Dear Janusz

The assessment of diaphragmatic function could be done in several ways. When you plan to evaluate the effectiveness of breathing exercises, the purpose of this intervention should be clear for you: whether you are planning to improve the strength of the diaphragm or to improve its dynamic quality of function (for example - breathing pattern which will support lumbar & pelvic stability).

For strength assessment you may use Maximal Inspiratory Pressure measurements (but you can't distinguish between diaphragmatic, intercostal or accessory muscle activity; it gives you a "total" inspiratory strength). McConnell Alison -  does a huge research on strength evaluation of inspiratory muscles in health and disease.  

For assessing dynamic qualities of diaphragmatic function - more qualitative measurements, like chest expansion with measuring tape in upper-middle-lower zones of chest could be made (Mary Massery has published artices regarding diaphragmatic breathing patterns to support movement & posture)

So I suppose once the objectives of your exercises are clear for you- you could easily find the better strategy to assess your intervention. 

Dear Irena

First of all, thank you for answer. The topic of research is impact of specific exercises with non respiratory maneuvers to diaphragm strength. In a numerous studies, transdiaphragmatic pressure (Pdi) is the basis for assessment of diaphragmatic function. My question refers how to calculate Pdi and other diaphragm parameters.

Kind Regards

Janusz

hope this can help but there are several other and more recent articles on this topic

J Appl Physiol (1985). 1985 May;58(5):1469-76.

Assessment of transdiaphragmatic pressure in humans.

Laporta D, Grassino A.

Abstract

Maximal force developed by the diaphragm at functional residual capacity is a useful index to establish muscle weakness; however, great disparity in its reproducibility can be observed among reports in the literature. We evaluated five maneuvers to measure maximal transdiaphragmatic pressure (Pdimax) in order to establish best reproducibility and value. Thirty-five naïve subjects, including 10 normal subjects (group 1), 12 patients with chronic obstructive pulmonary disease (group 2), and 13 patients with restrictive pulmonary disease (group 3), were studied. Each subject performed five separate maneuvers in random order that were repeated until reproducible values were obtained. The maneuvers were Mueller with (A) and without mouthpiece (B), abdominal expulsive effort with open glottis (C), two-step (maneuver C combined with Mueller effort) (D), and feedback [two-step with visual feedback of pleural (Ppl) and abdominal (Pab) pressure] (E). The greatest reproducible Pdimax values were obtained with maneuver E (P less than 0.01) (group 1: 180 +/- 14 cmH2O). The second best maneuvers were A, B, and D (group 1: 154 +/- 25 cmH2O). Maneuver C produced the lowest values. For all maneuvers, group 1 produced higher values than groups 2 and 3 (P less than 0.001), which were similar. The Ppl to Pdi ratio was 0.6 in maneuvers A and B, 0.4 in D and E, and 0.2 in C. We conclude that visual feedback of Ppl and Pab helped the subjects to elicit maximal diaphragmatic effort in a reproducible fashion. It is likely that the great variability of values in Pdimax previously reported are the result of inadequate techniques.

Strictly in spirometry, regarding the flow-volume curve, the strength of diaphragm influences only the ascending part of the curve, when the flow-limiting mechanism does not work yet. For experimental results and analysis see e.g.: R.E. Hyatt et al., J. Appl. Physiol. (1958) 13: 331-336; A.G. Polak, Comp. Biol. Med. (1998) 28: 613-625.

Try to access: 

Am J Respir Crit Care Med Vol 166. pp 518–624, 2002

https://www.thoracic.org/statements/resources/pft/respmuscle.pdf