The present study investigated the effect of chronic neck muscle pain (defined as trapezius myalgia) on neck/shoulder muscle function during concentric, eccentric and static contraction. Forty-two female office workers with trapezius myalgia (MYA) and 20 healthy matched controls (CON) participated. Isokinetic (-60, 60 and 180 degrees s(-1)) and static maximal voluntary shoulder abductions were performed in a Biodex dynamometer, and electromyography (EMG) obtained in the trapezius and deltoideus muscles. Muscle thickness in the trapezius was measured with ultrasound. Pain and perceived exertion were registered before and after the dynamometer test. The main findings were that shoulder abduction torque (at -60 and 60 degrees s(-1)) and trapezius EMG amplitude (at -60, 0 and 60 degrees s(-1)) were significantly lower in MYA compared with CON (p<0.001-0.05). Deltoideus EMG and trapezius muscle thickness were not significantly different between the groups. While perceived exertion increased in both groups in response to the test (p<0.0001), pain increased in MYA only (p<0.0001). In conclusion, having trapezius myalgia was associated with decreased strength capacity and lowered activity of the painful trapezius muscle. The most consistent differences-in terms of both torque and EMG-were found during slow concentric and eccentric contractions. Activity of the synergistic pain free deltoideus muscle was not significantly lower, indicating specific inhibitory feedback of the painful trapezius muscle only. Parallel increase in pain and perceived exertion among MYA were observed in response to the maximal contractions, emphasizing that heavy physical exertion provokes pain increase only in conditions of myalgia.