Razi Journal of Medical Sciences

  Background: The aim of this study was to investigate the effect of muscle fatigue on ankle joint moment and center of pressure during single-leg stance, perturbed by forward or backward platform perturbations.

  Methods : In this semi-experimental study fatigue induced to knee muscles by using an ergometer (monark). Surface elecromyographic signals were recorded from knee muscles during maximal voluntary contraction, sustained contraction and perturbation before and after muscle fatigue. Moreover, using force plate and video camera system, force and motion signals were simultaneously recorded during perturbations before and after muscle fatigue. To assess muscle reflex activity, the average rectified value of individual muscles was calculated over a fixed window, which was 180-ms after the onset of plate movement. Inverse dynamic method, ankle joint moment and center of pressure computed from kinetic and kinematics data after the onset of plate movement.

  Results : One way analysis of variance revealed that maximal knee extension contraction and muscle endurance (time to task failure) were significantly reduced after muscle fatigue (p< 0.05). Muscle reflex activities during post fatigue perturbations were significantly lower than the pre fatigue perturbations (p<0.05). A significant increase in ankle joint moments in the frontal and horizontal planes were also observed (p<0.05), which was accompanied by a significant increase in center of pressure in the frontal plane.

  Conclusion : The results of the current study demonstrated that muscle fatigue increases ankle joint moment and center of pressure in the frontal plane during single-leg stance, which in turn may increase the risk of lateral ankle injuries. These results partly explain that why lateral ankle sprains are common after heavy exercises.