r = 0.795, p < 0.05) and RF (r = 0.689, p < 0.05). Av@5000m and the RLS index strongly correlate for CM (r = 0.741, p < 0.01), and correlate less strongly for CF (r = 0.790, p < 0.05). There was no significant correlation for RM and RF. There was also no significant correlation between Av@5000m and Ct for the four groups. CONCLUSION AND The results show that Jh and the RLS index relate to Av@5000m for long-distance runners. Jh has particularly strong effects on Av@5000m. Ct does not relate to Av@5000m. Both Jh and RLS appear to be useful for evaluating long-distance running ability, while Ct appears not to be important. Further work will be required to evaluate the significance of explosive strength training in long-distance training.