Reliability of a trail running submaximal treadmill test and its relationship with flat surface running economy

Cost of exercise increases as a function of incline up to +15% and directly proportional to slope above +15%. Trail running racing consist of significant uphill locomotion at submaximal intensities, but the reliability of submaximal uphill treadmill running tests has not yet been established in ultra-trail runners.

Twenty-three male ultra-trail runners (maximal oxygen consumption (V̇O2max) 59.7±6.5 ml∙min^-1∙kg^-1) completed two uphill and one flat Submaximal Treadmill Running Test (STRT) separated by 7 days each. Uphill STRT test-retest reliability was measured via intraclass correlation coefficient (ICC), typical error of measurement (TEM), and typical error percentage (TEM%) for V̇O2, heart rate (HR), respiratory exchange ratio (RER), running economy (RE), and rating of perceived exertion (RPE). Relationships between uphill and flat STRT were determined via Pearson correlations.

Highest reliability and lowest TEM% were found in HR (ICC: 0.86-0.94; TEM%: 1.9-3.6%), while reliability in V̇O2 (ICC: 0.57-0.89; TEM%: 2.7-5.4%), RE (ICC: 0.57-0.89; TEM%: 2.7-5.4%), and RER (ICC: 0.67-0.86; TEM%: 1.8-2.6%) had greater variation between the five stages of the uphill STRT. RPE was least reliable (ICC: 0.57-0.70; TEM%: 9.6-13.3%). Moderate to strong correlations between the uphill and flat STRT HR were observed (r=0.69-0.89), with weaker correlations between flat and 5-10% uphill RE (r=0.45-0.81), and no relationship between flat and +25% RE.

The uphill STRT has good overall reliability, showing potential as a training monitoring tool. However, flat and steep uphill (+25%) RE were not correlated indicating the need for uphill testing amongst trail running populations.