Weakening of the respiratory muscles in patients with neuromuscular diseases (NMDs) threatens them with respiratory failure. The respiratory capacity of these patients is estimated by pulmonary functional tests. One of the most frequently used tests is the measurement of vital capacity (VC) and determination of the percentage share of the VC reference value (%VC). Different body parameters such as body height and body weight are used for determining the %VC. It is difficult to measure these parameters in wheelchair-bound NMD patients, especially those in more advanced stages of disease. Another reason for questioning the objectivity of spirometric testing in NMD patients is that many of them manifest severely weakened facial muscles and consequently have trouble with blowing. This article describes alternative testing of respiratory capacity and calculation (mathematical model) of predicted pulmonary functional values that is not based on the body height but on other more objectively measured anthropometrical parameters.