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:The phase 1, or cardiodynamic phase (Whipp et al. 1982), is followed by a primary and rapid exponential process (phase 2), with the third phase characterised by a delayed rise in _ VO2 (slow component) which causes _ VO2 to increase toward an elevated steady state, or else it is truncated by the achievement of peak _ VO2 (Gaesser and Poole 1996).
Category: