Exercise Thresholds on Trial: Are They Really Equivalent?
The interchangeable use of whole-body exercise thresholds and breakpoints (BPs) in the local oxygenation response, as measured via near-infrared spectroscopy (NIRS), has recently been questioned in scientific literature. Therefore, the present study aimed to longitudinally investigate the interrelationship of four commonly used exercise thresholds: critical power (CP), the respiratory compensation point (RCP) and BPs in muscle (m[HHb]BP) and brain (c[O2Hb]BP) oxygenation.Methods
Nine male participants (21.8±1.2 years) completed six weeks of cycling interval training. Prior to and following this intervention period, subjects performed a ramp incremental (RI) exercise protocol to determine RCP, m[HHb]BP and c[O2Hb]BP and four constant work rate (WR) tests to calculate CP.Results
WRs associated with CP, RCP, m[HHB]BP and c[O2Hb]BP increased by 7.7±4.2%, 13.6±9.0%, 9.8±5.7% and 11.3±11.1%, respectively. CP was lower (pre: 260±32W, post: 280±41W) (P < 0.05) than the WRs associated with RCP (pre: 281±28W, post: 318±36W) and c[O2Hb]BP (pre: 283±36W, post: 313±32W) which occurred concomitantly (P = 0.683). M[HHb]BP occurred at the highest WR and differed from all others (pre: 313±23W, post: 344±32W) (P < 0.05). Training-induced WR differences ([INCREMENT]WR) did not contrast between thresholds and initial parameter differences were not affected by the intervention (P = 0.253). Thresholds were partly correlated before (R = 0.67-0.85, P < 0.05) and after (R = 0.83-0.96, P < 0.05) training, but [INCREMENT]WR were not associated (P > 0.05).Conclusion
Results of the present study strongly question true equivalence of CP, RCP, m[HHb]BP and c[O2Hb]BP during RI exercise. Therefore, these exercise thresholds should not be used interchangeably.