Bone stress injuries (BSIs) are very common among collegiate cross country runners, occurring at a rate of 16 and 29 BSI per 100 000 athletic exposures for men and women, respectively. The average recovery time following a BSI in this population is 13 weeks. Therefore, identification of risk factors for BSI is vital fordeveloping injury prevention programmes, mitigating injury risk and maintaining athlete health.
Proposed risk factors for BSI include both biological and biomechanical risk factors. While biological contributions are well established, biomechanical contributions are less clear.
Running biomechanics are thought to influence BSI risk, however, minimal consensus exists regarding which specific characteristics contribute to this risk. Difference in vertical loading rate between those with and without prior BSI, spatiotemporal and kinematic metrics have shown inconsistent associations with BSI.
Given the repetitive bone loads associated with distance running, bone mineral density (BMD) has also been studied in relation to BSI, with mixed results.
Varied study designs may contribute to the mixed relationships found between running biomechanics, BMD and BSI. With limited prospective studies, it is difficult to discern if differences in running mechanics and BMD between groups are a causal factor of BSIs or rather a compensation developed following a BSI.
The purpose of this study was to determine if running biomechanics and BMD were prospectively associatedwith BSI occurrence in NCAA Division I cross country runners.
The study used 3 years of health and performance data routinely collected from 54 NCAA Division I crosscountry runners. Women (n=33) comprised 61% of the sample.
Running assessments followed a standardised testing protocol, with athletes running at their preferred speed. Data were recorded for 15 s. Whole body kinematics were collected using 42 reflective markers placed on the body segments, 23 were located on anatomical landmarks. Kinematic data from the running trials were recorded at 200 Hz using an 8-camera passive marker system. Three-dimensional GRF was synchronously recorded at 2000 Hz using an instrumented treadmill. A static standing position was also recorded to establishjoint centres and model scaling.
Spatiotemporal biomechanical variables of interest included preferred running speed (m/s) and step rate (steps/min). Kinematic variables included foot inclination angle (FIA) with respect to the ground at initial contact, normalised to standing posture (°); horizontal distance from centre of mass (COM) to heel marker (cm); COM vertical excursion over a gait cycle (cm); peak hip adduction during stance (°) and base of gait (BOG) at midstance (cm). GRF variables included peak VGRF (N/kg); impact peak (N/kg); VGRF impulse (Ns/kg); average vertical loading rate (N/kg/s) and braking impulse (Ns/kg). These variables are commonly used to assess an injured runner’s mechanics and are targets of gait retraining.
During annual preseason assessments, running biomechanics and dual X-ray absorptiometry (DXA) images were obtained on all athletes. A GE Healthcare Lunar iDXA densitometer was used for all examinations.Extracted variables from DXA images included lean mass, total leg BMD, total body bone mineral content (BMC) and BMD and total z-score BMD.
The primary outcome was BSI occurrence during the 12-month calendar year. A BSI was defined as a stress fracture or reaction confirmed via MRI by the presence of periosteal, marrow and/or cortical oedema. Generalised estimating equations were used to identify independent risk factors of BSI.
Thirty-two BSIs were recorded on 24 athletes (44.4%) over the study period. The number of athletessustaining a BSI per year ranged from 30% to 32%.
Univariably, BSI risk decreased by 4% (RR 0.96, 95% CI 0.92 to 0.99) with each step/min increase in step rate. Among kinematic variables, only COM vertical excursion was associated with BSI. The risk of BSI increased by 17% (RR=1.17, 95% CI 1.04 to 1.31) for each 0.5 cm increase in COM vertical excursion(p=0.01).
Low step rate was identified as a predictor of BSI in a multivariable model after adjusting for known BSI risk factors (history of BSI and sex). Step rate was the only variable significantly associated with BSI risk (p=0.008). A 1 step/min increase in step rate was associated with a 5% decreased risk of BSI (RR: 0.95; 95% CI 0.91 to 0.98).
No GRF variables were univariably associated with BSI.
Although not statistically significant, BMD z-score was an important covariate in the best-fitting model; the 95% CI suggests that larger BMD z-scores may be indicative of decreased BSI risk (RR: 0.93, 95% CI 0.85 to 1.03, unit=0.5).
To conclude, low step rate was identified as an independent risk factor, while BMD z-score was also determined to potentially influence BSI risk. Thus, monitoring step rate, along with BMD and its related factors, may be worthwhile as part of a comprehensive programme to treat and prevent BSIs in collegiate runners.
Kliethermes SA et al. (2021) Lower step rate is associated with a higher risk of bone stress injury: a prospective study of collegiate cross country runners. Br J Sports Med.
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