The Association Between Fatigue And Running Biomechanics During Treadmill And Outdoor Running

Like all endurance athletes, runners experience the impacts of fatigue, leading to changes in biomechanics and a decrease in performance. Fatigue assessment involves both subjective measures, such as ratings of perceived exertion (RPE), and physiological measures, like heart rate (HR). Previous studies have shown a more robust correlation between HR and RPE regardless of the running environment; however, we are unaware of any studies that have investigated this association with running biomechanics. PURPOSE: To investigate the association between RPE, HR and running biomechanics during treadmill and outdoor running. METHODS: Eligible participants were healthy males and females between the ages of 18-60 years-old who ran a minimum of 24.1 kilometers/week. Participants ran 8 km outdoors and on a treadmill at a self-selected speed. Polar Team Pro was used to measure HR and step rate (SR) while VICON IMeasureU Blue Trident inertial measurement units measured peak vertical (PV) and peak resultant (PRES) tibial acceleration. These data were captured continuously and averaged over every 1.6 km. Participants completed a Borg scale every 1.6 km to measure RPE. Separate linear mixed effects models were fitted to each outcome (SR, PV, PRES), for each location (treadmill vs outdoors), and for each measure of effort (RPE vs HR). RPE and HR were separated into their within-individual and between-individual effects, and both were included in the model. Subject was included as a random effect. RESULTS: Data were captured from 28 healthy adult runners (sex, females: 9, males: 19; age, 32.0 + 14.2 yrs). There was a significant, positive association between within-individual RPE and step rate (est [95% CI] = 0.85 [0.33 to 1.38], p = 0.002), PV (est [95% CI] = 0.14 [0.02 to 0.25], p = 0.021) and PRES (est [95% CI] = 0.27 [0.05 to 0.48], p = 0.015) while running outdoors but not on the treadmill. This suggests that higher RPE was associated with higher step rate, PV and PRES while running outdoors. There was no association between HR and step rate, PV, or PRES. CONCLUSIONS: Our results suggest that there is an association between fatigue measured using RPE and step rate and tibial acceleration and that this relationship differs between treadmill and outdoor running. This may have implications for assessing runners’ performance in different settings.