Duty Factor Dominates Stride Frequency to Modify Musculoskeletal Peak Loading in Running

ABSTRACT Background/purpose Adjusting running style can influence musculoskeletal loading, thereby altering injury risk. Duty factor, defined as the ratio of contact time to stride time, along with stride frequency, have been linked to peak loading in previous studies, although their specific influences remain unclear. This study elucidates how duty factor and stride frequency, both individually and in tandem, affect peak loading at an individualized constant speed, enhancing our understanding of how changes in running pattern affect musculoskeletal loading. Methods Nineteen female novice runners ran on treadmill at 90% of their individually preferred running speed. Subjects were instructed to adjust duty factor and/or stride frequency according to a specific protocol. Ground reaction forces and motion capture data were recorded. Peak loading was assessed through maximal vertical ground reaction force, maximal resultant joint reaction forces, and maximal extensor and flexor moments of the lower limb joints using an inverse dynamics approach. Mixed-effects models were utilized to analyze the individual and combined effects of duty factor and stride frequency on peak loading. Results and Discussion Increasing duty factor consistently reduced peak loading across all metrics except hip extensor and flexor moments, which showed an increase. In contrast, the relationship between stride frequency and peak loading varied across loading metrics. Increasing stride frequency reduced peak knee and hip extensor moments and had no effect on maximal vertical ground reaction force or peak joint reaction forces, but increased peak hip flexor moment. Surprisingly, when controlled for duty factor, stride frequency also became a determinant of maximal vertical ground reaction force and peak joint reaction forces, leading to unexpected increases in peak loading as stride frequency increased. Conclusions This study establishes duty factor as the primary factor in modulating peak loading in running. Stride frequency may also affect peak loading, but its effect varies based on individual characteristics and the metric considered.