Acute effects of fatigue of the plantarflexor muscles on different postural tasks.

Although the effect of muscle fatigue on posture is apparent, results are inconsistent across studies and this may be due to differences in the fatigue protocol, postural stance used and/or amount of visual information provided. The purpose of this study was to test the hypothesis that fatigue would have a more pronounced effect for more difficult compared with less difficult postural tasks. Twenty-three young adults (mean age 22 ± 3 years) were asked to perform three different postural tasks on a force-platform: feet together, semi-tandem and single-leg stance, before and immediately after fatiguing the plantarflexor muscles. Three 30-s trials were performed for each postural task with eyes open and eyes closed. The fatigue protocol consisted of an isometric contraction of the plantarflexor muscles, with subjects instructed to rise on their toes and maintain this position until exhaustion. The 95% ellipse area of the center of pressure (COP), as well as the COP sway amplitudes (standard deviation) and sway velocities in the antero-posterior and medio-lateral directions were calculated. All variables were greater for each postural task performed without vision compared to with vision, with the greatest difference found during the single-leg task. For both visual conditions, all variables increased as the difficulty of the task increased. Fatigue of the plantarflexor muscles mostly affected postural sway variables during single-leg and feet together tasks. In conclusion, the reduction in postural stability with muscle fatigue of the plantarflexors does not depend on the difficulty of the postural task.