Differences in the principal strain angles during activities performed on natural hilly terrain versus engineered surfaces.

Background: Tibial stress fractures in military recruits occur beginning with the fourth week of training. In and ex vivo tibial strain experiments indicate that the repetitive mechanical loading during this time may not alone be sufficient to cause stress fracture. This has led to the hypothesis that the development of tibial stress fracture is mediated by the bone remodeling response to high repetitive strains. This study assesses the differences in the strain and angle of the principal strain during military field activities versus common civilian activities.

Methods: In vivo strain measurements were made from a rosette strain gauge bonded to the midshaft of the medial tibia. Measurements of principal strains and their angles were made while performing level and inclined walking and running on an asphalt surface, while fast walking up and down stairs, while performing a standing vertical jump and while zig-zag running up and down a 30° inclined dirt hill.

Findings: The angle of the principal strain varied little (5.40° to +2.74°) during activities performed on engineered surfaces. During zig-zag running on a dirt hill the strain levels were higher (maximum shear = 4290 με). At the pivot points of zig-zag running the angle of the principal strain varied between -115° to -123° downhill and between -32.8° to -51° uphill.

Interpretation: Activities that mimic those performed by infantry recruits on irregular hilly surfaces result in higher tibial strains and have more variation in principal strain angles than activities of ordinary civilian life performed on engineered surfaces.