Myasthenia gravis (MG) is characterized by fatigue and fluctuating muscle weakness resulting from impaired neuromuscular transmission (NMT). The objective of this study was to quantify, by direct measurement of muscle force, the strength and fatigue of patients with MG. A maximal voluntary isometric contraction protocol of shoulder abductors was used in conjunction with conventional fatigue and disease-severity instruments. Results from patients with (D-MG) and without (ND-MG) decrement on repetitive nerve stimulation (RNS) of the spinal accessory and axillary nerves were compared with healthy controls. Patients with MG reported greater fatigue than controls. Muscle strength was lowest in the D-MG group, followed by the ND-MG group and controls. Normalized shoulder abduction fatigue and recovery values did not differ between the D-MG and ND-MG groups or controls. The RNS decrement appears to relate best to disease severity and muscle weakness but not to objective measures of fatigue in this population.