Effect of load with dynamic contractions on contralateral homologous muscle performance

摘要

Fatigue is defined as a decrease in force and power production and is normally classified as being central or peripheral in origin. A reduction in neural output either from spinal or supraspinal levels characterizes central fatigue, while physiological changes at or distal to the neuromuscular junction characterizes peripheral fatigue. Additionally, peripheral fatigue induced in a specific muscle can result in the development of central fatigue and create in a non-local response. This non-local response is referred to as crossover fatigue and affects non-fatigued muscles.udResearch exploring crossover fatigue is in its infancy and appears to be conflicting. Several studies have not shown significant crossover fatigue effects while others support its existence. Despite all studies claiming that their subjects were brought to temporary exhaustion unilaterally, the presence of crossover fatigue phenomenon is not consistent. The inconsistent findings may be related to the differences in the unilateral fatiguing protocols, which include variations in parameters such as exercise intensity, volume and method of muscular contraction.udBased on the inconsistencies in the literature, the following experiment was conducted. Two different fatiguing protocols consisting of dynamic knee extension contractions were implemented to create localized fatigue in the ipsilateral knee extensors. Two different exercise intensities (40% and 70% MVC) were used and pre- and post-intervention measures (e.g. MVC, F100, EMG, median frequency and submaximal endurance performance) were recorded from the contralateral non-fatigued knee extensors. It was hypothesized that a higher intensity of unilateral dynamic exercise will lead to greater crossover fatigue effects, demonstrated by detriments in muscle strength and endurance performance.udIt was found that both 40% and 70% of MVC dynamic fatiguing protocols caused a decrease in subsequent MVC and instantaneous force with greater detriments from the 70% condition. Additionally, the voluntary force produced during the initial 10 seconds of the submaximal endurance test showed a moderate magnitude of variability in both the 40% and 70% conditions compared with the control condition thus, indicated neuromuscular function was impaired.