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:Muscle enzymes (lactic dehydrogenase or LDH and creatine kinase or CK) were measured by enzymatic assays (Olympus 640 kits and equipment, Olympus Diagnostica, Hamburg, Germany) only in the marathon protocol, since they were previously found to be unchanged after short-lived exercise (22).
Category: