Increasing step rate reduces insole force and cumulative load in college runners

Competitive distance runners are at high risk of injuries, which result in financial burdens and impede performance. Running injuries are usually overuse injuries resulting from mechanical fatigue in repetitively loaded musculoskeletal structures. Mechanical fatigue refers to the accumulation of microstructural damage due to a combination of the number of loading cycles and load magnitude, potentially resulting in mechanical property degradation and ultimately failure. Increasing step rate (the number of steps per minute) has been proposed to reduce the risk of running injuries by reducing load magnitude during running. Based on fatigue failure behavior of musculoskeletal structures, the reduction in structure load magnitude in response to increased step rate should reduce the amount of damage accumulated for a given running speed and distance despite the increased number of loading cycles. Testing this hypothesis is difficult because of the difficulty in quantifying fatigue damage. However, in level running at submaximal speeds, muscle-tendon and bone forces increase with increasing vertical ground reaction force (vGRF). Therefore, changes in vGRF in response to increased step rate may reflect changes in structure-specific loads and cumulative damage. Further, peak vGRF can be approximated using force sensing insoles, allowing ecologically valid observations. To assess the efficacy of increasing step rate to reduce peak vGRF and potential cumulative damage in college runners, we examined changes in peak insole force and cumulative weighted peak force (CWPF), based on fatigue failure behavior of musculoskeletal structures, with increased step rate. We also evaluated the use of sacral acceleration, a correlate of vGRF, to detect these changes. 12 collegiate distance runners ran on an outdoor track at 3.83 m/s ± 5% for 1000m at their preferred step rate and at a 10% increased step rate while insole force and sacral acceleration were recorded. Average peak insole force and CWPF per kilometer decreased significantly (p < 0.001) with increased step rate, suggesting increasing step rate can reduce peak vGRF and a general measure of cumulative damage in college runners. Changes in sacral acceleration measurements were consistent with changes in force measurements and peak acceleration correlated with peak insole force on an individual basis (mean r = 0.62), supporting the use of sacral acceleration to detect changes in vGRF and potential relative changes in structure loads and cumulative damage with increased step rate. These results suggest clinicians should consider interventions targeting an increase in step rate to help reduce the risk of injury in college runners and that the potential efficacy of those interventions can be evaluated using field-based accelerometry as a more accessible alternative to measuring forces