Near-infrared Spectroscopy More Accurate Than Heart Rate For Monitoring Intensity During Cycling Sprints: Pilot Data

INTRODUCTION: Heart rate (HR) is commonly used to measure exercise intensity, however tissue-saturation index (TSI), via near-infrared spectroscopy (NIRS), may be a more accurate measurement of intensity during short duration cycling sprints due to the rapid response in TSI. PURPOSE: To investigate the cardiorespiratory, metabolic, and muscle oxygen response during cycling sprints and to evaluate whether heart rate (HR) adequately reflects short-duration exercise intensity or if the tissue-saturation index (TSI) could provide a more accurate measure of short-duration cycling sprint intensity. METHODS: In a randomized, counterbalanced study, six healthy, young adults (20±1 years) completed two visits separated by 48 hours. Visit 1 consisted of a VO2max test and equipment/protocol familiarization. Metabolic data (VO2), HR and muscle oxygen saturation measured by TSI were continuously collected during a 10-minute cycling warmup followed by a series of four 15-second sprints (Sp1, Sp2, Sp3, and Sp4), interspersed with a 45-second low-intensity recovery. A linear mixed effect model was used to determine if physiologic (HR, VO2, and TSI) or performance (power) parameters changed across the four sprint trials. Sprint trial (1-4) served as a continuous covariate, and participant served as a random factor. Spearman correlations evaluated the relationship between variables during the sprints. RESULTS: HR increased from Sp1 to Sp4 (150±3, 156±3,159±2 and 161±2 bpm; respectively, p<0.05), while TSI (52±10, 47±11, 47±12, and 47±13%; respectively, p=0.79), VO2 (35±4, 36±4, 37±3, 38±3 ml/kg/min; respectively, p=0.51), and power (216±41, 223±47, 228±58, 250±68 watts; respectively, p=0.68) did not. Upon completion of Sp4, VO2 vs. power had the strongest correlation (0.83, p<0.05) followed closely by VO2 vs TSI (-0.77, p=0.07) while VO2 was not related to HR (0.03, p=0.33). CONCLUSION: HR increased throughout four cycling sprints while power and VO2 remained similar across all sprints. In the absence of a method to measure intensity via metabolic data or power, TSI may be a better predictor of intensity than HR, given the consistency in TSI while HR increases over time. A NIRS-based variable may offer a more accurate alternative to HR when monitoring cycling sprint intensity of short duration.