The Validity of Stryd Leg Stiffness Against the Morin (2005) Sine-Wave Method: A Level-1 Assessment of Flat and Uphill Treadmill Running

This study evaluated the validity of the leg stiffness metric provided by the Stryd running power meter against the Morin (2005) sine-wave spring–mass model. Twenty-three highly trained trail runners (11 women) completed a 12 min uphill time trial at +12% grade and one hour of submaximal level running. Leg stiffness was calculated from contact time, flight time, running speed, and leg length using Morin’s method, and compared with Stryd values. Agreement was assessed following the Dhahbi and Chamari Level-1 analytical framework, including intraclass correlation coefficient (ICC_2,1), Bland–Altman analysis, mean absolute percentage error (MAPE), and paired t-tests. Stryd and Morin estimates showed excellent agreement in both conditions: uphill running: ICC_2,1 = 0.96 (95%CI: 0.91–0.98), bias = −0.02 kN·m⁻¹, limits of agreement (LoAs) = [−0.61, 0.58] kN·m⁻¹, MAPE = 2.5% (p = 0.803); and level running: ICC_2,1 = 0.97 (95%CI: 0.93–0.99), bias = −0.04 kN·m⁻¹, LoAs = [−0.62, 0.54] kN·m⁻¹, MAPE = 2.6% (p = 0.505). The Stryd sensor provides valid leg stiffness estimates in highly trained trail runners on both level and inclined terrain. The negligible systematic bias and narrow limits of agreement support the use of Stryd for leg stiffness monitoring in field and laboratory settings.