INTRODUCTION
The control of upright posture is achieved through the integration of a complex set of reflex and perceptual processes that ensure the maintenance and recovery of balance. Such processes depend on the uninterrupted flow of afferent impulses that originate in proprioceptive, vestibular, and visual structures. As a consequence, perception deficits may hinder postural control and influence balance. According to some studies, orthopedic injuries, vestibular disorders and fatiguing exercises4 can affect proprioception and therefore also postural control.
Changes in proprioception that occur in response to fatigue8 - 10may affect neuromuscular control and increase the incidence of injuries11. Disorders of the perceptual system can also increase the risk of ligament injury, especially around the knees and ankles given the relevant role of these joints in postural control and the maintenance of balance.
The ability of muscle to generate force and power decreases under fatigue conditions13, resulting in motor performance deficits and increasing oscillations during static posture. The effects of fatigue on balance can appear immediately after the performance of exercise and tend to decrease over time. Therefore, balance and the performance of functional tasks decrease following fatiguing exercise.
Balance depends on the ability of the neuromuscular system to react quickly to perturbations that introduce destabilizing forces, the effects of which are minimized by the adjustment of posture18. However, a reduction in the ability of the musculoskeletal system to respond quickly to the forces that act on the body and hinder its balance may affect perceptual and effector mechanisms and thus alter the response of the neuromuscular system, resulting in interference with body stability. According to certain studies, trunk and lower limb muscle fatigue in young adults is associated with a reduction in postural control .
Bisson et al. found no differences between two types of fatiguing protocols involving isometric and isokinetic contractions of the plantar flexor muscles, although both protocols altered the center of pressure (CoP) parameters. In addition, increases in the CoP excursion area, velocity, and variability were found in both the anteroposterior and mediolateral planes. Although the muscles affecting the ankles play a crucial role in the control of the quiet upright posture, the knee extensor muscles are proportionately much more greatly used and are therefore also more exposed to fatigue during the performance of activities of daily living and may exhibit responses different from those observed when fatiguing activities are imposed exclusively on the more distal muscles of the lower body. Therefore, the aim of the present study was to analyze the effect of muscle fatigue induced by resistance training on the knee extensor muscles as well as the stabilometric responses of young individuals in the single-leg stance.