Fig. 2. Diving stroke volume (VS) correlates with the degree of bradycardia expressed during the dive [ratio of mean diving heart rate (fH) to mean pre-dive fH (P=0.01, r²=0.98; y=-47.83x+150.56)]. Although the degree of bradycardia is correlated with VS, the absolute fH during the dive is not, suggesting that it is the degree of cardiac response to diving that is driving the increase in VS rather than the absolute rate of contraction.