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:In a study that determined the response of the spine to sudden loads causing sagittal flexion, it was found that abdominal muscles rapidly responded to the sudden load with a large increase in muscle activation (Krajcarski et al., 1999). Also, Krajcarski et al. concluded that increases in EMG amplitudes of abdominal and trunk extensor muscles were determined predominantly by the added load magnitude and not the pre-loads. Chiang (in press) conducted a similar study with the difference being the trunk motion investigated was lateral bend instead of trunk flexion. Similar to Krajcarski et al. (1999), Chiang (in press) concluded that increased muscular activity due to pre-loads leads to increases in trunk muscle stiffness and spine stability.
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