Vertical Loading Rate Is Not Associated with Running Injury, Regardless of Calculation Method

Introduction Loading rate (LR), the slope of the vertical ground reaction force (vGRF), is commonly used to assess running-related injury risk. However, the relationship between LR and running-related injuries, including bone stress injuries (BSI), is unclear. Inconsistent findings may result from the numerous LR calculation methods that exist and their application across different running speeds. Purpose This study aimed to assess the influence of calculation method and running speed on LR values and to determine the association of LR during healthy running with subsequent injury. Methods Healthy preseason running data and subsequent injury records from Division I cross-country athletes (n = 79) over four seasons (2015-2019) at 2.68 m center dot s(-1), preferred training pace, and 4.47 m center dot s(-1) were collected. LR at each speed was calculated four ways: 1) maximum and 2) average slope from 20% to 80% of vGRF magnitude at impact peak (IP), 3) average slope from initial contact to IP, and 4) average slope from 3% to 12% of stance time. Linear mixed effects models and generalized estimation equations were used to assess LR associations. Results LR values differed depending on speed and calculation method (P value <0.001). The maximum slope from 20% to 80% of the vGRF at 4.47 m center dot s(-1) produced the highest LR estimate and the average slope from initial contact to IP at 2.68 m center dot s(-1) produced the lowest. Sixty-four injuries (20 BSI) were observed. No significant association was found between LR and all injuries or BSI across any calculation method (P values >= 0.13). Conclusions Calculation method and running speed result in significantly different LR values. Regardless of calculation method, no association between LR and subsequent injury was identified. Thus, healthy baseline LR may not be useful to prospectively assess running-related injury risk.