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Effects of Stride Length and Running Mileage on a Probabilistic Stress Fracture Model

EDWARDS, W. BRENT1; TAYLOR, DAVID2; RUDOLPHI, THOMAS J.3; GILLETTE, JASON C.1,; DERRICK, TIMOTHY R.1

Medicine & Science in Sports & Exercise: December 2009 - Volume 41 - Issue 12 - pp 2177-2184
doi: 10.1249/MSS.0b013e3181a984c4
Applied Sciences

ABSTRACT: The fatigue life of bone is inversely related to strain magnitude. Decreasing stride length is a potential mechanism of strain reduction during running. If stride length is decreased, the number of loading cycles will increase for a given mileage. It is unclear if increased loading cycles are detrimental to skeletal health despite reductions in strain.

Purpose: To determine the effects of stride length and running mileage on the probability of tibial stress fracture.

Methods: Ten male subjects ran overground at their preferred running velocity during two conditions: preferred stride length and 10% reduction in preferred stride length. Force platform and kinematic data were collected concurrently. A combination of experimental and musculoskeletal modeling techniques was used to determine joint contact forces acting on the distal tibia. Peak instantaneous joint contact forces served as inputs to a finite element model to estimate tibial strains during stance. Stress fracture probability for stride length conditions and three running mileages (3, 5, and 7 miles·d−1) were determined using a probabilistic model of bone damage, repair, and adaptation. Differences in stress fracture probability were compared between conditions using a 2 × 3 repeated-measures ANOVA.

Results: The main effects of stride length (P = 0.017) and running mileage (P = 0.001) were significant. Reducing stride length decreased the probability of stress fracture by 3% to 6%. Increasing running mileage increased the probability of stress fracture by 4% to 10%.

Conclusions: Results suggest that strain magnitude plays a more important role in stress fracture development than the total number of loading cycles. Runners wishing to decrease their probability for tibial stress fracture may benefit from a 10% reduction in stride length.

1Department of Kinesiology, Iowa State University, Ames, IA; 2Trinity Centre for Bioengineering, Department of Mechanical and Manufacturing Engineering, Trinity College Dublin, Dublin, IRELAND; and 3Department of Aerospace Engineering, Iowa State University, Ames, IA

Address for correspondence: William Brent Edwards, M.S., Department of Kinesiology, 283 Forker Building, Iowa State University, Ames, IA 50011-1160; E-mail: edwards9@iastate.edu.

Submitted for publication January 2009.

Accepted for publication April 2009.

©2009The American College of Sports Medicine