The purpose of this study was to determine whether Critical Stroke Rate (CSR) is effective as an index of intensity endurance interval training in competitive swimmers. CSR was expressed as the slope of the regression line between the number of stroke cycles and time. Seven collegiate competitive swimmers performed maximal 200 m and 400 m front crawl trials for CSR and Critical Swimming Velocity (CV). Subsequently, swimmers were instructed to perform a 4 × 400 m swimming test at CSR using a tempo trainer (interval test). Mean swimming velocity during the interval test was correlated significantly with CV (r=0.880, p<0.05). In the interval test, stroke length, blood lactate and RPE were also significantly increased (2.51±0.10 to 2.55±0.11m/stroke, 3.1±0.4 to 4.8±0.7mmol/L, 13.6±0.3 to 16.7±0.7, respectively) with the increase in swimming velocity. CSR is considered to be effective as an index for endurance interval training. Moreover, it is also suggested to be able to monitor stroke length in endurance interval training at CSR.

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 In the 1950s, there were miserable water accidents that many children had drowned in Japan. As measures to such accidents, the Japanese government promoted swimming education and made swimming a compulsory subject in the elementary and ju-nior high school. Because many of the victims were a clothing state, some swimming leaders and researchers started studies on the swimming with clothes on in 1980s.
 In the swimming with clothes on, there are two major purposes. One is to train basic swimming abilities not to be drowned, available even in the clothing state. Another is to acquire survival abilities to stay floating safely until a rescue.
 According to the history of swimming education in Japan and the actual situation of the instruction about water safety in the world, clothed swimming is a part of the method to obtain water safety and should be planned in conjunction with developing individual abilities not to be drowned, which are included in the basic and fundamental swimming skills.

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