The influence of muscle fatigue on electromyogram and plantar pressure patterns as an explanation for the incidence of metatarsal stress fractures.

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.