Near-Infrared Spectroscopy: More Accurate Than Heart Rate for Monitoring Intensity in Running in Hilly Terrain

Abstract

Purpose: To investigate the cardiorespiratory and metabolic response of trail running and evaluate whether heart rate (HR) adequately reflects the exercise intensity or if the tissue-saturation index (TSI) could provide a more accurate measure during running in hilly terrain.

Methods: Seventeen competitive runners (4 women, V̇O₂max, 55 ± 6 mL · kg^-1 · min^-1; 13 men, V̇O₂max, 68 ± 6 mL · kg^-1 · min^-1) performed a time trial on an off-road trail course. The course was made up of 2 laps covering a total distance of 7 km and included 6 steep uphill and downhill sections with an elevation gain of 486 m. All runners were equipped with a portable breath-by-breath gas analyzer, HR belt, global positioning system receiver, and near-infrared spectroscopy (NIRS) device to measure the TSI.

Results: During the trail run, the exercise intensity in the uphill and downhill sections was 94% ± 2% and 91% ± 3% of maximal heart rate, respectively, and 84% ± 8% and 68% ± 7% of V̇O₂max, respectively. The oxygen uptake (V̇O₂) increased in the uphill sections and decreased in the downhill sections (P < .01). Although HR was unaffected by the altering slope conditions, the TSI was inversely correlated to the changes in V̇O₂ (r = -.70, P < .05).

Conclusions: HR was unaffected by the continuously changing exercise intensity; however, TSI reflected the alternations in V̇O₂. Recently used exclusively for scientific purposes, this NIRS-based variable may offer a more accurate alternative than HR to monitor running intensity in the future, especially for training and competition in hilly terrain.

Keywords: athlete; elite; tissue saturation; undulating terrain.