An Accelerometer-Equipped Running Watch Provides a Valid Estimate of Peak Vertical Ground Reaction Force and Cumulative Weighted Damage: A Cross-Sectional Laboratory Study.

We validated a wearable accelerometer and running watch for estimating peak vertical ground reaction force (vGRF) and cumulative weighted damage (CWD). First, 18 well-trained runners of equal sex distribution (3.3 ± 0.3 m/s habitual training pace) ran a range of speeds (2.7-4.0 m/s, plus training pace) on an instrumented treadmill as reference standard vGRF and wearable data were collected. Wearable data were inputted into a published equation to estimate peak vGRF. The wearable and equation systematically underestimated peak vGRF compared with the instrumented treadmill (mean bias: -0.22 BWs with LOA: -0.44 to 0.01; 9.4-10.1%RMSE). A linear regression model was used to derive a corrected equation to improve the estimate of peak vGRF. In the validation stage, the wearable and corrected equation were validated in 10 runners in an independent sample of equal sex distribution (3.2 ± 0.3 m/s habitual training pace) yielding excellent relationships and trivial errors for peak vGRF versus the reference standard (ICC3,1 = 0.87-0.97; p < 0.001; mean bias -0.008 BWs with LOA: -0.15 to 0.14 BWs; 1.5%-4.0% RMSE). To illustrate how these methods can be applied to runners at risk for a bone stress injury, CWD per km was estimated, resulting in trivial errors by the wearable versus the instrumented treadmill (ICC3,1 = 0.85-0.97; p < 0.001; 1.5-4.1%RMSE). In summary, a commercial wearable and corrected equation were valid estimates of peak vGRF and CWD. These methods have potential for monitoring training loads in runners at risk for bone stress injuries. An online calculator is provided to estimate peak vGRF and CWD from the running watch and wearable.