START-TO-LOW-RUN: The effect of a 10 week instructed running program on duty factor to reduce loading magnitude

This study aimed to evaluate the effectiveness of a 10-week start-to-run program using auditive instructions to increase duty factor (DF), defined as contact time divided by stride time. Forty novice runners were assigned to either an intervention or a control group and completed a 10-week start-to-run program. The intervention group received instructions to reduce vertical oscillation (‘go less up and down’), while the control group received a sham instruction. DF and maximal vertical ground reaction force (FzMax) were measured before, immediately after, and 16 weeks post-program while running overground at 2.36 m · s−1. To assess recall effects, instructions were repeated at the post- and retention testing sessions. Both groups showed a significant increase in DF after the program (intervention: +1.524%, p < 0.001; control: +1.025%, p = 0.041). However, there were no significant differences between groups, and gains were not retained after 16 weeks. After the instruction was reiterated, only the intervention group again showed a significant DF increase (post: +1.354%, p < 0.001; retention: +0.903%, p = 0.001) and a decrease in FzMax (post: −0.063 BW, p = 0.001; retention: −0.045 BW, p = 0.040). Overall, DF improved post-intervention, but without between-group differences or long-term retention.