Leg Stiffness Features In Runners With Single And Multiple Stress Fracture Histories

The relationship of leg stiffness on running related injury patterns is not clear, especially among runners with bony injuries. PURPOSE: To determine differences in leg stiffness values and key biomechanical features between runners with recent history of single or multiple lower body stress fractures. METHODS: This was a cross-sectional study design of community runners (31.4 ± 13.9 yrs; 50.2% female; body mass index 22.4 ± 9.3 kg/m2; 9.1 ± 9.5 yrs experience) without injury (n = 212) and who were cleared to run after lower-body running-related stress fractures at one site (n = 75) or multiple sites (n = 20). Fractures were confirmed radiographically in locations from pelvis to the foot. All runners completed running history forms and performed a 3D gait analysis. Kinematic data were collected at 120 Hz using a motion capture system and normalized to a gait cycle, while participants ran on level grade at self-selected speed on an instrumented treadmill. Ground reaction forces (GRF) were collected at 1200 Hz. Vertical average loading rate (VALR) and vertical stiffness (Kvert = ΔGRF/ Δcenter of mass vertical displacement) were calculated. Running speed was similar across groups (10.4 km/r). RESULTS: After adjusting statistical models for age, speed and body weight, both single and multisite fracture groups had lower Kvert values compared to healthy runners (162 ± 39 N/cm and 167 ± 47 N/cm versus 183 ± 35 Ncm, respectively; p < .001). Pelvic anterior tilt (in degrees) at foot strike was highest in multisite fracture group compared to single site and healthy runners (19.2 ± 6.3 versus 16.0 ± 5.3 17.2 ± 5.5; respectively; p = .03) in the no injury group, but pelvis transverse rotation excursion (in degrees) was least (11.6 ± 3.5 versus 13.9 ± 4.2 and 14.0 ± 3.5; p = .004). Knee flexion excursion was greatest in the multisite fracture group (p = .028). Peak GRF, VALR and temporal spatial parameters were not different among groups. CONCLUSIONS: Two interpretations of our findings are that: 1) Kvert may be involved in the onset of a lower body stress fracture injury, 2) Increased reliance knee flexion with less pelvis rotation combined with lower Kvert may be functionally protective response to previous injury. Electromyography of leg, hip and core musculature during running in healthy and recovering runners could provide insight to this issue.