Power or speed: Which metric is more accurate for modelling endurance running performance on track?

This study aimed to compare the accuracy of the power output, measured by a power meter, with respect to the speed, measured by an inertial measurement unit (IMU) and a global navigation satellite system (GNSS) sport watch to determine the critical power (CP) and speed (CS), work over CP (W') and CS (D'), and long-duration performance (i.e., 60 min). Fifteen highly trained athletes randomly performed seven time trials on a 400 m track. The CP/CS and W'/D' were defined through the inverse of time model using the 3, 4, 5, 10, and 20 min trials. The 60 min performance was estimated through the power law model using the 1, 3, and 10 min trials and compared with the actual performance. A lower standard error of the estimate was obtained when using the power meter (CP: 2.7 [2.1-3.3] % and W': 13.8 [10.4-17.3] %) compared to the speed reported by the IMU (CS: 3.4 [2.5-4.3] %) and D': 20.7 [16.6-24.7] %) and GNSS sport watch (CS: 3.4 [2.5-4.3] % and D': 20.6 [16.7-24.7] %). A lower coefficient of variation was also observed for the power meter (4.9 [3.7-6.1] %) Regarding the speed reported by the IMU (10.9 [7.1-14.8] %) and GNSS sport watch (10.9 [7.0-14.7] %) in the 60 min performance estimation, the power meter offered lower errors than the IMU and GNSS sport watch for modelling endurance performance on the track. Recent advances in wearable technology are creating new opportunities to accurately monitor running intensity in contexts where precision is essential (i.e., track running) for athletes' (e.g., pacing) and coaches' (e.g., performance analysis) tasks. The track mode of recent global navigation satellite system (GNSS) sport watches seems to offer a decent improvement. Alternatively, new inertial measurement units (IMUs) designed specifically for running have emerged offering the capability to monitor distance in specific contexts such as treadmills and indoor tracks. Additionally, these IMUs provide new metrics of the external load. The power meter provided by the Stryd inertial measurement unit (IMU) demonstrated lower estimation errors in modelling endurance running performance on the track compared to speed data reported by the same IMU and a Garmin GNSS sports watch (using the track mode).