Increases in local pressure pain thresholds after muscle exertion in women with chronic shoulder pain

Objective: To examine the relation between muscular tenderness and local muscular fatigue. Design: Before-after trial, measuring pressure pain thresholds and signs of local muscular fatigue by using surface electromyography before, after, 10 minutes after, and 20 minutes after intervention. Setting: University rehabilitation department in Sweden. Participants: Nineteen female hospital cleaners with unilateral chronic shoulder pain for 1 year. No previous trauma, surgery, or signs of systemic or neurologic disease. Intervention: Static abduction endurance test with submaximal unilateral activation of the trapezius and deltoid muscles. Main Outcome Measures: Pressure pain thresholds and electromyographic fatigue parameters (root mean square [RMS]; mean power frequency [MPF]). Results: Mean normalized pressure pain threshold values increased significantly (95% confidence interval [CI]) and lasted for 20 minutes: in the trapezius, threshold values increased to 115% to 120% (95% CI, 100%-140%; P=.04-.05); and in the deltoid, the threshold increased to 112% to 115% (95% CI, 100%-130%; P=.02-.05). Development of significant electromyographically defined fatigue was seen: in the trapezius, the RMS increased to 130% (95% CI, 119%-141%), and in the deltoid, the MPF decreased to 78% (95% CI, 74%-82%), but normalized within 15 seconds to 10 minutes. Conclusion: Lasting bilateral increases in pressure pain thresholds but transient local muscular fatigue were seen after a unilateral static endurance test. The sensitivity of the sensory nervous system may change during a static muscle contraction and sometimes contributes to a localized sensation of numbness.