Subjects reporting neck/shoulder pain have been shown to generate less force during maximal voluntary isometric contractions (MVC) of the shoulder muscles compared to healthy controls. This has been suggested to be caused by a pain-related decrease in voluntary activation (VA) rather than lack of muscle mass. The aim of the present study was to investigate VA of the trapezius muscle during MVCs in subjects with and without neck/shoulder pain by use of the twitch interpolation technique. Ten cases suffering from pain and ten age and gender matched, healthy controls were included in the study. Upper trapezius muscle thickness was measured using ultrasonography and pain intensity was measured on a 100mm visual analog scale (VAS). VA was calculated from five maximal muscle activation attempts. Superimposed stimuli were delivered to the accessory nerve at peak force and during a 2% MVC following the maximal contraction. Presented as mean±SD for cases and controls, respectively: VAS; 16.0±14.4mm and 2.1±4.1mm (P=0.004), MVC; 545±161N and 664±195N (P=0.016), upper trapezius muscle thickness; 10.9±1.9mm and 10.4±1.5mm (P=0.20), VA; 93.6±14.2% and 96.3±6.0% (P=0.29). In spite of significantly eight-fold higher pain intensity and ∼20% lower MVC for cases compared to controls, no difference was found in VA. Possible explanations for the reduction in MVC could be differences in co-activation of antagonists and synergists as well as muscle quality.