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:Since, muscle fatigue has an effect on the ability of the spine to maintain stability, it would be of significant interest to determine whether a fatigued trunk system responds differently to the pre- and added loads that were described by Krajcarski et al (1999).
Category: