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:Using simulations based on our PK model, we designed an infusion protocol of two consecutive constantrate infusions, the first of 125 mg kg 1 h 1 for 15 min followed by 25 mg kg 1 h 1 with the subjects at rest for 35 min, thereafter continuing the infusion during exercise until this phase was terminated by subject fatigue [2–4].
Category: