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:Optimization
 We used three objective functions that were employed previously to study multi-finger prehension in human performers (Hershkovitz et al. 1995, 1997; Zatsiorsky et al. 2002b; Pataky et al. 2004a, b).
Category: