Fatigue-related loading imbalance on the shank in running: a possible factor in stress fractures

In previous reports we have shown that in long distance running the impact acceleration on the shank increases with progressing fatigue. The aim of th e present study was to test whether, in parallel to this increase, an imbal ance in the activities between the ankle plantar and dorsi flexor muscles d evelops. The tests were made on fourteen subjects during 30 min treadmill r unning above their anaerobic thresholds. Respiratory data were collected to determine the anaerobic threshold speed and to indicate the progressively developing metabolic fatigue. Surface electromyogram (EMG) was monitored to indicate the changing activity of the shank muscles. In the tibialis anter ior the average integrated EMG (iEMG) and the mean power frequency (MPF) si gnificantly decreased from the beginning to the end of running. In the gast rocnemius iEMG did not change, while MPF increased during the course of run ning. The impact acceleration, measured by means of an accelerometer attach ed to the tibial tuberosity, significantly increased during the course of r unning. It was concluded that, with developing fatigue, an imbalance in the contraction of the shank muscles develops in parallel to an increase in sh ank shock acceleration. The combination of these two changes may hamper the loading balance on the tibia since the bone becomes exposed to excessive b ending stresses and to higher risk of stress injury. (C) 2000 Biomedical En gineering Society.