Isokinetic muscle testing (IMT) allows precise and reliable measurement of the force produced by the skeletal muscle during exercise at constant velocity and accommodating resistance. This study reports on some clinical situations that illustrate the difference between manual muscle testing (MMT) and IMT in neuromuscular patients. IMT was performed by a special method (continuous passive motion plus gravity compensation) which allowed the measurement of very weak forces, such as in the four patients described in this study. It is important to note that for the same MMT grading the corresponding isokinetic force values were very different. Therefore there is an obvious correspondence between the isokinetic measurement of muscle strength and the morphological change in the muscle on the CT scan of the thigh, which did not show up on MMT. IMT could be extremely important for research into neuromuscular disorders, where the detection of even the tiniest variations in strength is relevant.