Cardiac autonomic responses to repeated shuttle sprints

Team sport match play requires athletes to perform a number of repeated shuttle sprints. However, the acute effects of these repeated sprint sequences on lactic acidosis and resulting autonomic state perturbation are not known. The aim of this study was to observe and compare the blood lactate and post-exercise cardiac autonomic responses of a repeated shuttle-sprint ability test with the 30-15 Intermittent Fitness Test (30-15 (IFT)); the latter test representing a standard for exhaustive supramaximal effort. Thirteen adult team sport players performed the repeated shuttle-sprint ability test and the 30-15 (IFT) on separate days in a counter-balanced order. The repeated shuttle-sprint ability test consisted of six repetitions of maximal 2x15 m shuttle sprints ( approximately 5 s) departing every 20 s, while the 30-15 (IFT) involved progressive 30 s shuttle runs interspersed with 15 s of passive recovery until exhaustion. Blood lactate was measured before and after the tests, while autonomic responses were assessed using immediate heart rate recovery and heart rate variability indices. Peak blood lactate (10.6+/-2.1 vs. 10.2+/-2.8 mM) and heart beats recovered in one minute after exercise cessation (36.4+/-7.8 vs. 39.3+/-7.9 bpm) were similar after both the repeated shuttle-sprint ability test and the 30-15 (IFT). With the exception of the vagal-related time-varying root mean square of successive R-R interval differences at each 30 s, which recovered earlier after the repeated shuttle-sprint ability test compared with 30-15 (IFT), all heart rate variability indices decreased similarly after both tests in comparison to baseline values. In conclusion, the repeated shuttle-sprint ability test was shown to induce comparable levels of lactic acidosis and post-exercise autonomic state as the 30-15 (IFT). These levels of metabolic and autonomic states are likely to occur during team sport match play.