More days of either aerobic or strength exercise were associated with higher levels of creatinine in both men and women. As creatinine is a metabolic product of creatine breakdown, active individuals and those with higher muscle mass would be expected to have higher creatine turnover resulting in higher serum creatinine levels. Prior research has similarly shown higher serum creatinine in athletic populations , particularly in sports involving strength and power . As our results and prior research have shown , reference intervals for creatinine for the general healthy population may not be appropriate for the active population.