Transitioning runners from a rearfoot strike (RFS) to a non-rearfoot strike (NRFS) pattern has been shown to influence several factors related to running injuries, to include a reducing average vertical loading rate . However there are likely other factors influencing running related injuries, one potential element being total impulse.
The purpose of this study was to analyze the effect of a transitional NRFS running program on impulses per unit distance. We hypothesized that impulse per unit distance will remain constant following a running transition program.
Twenty-seven runners recovering from lower extremity injuries at the US Military Academy were recruited to participate in this study after providing IRB approved consent (14 males, 13 females; age: 25.3±9.9 years; height: 172.1±10.3 cm; mass 73.0±12.0 kg).
Participants met their study assigned medical provider once a week post-training for the first 4 weeks, then at weeks 6, 8, and 10. Initial (RFS) and 10 week (NRFS) assessments were conducted before and after the training program. Assessments consisted of subjects running on an instrumented treadmill, collecting ground reaction forces at 1000 Hz [Bertec; Columbus, OH], at a self-selected pace for five minutes. Speed was kept constant for both evaluations.
Impulse was calculated for five stance phases on each foot, normalized by bodyweight, in the vertical, anterior, posterior, medial, and lateral directions. Impulse per distance was found by multiplying average impulse by cadence and divided by self-selected treadmill speed for each subject and multiplied by 1000 meters to yield BW-s/km.
Paired t-tests with a significance level of alpha=0.05 were used to identify differences between the initial and 10 week.
All subjects successfully transitioned from a RFS to NRFS pattern and cadence significantly increased from 168.7±8.2 to 173.6±7.5 steps/min (p<0.01). Vertical (p=0.38) and medial (p=0.30) impulses were not significantly different from the initial RFS assessment to the subsequent NFRS assessment, 352.4±41.5 vs 355.2±41.6 BW-s/km and 9.8±3.0 vs 9.2±2.5 BW-s/km respectively. Contrary to our hypothesis, significant changes in impulse per kilometer were found in anterior (p<0.01), posterior (p<0.001), and lateral directions (p<0.01) (Figure 1). Anterior and posterior impulses per kilometer increased by 12.2% and 12.0%, respectively, while lateral impulse increased by 77.5%.
The effects of total impulses on long term training and injury risk remain unknown. Our results also indicate that braking, propulsive, and lateral impulses increase per unit distance following a foot strike transition program. Additional research is required to determine the effects of a higher cadence and altered impulses on injury risk.
1. Van Gent RN, et al., Br J Sports Med 41, 469-480, 2007.