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 type I–selective nature of PPARa activation of transcription in skeletal muscle is particularly interesting given that statin-induced muscle degeneration occurs preferentially in type II skeletal muscle fibers with a reduced effect observed in type I fibers (Reijneveld et al., 1996; Smith et al., 1991; Waclawik et al., 1993; Westwood et al., 2005).
Category: