No extra sensors needed: estimating vertical ground reaction force using IMUs at the smartwatch and heartrate strap location.

Understanding the factors contributing to running-related injuries is important, since such injuries are common among runners. Vertical ground reaction forces (vGRFs) can help quantify biomechanical load during running, but are typically measured using force plates in laboratory settings. To allow continuous monitoring outdoors, wearable sensors such as inertial measurement units (IMUs) may offer a practical alternative. These sensors are often also embedded in smartwatches and heart rate monitor belts. This study aimed to predict vGRF using IMUs placed on the wrist and sternum-mimicking smartwatch and heart rate strap positions-combined with smartwatch-derived variables. Eleven rearfoot strike runners completed twelve 90-s treadmill trials at four speeds (8, 10, 12, 14 km/h) and three cadences (preferred ±10%). IMUs captured 3D acceleration and angular velocity; pressure insoles provided vGRF estimates used for outdoor validation. Subsequently, participants ran outdoors on an athletics track at the same four speeds. A long short-term memory (LSTM) neural network was trained to predict vGRF from IMU data. Feature importance analysis showed the sternum IMU contributed most to prediction accuracy. Leave-one-subject-out cross-validation, using all features, yielded an RMSE of 0.10 ± 0.024 body weight compared to indoor force plate measurements. Outdoor validation showed no significant performance drop. These results suggest that wearable devices enable meaningful vGRF monitoring during outdoor running.