Purpose:

To 1) investigate the cardiorespiratory and metabolic response of trail running and 2) evaluate whether heart rate (HR) adequately reflects the exercise intensity or whether the tissue saturation index (TSI) could provide a more accurate measure when running in hilly terrain.

Methods:

Seventeen competitive runners (female: n=4, V’O_2max: 55±6 mL·kg⁻¹·min⁻¹; male: n=13, V’O_2max: 68±6 mL·kg⁻¹·min⁻¹) performed a time trial on an off-road trail course. The course was made up of two laps covering a total distance of 7 km and included six steep up- 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 within the uphill and downhill sections was 94±2% and 91±3% of HR_max, 84±8% and 68±7% of V’O_2max, respectively. The oxygen uptake (V’O₂) increased within the uphill and decreased within the downhill sections (P< .01). While HR was unaffected by the altering slope conditions, the TSI was inversely correlated to the changes in V’O₂ (r = - .70, P< .05).

Conclusions:

The HR was unaffected by the continuously changing exercise intensity, however, the TSI reflected the alternations in V’O₂. Recently used exclusively for scientific purpose, this NIRS based variable may offer a more accurate alternative to HR to monitor running intensity in the future, especially for training and competition in hilly terrain.