Muscular endurance and electromyographic fatigue in myofascial shoulder pain

An endurance test in shoulder flexion was made in 10 patients (median age 30 yr) with a history of shoulder-neck pain of at least one year's duration. No signs of inflammatory, rheumatic, or neuromuscular diseases were present among the patients but there was localized muscular pain and tenderness in the shoulder-neck region. In a sitting position the patients performed an endurance test of a 30 degree isometric shoulder flexion on both the left and right sides. EMG was recorded from the descending part of the trapezius muscle by surface electrodes and from the supraspinatus muscle by intramuscular wire electrodes. Myoelectric amplitude increase and mean power frequency (MPF) decrease with time during the endurance test were used as measures of fatigue development. All patients had a shorter duration of endurance on the more painful side. The endurance time was short in relation to the load. The regression between endurance time and MPF decrease was similar to that for healthy subjects. The short endurance time for the patients appears to be an effect of increased physiologic fatigability of the muscle which may be due to a change in its work capacity.