Effects of altered stride frequency and contact time on leg-spring behavior in human running

Many studies have demonstrated that contact time is a key factor affecting both the energetics and mechanics of running. The purpose of the present study was to further explore the relationships between contact time (tc), step frequency (f) and leg stiffness (kleg) in human running. Since f is a compound parameter, depending on both contact and aerial time, the specific goal of this study was to independently vary f and tc and to investigate their respective effects on spring–mass characteristics during running, seeking to determine if the changes in kleg observed when running at different f are mainly due to inherent changes in tc. We compared three types of constant 3.33 m s−1 running conditions in 10 male subjects: normal running at the subjectʹs freely chosen f, running with decreased and increased f, and decreased and increased tc at the imposed freely chosen f. The data from the varied f trials showed that the variation of tc was strongly correlated to that of kleg (r2=0.90), and the variation of f was also significantly correlated to that of kleg (r2=0.47). Further, changes in tc obtained in various tc conditions were significantly correlated to changes in kleg (r2=0.96). These results confirm that leg stiffness was significantly influenced by step frequency variations during constant speed running, as earlier demonstrated, but our more novel finding is that compared to step frequency, the effect of contact time variations appears to be a stronger and more direct determinant of kleg. Indeed, 90–96% of the variance in kleg can be explained by contact time, whether this latter parameter is directly controlled, or indirectly controlled through its close relationship with step frequency. In conclusion, from the comparison of two experimental procedures, i.e. imposing various step frequency conditions vs. asking subjects to intentionally vary contact time at their freely chosen step frequency, it appears that changes in leg stiffness are mainly related to changes in contact time, rather than to those in step frequency. Step frequency appears to be an indirect factor influencing leg stiffness, through its effect on contact time, which could be considered a major determinant of this spring–mass characteristic of human running.