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:Bimodality has been described in the propulsion literature with the first peak, termed the impact spike, caused by the user forcefully contacting the pushrim at the beginning of the push phase (Robertson et al., 1996; Cooper et al., 1997).
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