Duty factor is a more prominent modulator of external vertical loading than step frequency during running: a within-subject experimental study

Peak vertical ground reaction force (F_v,max) and vertical average loading rate (VALR) are commonly used to characterize external vertical loading during running. Although increasing step frequency (SF) is frequently used to modify these measures, the observed effects may partly reflect concurrent changes in duty factor (DF). This study determined the independent effects of manipulating DF and SF on F_v,max and VALR during level treadmill running. Fifteen healthy runners completed a preferred-condition trial and eight randomized 5-min trials at 11 km/h. DF was manipulated by ± 16% and ± 8% while SF was maintained at its preferred value, and SF was manipulated by ± 12% and ± 6% while DF was controlled. Preferred DF was strongly and inversely associated with F_v,max (R² = 0.81, p < 0.001) but not with VALR (R² = 0.21, p = 0.09). Preferred SF was not associated with either loading measure (p ≥ 0.70). DF manipulation affected F_v,max and VALR (both p ≤ 0.001), whereas SF manipulation affected F_v,max (p < 0.001), driven by the - 12% condition, but not VALR (p = 0.11). Normalized within-participant slopes were more negative for DF than SF for F_v,max (-1.03 ± 0.20 vs 0.07 ± 0.20% per 1% relative change; p < 0.001) and VALR (-0.97 ± 0.94 vs 0.32 ± 1.11%; p = 0.02). During level treadmill running at the submaximal speed investigated, DF produced more consistent and quantitatively larger changes in external vertical loading than SF when the other temporal variable was controlled. These findings identify DF as a more prominent temporal determinant of external vertical loading than SF under the conditions investigated and demonstrate, at both the between- and within-subject levels, the importance of DF in modulating vertical loading.