The effect of 4-week individualized rehabilitation on shoulder muscle strength and fatigability was evaluated in 10 patients with frozen shoulder syndrome (FSS) in comparison of 10 age- and gender-matched healthy control subjects. Isometric maximal voluntary contraction (MVC) force of the shoulder flexors was measured by hand-held dynamometer. Isometric endurance of the shoulder muscles was characterized by endurance time and net impulse (NI) assessed during weight (30% MVC) holding in hand until exhaustion. Fatigability of deltoideus, infraspinatus and trapezius muscles during isometric endurance test was assessed by electromyogram (EMG) power spectrum median frequency (MF) slope per minute. Rehabilitation in patients with FSS consisted of exercise therapy in swimming pool and gymnasium, electrical therapy and massage. Before rehabilitation, patients with FSS had less (p < 0.05) isometric MVC force and NI during endurance test compared to the control. MF slope in patients with FSS for involved extremity was higher (p <0.05) for the deltoideus muscle and less for the infraspinatus muscle before rehabilitation compared to the controls. Shoulder pain was decreased (p < 0.05) and isometric MVC force and NI in endurance test in patients for involved extremity were increased after rehabilitation. No significant changes in endurance time and MF slope for infraspinatus and trapezius muscles in patients for involved extremity were observed after rehabilitation, whereas MF slope for deltoideus muscle was increased. It was concluded that in patients with FSS, 4-week rehabilitation decreased shoulder pain and improved MVC force of the shoulder flexors and isometric working capacity of these muscles during endurance test.