The current issue of Medicine and Science in Sports and Exercise (MSSE) has a study which indicates that the frontal plane force vector of the limb is directed more medially (toward the midline of the body) in those who have sufferred tibail stress fractures. The magnitude of forces in the control group were the same as those in the group that had sufferred injury. The conclusion of the authors was that the direction of the forces may be a contributor in the development of tibial stress fractures. This add a specific biomechanical risk factor for tibial stress fractures.
Within a narrow to medium range of normal this may be very significant. At greater deviations from normal, such as with severe genu valgum or varum (knock kneed or bow legged), my guess is that the forces will be too far away from the affected area to matter. So one day another study, with more extremes will have a different result than the current one. Study design and sample populations play a large role in study results.
Studies have already demonstrated the efficacy of the Pneumatic Walker in aiding and speeding healing of tibial stress fractures. Excessive pronation plays a role in overload of the medial muscles of the leg and also will shift the force vectors medially. Well designed custom foot orthotics should effectively shift the vector laterally and could be useful in preventing recurrence. No one factor alone though should be considered enough. Bone density, calcium and Vitamin D needs, and training must be evaluated.
Already, the warnings are in for avoiding too much, too soon and to allow your body, and your bones to adapt to the stresses you want to place them under. Increase your training slowly, gradually, and carefully and make certain to drop back every 2-3 weeks while increasing the training volume (and load).
External Frontal Plane Loads May Be Associated with Tibial Stress Fracture.
Medicine & Science in Sports & Exercise. 40(9):1669-1674, September 2008.
CREABY, MARK W. 1,2; DIXON, SHARON J. 2
Clinical Biomechanics Volume 19, Issue 1, January 2004, Pages 71-77
The role of selected extrinsic foot muscles during running
Kristian M. O’Connor and Joseph Hamill
Effect of Inverted Orthoses on Lower-Extremity Mechanics in Runners.
Medicine & Science in Sports & Exercise. 35(12):2060-2068, December 2003.
WILLIAMS , DORSEY S. III 1; MCCLAY DAVIS, IRENE 2 3; BAITCH, STEPHEN P. 4