Speed in aquatic locomotion is determined by stroke distance and stroke rate, but it does not always follow that an increase instroke rate will lead to an increase in speed. Kleshnev (2006) developed a method to evaluate the relationship between speedand stroke rate during rowing- the effective work per stroke. In this case study, the effective work per stroke was determinedfor a male world-class 100-m breaststroke swimmer for seven races in major championships and compared between: each ofthe seven races; each quarter within each race; and the best swims of this case study and seven other world-class swimmers.The effective work per stroke was related to race performance, with the fastest race having the highest effective work perstroke and lowest stroke rate, with slower races having low effectiveness and high stroke rate (R²=0.85). The effective workper stroke was reduced in a race as the swimmer fatigued. The within-race standard deviation of effectiveness was lower infast swims (R²=0.84). This analysis has identified some characteristics of fast swimming: high effectiveness, optimal strokerate, and a flat effectiveness profile. Training and racing strategies can now be devised to improve performance by increasingthe sensitivity of assessment of strengths and weaknesses in individuals.