Background: Stress fractures are common overuse injuries in runners and appear most frequently in the metatarsals.
Purpose: To investigate fatigue-related changes in surface electromyographic activity patterns and plantar pressure patterns during treadmill running as potential causative factors for metatarsal stress fractures.
Study design: Prospective cohort study with repeated measurements.
Methods: Thirty experienced runners volunteered to participate in a maximally exhaustive run above the anaerobic threshold. Surface electromyographic activity was monitored for 14 muscles, and plantar pressures were measured using an in-shoe monitoring system. Fatigue was documented with blood lactate measurements.
Results: The results demonstrated an increased maximal force (5%, P < .01), peak pressure (12%, P < .001), and impulse (9%, P < .01) under the second and third metatarsal head and under the medial midfoot (force = 7%, P < .05; pressure = 6%, P < .05; impulse = 17%, P < .01) toward the end of the fatiguing run. Contact area and contact time were only slightly affected. The mean electromyographic activity was significantly reduced in the medial gastrocnemius (-9%, P < .01), lateral gastrocnemius (-12%, P < .01), and soleus (-9%, P < .001) muscles.
Conclusion: The demonstrated alteration of the rollover process with an increased forefoot loading may help to explain the incidence of stress fractures of the metatarsals under fatiguing loading conditions.