Power‐Based Running Durability in Trained Runners: Preliminary Evidence of Speed‐Adjusted Mechanical Drift as a Marker of Fatigue

ABSTRACT

This study examined wearable‐derived running power and biomechanical variables as indicators of durability during prolonged dual‐intensity exercise in trained trail runners. Twelve trained trail runners completed a 216‐min treadmill protocol of three repeated 72‐min blocks (60 min steady‐state running at 85% of lactate threshold speed plus a 12‐min uphill time trial [+12% incline]), plus an unfatigued control trial. A foot‐mounted running power metre (Stryd Next Gen, Stryd Inc., CO, USA; firmware version 2.1.32.1.1) continuously recorded power output, duty factor, ground contact time, form power, cadence, vertical oscillation and leg stiffness. Repeated‐measures ANCOVA with running speed as a covariate compared biomechanical across time trials at a common, model‐adjusted running speed. During the uphill time trials, duty factor, ground contact time, leg stiffness, vertical oscillation and form power changed significantly after controlling for speed (all p  < 0.05; η ² p  = 0.23–0.41), reflecting neuromechanical fatigue and a shift towards a lower‐stiffness, higher‐contact‐duration gait. In contrast, absolute power, normalised power and %CP declined in proportion to running speed and did not reach significance after covariate adjustment. During steady‐state running, biomechanical and physiological variables remained stable except duty factor, which increased modestly but significantly from 60 to 180 min (71.3 ± 1.6% to 72.7 ± 1.7%; p  = 0.014, η ² p  = 0.32), the sole wearable‐detected indicator of mechanical drift at submaximal intensity. Running cost remained preserved throughout both exercise conditions. These findings suggest that the duty factor is the most sensitive wearable‐derived marker of fatigue‐related mechanical reorganisation during prolonged trail running, and that a wearable device can capture both the neuromechanical and performance dimensions of durability.