The impact of swimming speed on respiratory muscle fatigue during front crawl swimming: a role for critical velocity?

The Critical velocity (Vcrit) represents a holistic swimming fatigue threshold and critical stroke rate is thought to coincide with Vcrit. Whether Vcrit, and in-turn critical stroke rate, also represent an inspiratory muscle fatigue threshold is not known. Following the determination of Vcrit and critical stroke rate via a two-parameter 200-m and 400-m model, 17 well-trained competitive swimmers (9 males and 8 females) undertook three experimental 200-m front crawl (FC) swims on separate occasions. One experimental swim corresponded to Vcrit, one was 5% slower than Vcrit (Vcrit5%+) and one 5% faster (Vcrit5%-) than Vcrit. Swim time, inspiratory and expiratory mouth pressure (PImax and PEmax, respectively), stroke rate, stroke length and breathing frequency were recorded during each experimental swim and were compared between swims as well as between genders. Baseline PImax and PEmax did not differ between experimental swims or genders. Although PEmax decreased significantly by a similar magnitude after each swim (range 5-8%), PImax was significantly lower in response to swimming at Vcrit (7%) and Vcrit5%- (22%) thus indicating inspiratory muscle fatigue. However, the difference in magnitude between velocities was not significant. Only stroke rate and relative swimming velocity were significantly related to the fall in PImax and no correlations were observed between PEmax and any other variable. The development of inspiratory muscle fatigue at and above Vcrit is consistent with substantial swim induced metabolic strain with the response similar between males and females. Our data suggest that Vcrit represents a FC inspiratory muscle fatigue threshold when using this two-parameter Vcrit model.