<p>Surface electromyography (SEMG) can objectively evaluate the electrical activity of muscles under a variety of functional conditions. The aim of this study was to use change-point analysis to assess variations in muscle activation levels in the shoulder region during maximal voluntary isometric contraction. Using SEMG, the muscle activity of the right and left upper trapezius of 30 healthy participants was recorded. Significant changes characterized by a decreased electromyographic amplitude during the muscle contraction were observed in 27 subjects. Varied time location of detected changes indicated that there was not a common onset of muscular activity reduction. The change-point method can be a useful tool for the measurement of changes during muscle contractions. However, further research is necessary.</p><p> </p>