Oxygen uptake, oxygen debt, blood lactate concentration and biomechanical parameters, i.e. running velocity, stride length and stride rate, in a 400m sprint running were measured in one untrained and four trained subjects to reveal the physiological factors of limitation of sprint performance. Significant correlations between peak blood lactate concentration and mean running velocity, stride rate, and a decrease in stride length were found. The results indicate that the higher mean running velocity or stride rate may be an important factor producing a high blood lactate concentration, which might induce a decrease of stride length primarily at the later periods of 400m sprinting than that of stride rate.
　In order to improve 400m sprint performance, the following conclusions could be considered:
1) to delay accumulating lactic acid when running velocity or stride rate is higher, 2) to reduce decreasing rate of stride length at a given higher concentration of lactic acid.

DESCENTE SPORTS SCIENCE Vol.1/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE