Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…breathing test during which subjects were required to breathe at approximately 60% MVV using tidal volumes that
were about 50% of VC at a breathing frequency of approximately 30 breaths/min until exhaustion (defined as inability to maintain the target ventilation) (Boutellier et al. 1992).
Category: