Impact Mechanics in Female Runners with Single and Multiple Stress Fractures Following Fatigue

Abstract

Stress fractures are common injuries in runners and military recruits, with females at greater risk than their male counterparts. Impact variables including peak axial tibial shock and vertical average load rate during running have been shown to be higher in females with a history of tibial stress fractures and increase with fatigue. However, the relationship between mechanics and injury in those with multiple lower extremity stress fractures has not been examined. PURPOSE: To investigate whether impacts increase with fatigue in runners with no history of stress fractures (CON), one (1SFX), and 3 or more (3SFX) stress fractures. METHODS: Impact variables were calculated for 43 females (14 CON, 14 1SFX, and 15 3SFX) at a speed of 2.67 m/s before and after a fatigue run. Variables included peak axial and resultant tibial shock (VTA, RTA) and vertical average and instantaneous loading rates (VALR, VILR). The fatigue run was performed at a 5 km predicted pace and stopped when a RPE reached ≥ 18. Absolute change comparisons were made using one-way ANOVAs and post-hoc tests. RESULTS: 3SFX had larger changes (i.e. increased values) after fatigue than CON or 1SFX for all variables (Table 1), although after post-hoc tests only VILR for 3SFX was statistically higher than 1SFX (p<0.05), with VALR, VTA, and RTA not reaching significance. Additionally, subjects with the largest changes in these variables exhibited a change in their foot strike towards a more posterior strike pattern. CONCLUSION: Females with multiple stress fractures showed larger changes in load rates after fatigue, compared to those with history of one stress fracture. Change in foot strike pattern greatly influences impact mechanics before and after fatigue. Supported by U.S. Department of Defense, Defense Health Program, and Joint Program Committee W81XWH-16-1-0652 The views expressed are those of the authors and do not reflect the official policy of the Department of Army, Department of Defense, or the U.S. Government