INTRODUCTION: A recent study reported that blood glucose threshold (GT) could be observed during an incremental exercise test and that GT was correlated with blood lactate threshold (LT) (1). The relationship between GT and LT during a running exercise was discussed, however, no study has investigated GT during swimming. The purpose of the present study was to investigate whether GT could be determined in swimmers early in the swimming season. METHODS: Seven university swimmers participated in this study and performed an incremental swimming test in a swimming flume. The test was conducted at 4-5 weeks after the beginning of the winter swimming season. Swimming velocity was increased incrementally from 55% to 85% of the individual`s 400 m freestyle average swimming speed. Each stage included swimming for three minutes with two minutes rest for blood sampling. Blood glucose (Glu) and blood lactate (Bla) was measured after each swimming stage, and the velocity corresponding to GT and LT was analyzed. The subjects were instructed to finish their last meal 3 h before testing. RESULTS: LT was determined in all swimmers (1.24±0.05 m/sec). Glu did not increase during the high intensity stage and GT was not determined. DISCUSSION: The present study investigated whether GT could be determined in swimmers early in the swimming season. Our results showed that GT could not be determined during an incremental swimming test. GT is thought to occur by the elevation of catecholamine and glucagon activity during running, thus, a difference in hormonal response during swimming may explain the present result. Flynn et al. (2) reported that Glu and Glucagon:Insulin ratio reacted differently between continuous 45 min (75%VO2max) swimming and running. The low Glu observed during high intensities may be due to high glucose uptake by the working muscle. However, since the experiment was conducted early in the swimming season, the low endurance capacity of the swimmer may also affect the Glu response in this study. Further study investigating the hormonal responses and carbohydrate oxidation rate during an incremental swimming test is warranted.
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