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

BACKGROUND: 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. METHODS: ) 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. RESULTS: 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. CONCLUSIONS: 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.