A model for the training effects in swimming demonstrates a strong relationship between parasympathetic activity, performance and index of fatigue.
Chalencon S, Busso T, Lacour JR, Garet M, Pichot V, Connes P, Gabel CP, Roche F, Barthélémy JC. PLoS One. 2012; 7(12):e52636. Epub 2012 Dec 20.