Using near-infrared spectroscopy to determine maximal steady state exercise intensity.

Maximal steady state (MSS) speed can be determined from blood lactate concentration (HLa); however, this method is not optimal. The purpose of this study was to determine whether near-infrared spectroscopy (NIRS) technology could be used to detect a breakpoint in percent oxygen saturation (StO2) of the muscle and whether the determined breakpoint exercise intensity could be used to determine MSS exercise intensity. Sixteen distance runners and triathletes (men = 9, VO2max = 64.9 +/- 4.9 ml x kg(-1) x min(-1), women = 7, VO2max = 50.8 +/- 7.0 ml x kg(-1) x min(-1)) completed an incremental exercise test. A change from linearity when plotting StO2 or HLa vs. running speed was defined as the breakpoint. The subjects then completed constant speed runs to determine maximal lactate steady state (MLSS). In 12 subjects, breakpoints were identified for both HLa and StO2 values. Predicted MLSS velocities from HLa breakpoint (12.76 +/- 1.63 km x h(-1)), StO2 breakpoint (12.84 +/- 1.58 km x h(-1)), and 4 mM HLa (13.49 +/- 1.71 km x h(-1)) methods from the incremental test did not differ from MLSS speeds (13.04 +/- 2.03 km x h(-1)). A Bland and Altman analysis of agreement between the MLSS and the StO2 breakpoint speeds resulted in a mean difference of 0.14 +/- 0.36, whereas the mean difference between MLSS and HLa breakpoint speeds was 0.19 +/- 0.43. During the incremental test, no StO2 breakpoint was determined in 2 subjects, whereas 2 subjects had no HLa breakpoint. The results of this study lead us to conclude that the NIRS determination of StO2 is a noninvasive technique that is comparable with HLa in determining MSS intensity and therefore appropriate for use in determining exercise training intensity.