INTRODUCTION Critical velocity (Vcri) determined by the relationship between the swimming distance and the swimming time, has been defined as the maximal speed which could theoretically be maintained without exhaustion during swimming. On the other hand, since an interval swims includes rest period, the critical velocity for the interval swim (Vcri-IS), which is defined as the maximal average speed to be able to swim without exhaustion, should not be identical to the conventional Vcri, and also it would vary with the rest duration. Therefore, the purpose of this study was to determine Vcri-IS for interval training at the swimming velocity with the different rest periods. METHODS The subjects were 14 well-trained college male swimmers. Experiments were carried out in the 50 m pool. In this experiments, the maximal swim test (Tmax) and 50 m interval swim test (Tint) were performed. In Tmax, the subjects swam 50 m and 2000 m at maximal effort, and the mean velocity of each swim was determined (V50, V2000). The swimming velocity in Tint was set at the pace of V50 % for each subject, which was calculated by the following equation, V50%=0.5(V50-V2000) + V2000. A light pace marker was used to ensure compliance with predetermined velocity. The rest time in Tint was, first, set 5 s. However, if the subject could not complete interval swim 30 times with 5 s rest time, the rest time was increased by 5 s in the next trial, until the subjects could complete Tint. Finally, The subjects performed four to six sets of Tint, and thus, the rest time ranged from 5 s to 30 s. The total time (tT) of Tint including interval swims and rest periods and the total swim distance (Dint) of Tint were determined. Ccri-IS could be determined by the relationship between Dint and tT. RESULTS The relations between Dint and tT were expressed in the general form, Dint=a+b*tT, with r value showing higher than 0.997 (p<0.05) in all subjects. Vcri-IS ranged from 0.730 to 1.017 m/s, with a mean of 0.924 m/s (SD=0.076). The rest time (tR) which can theoretically be repeated the interval swims without exhaustion in Tint at V50 %, can be estimated by using Vcri-IS. tR ranged from 17.5 to 31.1 s with a mean of 22.0 s (SD=3.7). There was a tendency that tR of the long distance swimmers was shorter than that of the short distance swimmers. DISCUSSION It was thought that the concept of the conventional Vcri could be applied to the interval swim training. The Vcri-IS obtained by this method could be applied as an index for setting the combination of swim velocity and rest time for the interval training, corresponding to the fatigue characteristics and the performance level of the swimmers.
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