  This study introduces a novel muscle activation analysis based on surface
electromyography (sEMG) signals to assess the muscle's after-fatigue condition.
Previous studies have mainly focused on the before-fatigue and fatigue
conditions. However, a comprehensive analysis of the after-fatigue condition
has been overlooked. The proposed method analyzes muscle fatigue indicators at
various maximal voluntary contraction (MVC) levels to compare the
before-fatigue, fatigue, and after-fatigue conditions using amplitude-based,
spectral-based, and muscle fiber conduction velocity (CV) parameters. In
addition, the contraction time of each MVC level is also analyzed with the same
indicators. The results show that in the after-fatigue condition, the muscle
activation changes significantly in the ways such as higher CV, power spectral
density shifting to the right, and longer contraction time until exhaustion
compared to the before-fatigue and fatigue conditions. The results can provide
a comprehensive and objective evaluation of muscle fatigue and recovery, which
can be helpful in clinical diagnosis, rehabilitation, and sports performance.
