The effects of two levels of muscular work on urinary creatinine excretion

It has been suggested by some investigators that the quantity of creatinine excreted in the urine is increased as a result of muscular work. The purpose of this study was to determine whether there is a substantial increase in urinary creatinine excretion (UCE) as a result of muscular work and specifically, to determine whether any increase is related to the intensity of the muscular work.

After an initial determination of their maximum oxygen uptake (max VO₂), six male subjects engaged in four separate tests, two on designated control days (no exercise) and the remaining two on designated exercise days. On the exercise days, the subjects ran on a treadmill for one hour at a percentage of their max VO₂ .On one of the exercise days, the subjects ran at 50% of their max VO₂ and on the other exercise day they ran at 70% of their max VO₂

The order of the four test days was randomly assigned to each subject, and all subjects were in the post-absorptive state until 3:00 P.M. on each test day. Urine samples were collected at specified times on each day and later analyzed for creatinine content. Values were obtained for the quantity of creatinine excreted per hour for all four test days.

The results of this study indicate that the hourly excretion rate of creatinine in the urine is highly variable not only among individuals but within individuals as well. It was also found that the quantity of creatinine excreted as a result of the 70% workload was significantly (p<.O1) decreased when compared to the UCE on the two control days and the 50% workload day. The quantity of UCE resulting from the 50% workload was not significantly different from the two control days.

The large variability in the hourly rates of UCE among and within individuals is of importance to investigators who have used this measurement in the determination of the excretion of other metabolites.