The effects of standard issue Royal Marine recruit footwear on risk factors associated with third metatarsal stress fractures

Abstract

Purpose: The relatively high incidence rate of third metatarsal (MT3) stress fractures in Royal Marine (RM) recruits may be linked to the footwear worn during training. The present study investigated the effect of standard issue RM recruit footwear on biomechanical variables linked with MT3 stress fracture risk.

Methods: Seven male volunteers (age 18.3 ± 0.4 years, mass 81.1 ± 8.2 kg) ran at 3.6 m s⁻¹ in a laboratory while wearing a combat assault boot (CAB) and a neutral gym trainer (GT). In-shoe plantar pressure was assessed using pressure insoles (RSScan, 500 Hz). Two-dimensional ankle kinematics and kinetics were assessed at 120 Hz (Peak Motus). Horizontal ground reaction force characteristics were investigated using an AMTI force plate (960 Hz).

Results: Peak plantar pressure, impulse and loading rate were significantly greater at the MT3 head in the CAB (P < 0.05). Further significant differences with the CAB were a smaller and earlier peak ankle dorsiflexion, a later heel-off, and greater magnitudes of peak plantarflexion moment and ankle joint stiffness (P < 0.05). At the instant of peak horizontal braking force, the resultant horizontal force vector was applied significantly more laterally in the CAB than the GT.

Conclusions: The higher magnitude for several risk factors for MT3 stress fracture when wearing the CAB compared with the GT suggests that wearing the CAB contributes to the high incidence of MT3 stress fracture in RM recruits. Increased ankle stiffness was attributed to above-ankle support and increased eccentric muscular control in the CAB, contributing to the higher plantar loading at the MT3 head. The higher plantarflexion moment and associated increased muscular demand in the CAB may have implications for fatigue-related MT3 stress fracture mechanisms, although further research is required to support this.

Keywords:

• plantar pressure
• third metatarsal
• stress fractures
• military boots
• ankle kinematics
• ankle kinetics

Acknowledgements

The authors would like to thank Dr. Matt Carre and Dr. James Clarke, from the Sports Engineering Resarch Group, Department of Mechanical Engineering, University of Sheffield, UK, for carrying out mechanical stiffness testing of the study footwear.