I found this information in “UpToDate” which is an online medical textbook:
Comparing measures — The vital capacity (VC) and the forced vital capacity (FVC) are widely used alternatives to the MIP and MEP that are used to follow the disease course. The difference between the VC and the FVC should be clearly understood. The VC is the maximum volume of gas that can be expelled from full inspiration. The FVC is also the maximum volume of gas that can be expelled from full inspiration, but it is measured when the patient is exhaling with maximal speed and effort. The VC is usually higher than the FVC, with the difference being directly related to the degree of obstruction.
Direct comparison of the MIP and FVC demonstrates the following differences:
●When measured in the upright position, a decreased FVC is less specific for respiratory muscle strength (it can also be decreased in interstitial lung disease, chest wall disease, and other conditions) and less sensitive for changes in inspiratory muscle strength, compared to the MIP.
●Because relatively small pressures are required to fully inflate the normal lung the inspiratory muscle can be substantially weakened before the VC is much reduced. The VC falls late in progressive neuromuscular disease like ALS. In contrast, maximum pressure generation falls in line with disease progression.
●A fall in the FVC when the patient moves from the upright to the supine position indicates significant diaphragm weakness more reliably than a low MIP. Significant falls in VC when supine only occur when diaphragm strength is greatly reduced.
Comparison of the MIP and VC demonstrates similar sensitivity and specificity for detecting hypercapnic (high CO2) respiratory failure . Specifically, the MIP had a sensitivity and specificity of 55 and 83 percent, respectively, while the VC had a sensitivity and specificity of 53 and 89 percent, respectively.
Taken together, the data indicate that no single measure is best for assessing respiratory muscle weakness. In our practice, we make clinical decisions after measuring the MIP, SNIP, MEP, and FVC. We measure upright and supine FVC. When doubt persists about whether a patient has respiratory muscle weakness, we perform more complex tests.
(They also note that MIP and MEP are highly impacted by patient effort and ability to form a tight seal on the mouthpiece.)