Aim. This study investigates consequences of chronic neck pain on muscle function and the rehabilitating effects of contrasting interventions. Methods. Women with trapezius myalgia (MYA, n=42) and healthy controls (CON, n=20) participated in a case-control study. Subsequently MYA were randomized to 10 weeks of specific strength training (SST, n=18), general fitness training (GFT, n=16), or a reference group without physical training (REF, n=8). Participants performed tests of 100 consecutive cycles of 2 s isometric maximal voluntary contractions (MVC) of shoulder elevation followed by 2 s relaxation at baseline and 10-week follow-up. Results. In the case-control study, peak force, rate of force development, and rate of force relaxation as well as EMG amplitude were lower in MYA than CON throughout all 100 MVC. Muscle fiber capillarization was not significantly different between MYA and CON. In the intervention study, SST improved all force parameters significantly more than the two other groups, to levels comparable to that of CON. This was seen along with muscle fiber hypertrophy and increased capillarization. Conclusion. Women with trapezius myalgia have lower strength capacity during repetitive MVC of the trapezius muscle than healthy controls. High-intensity strength training effectively improves strength capacity during repetitive MVC of the painful trapezius muscle.