It is advisable to perform MiPs & MEPs alng with lung volumes by plethysmography, not just spirometry.
This is from Chapter 11 of the ATS Manual:
To adjust for differences in body size and varied levels of lung capacity, caused by disease states, pressures can be presented as percent of predicted TLC at which they were measured.
To facilitate the above corrections, it is preferable to perform lung volume testing at the same time as the maximal respiratory pressures, using a body plethysmograph to obtain functional residual capacity (FRCpleth). This allows correlation of respiratory muscles forces with the degree of hyperinflation at which they were generated, which can be especially important with patients with obstructive lung disease.
Marcy Petrini, Ph.D. FCCP
Medical Director
Pulmonary Function Laboratory
University of Mississippi Medical Center