May The Force Be With You: Acceleration-based Estimates Of Vertical Ground Reaction Forces During Running

Abstract

Running-related injury (RRI) may be caused by combinations of load magnitudes and numbers exceeding musculoskeletal structure capacity. Few methods exist, however, to quantify external loads (vertical ground reaction forces; vGRFs) during running in ecologically valid settings. PURPOSE: Develop models to accurately estimate vGRF second (“active”) peaks during running from iliac crest and sacrum accelerations. METHODS: Anthropometric and sex data were collected from 40 runners. Runners wore inertial measurement units (IMUs) (±100g, ±2000deg/s, 1000Hz) secured to their iliac crests and sacra while they ran a 25m track with embedded force plate (1000Hz). Speed, IMU accelerations, and force plate data were synchronously recorded for ten stances per foot at “slow”, “typical”, and “fast” self-selected speeds. Accelerations were transformed to a segment coordinate system. Force and acceleration signals were 50Hz low-pass filtered and divided into 0-8Hz low frequency (LoF) and ≥10Hz high frequency (HiF) signals. Acceleration and vGRF peaks were extracted from the original, LoF, and HiF signals. Two multiple linear regressions were created to estimate log-transformed vGRF second peak: One used sacrum accelerations to predict bilateral forces, the other used iliac crest accelerations to predict ipsilateral forces. Each model included sex and limb lengths as fixed effects and was validated using an eight-fold cross-over. RESULTS: Both models predicted observed vGRF second peaks well (r2=0.78, mean absolute error <7%). Addition of participant as a random effect (r2≥0.93, mean absolute error <4%) or speed as a fixed effect (r2≥0.83, mean absolute error <6%) further improved results. CONCLUSIONS: The models developed here demonstrate a single IMU secured over the iliac crest or sacrum can estimate ipsilateral or bilateral vGRF second peak, respectively, with high accuracy. This approach could greatly impact our understanding of RRI by facilitating quantification of the step-by-step external forces experienced by runners over long time periods in ecologically valid settings. Supported by an ACSM Doctoral Student Research Grant, a Sigma Xi Grant-in-Aid of Research, the Maury Hull Endowed Fellowship for Musculoskeletal Biomechanics Research, and an NSERC Post-Graduate Scholarship.