Effect of Body Weight Variation on Swimming Exercise Workload in Rats With Constant and Size-Adjusted Loads

In swimming animal models, weights are added according to some percentage of body weight (%BW) or as a constant load (CL) to equalize the workload of each animal or to reduce the time in swimming-to-exhaustion endurance tests. The objective of the present study was to evaluate the effect of body weight variation on swimming exercise workload through the reliability analysis of swimming-to-exhaustion endurance tests. We examined the reliability by comparing the mean time to exhaustion (TEx) in trials performed on the 30(th), 60(th), 90(th) 120(th) and 150(th) days of life of Wistar rats using three %BW and CL workloads (4%, 6% and 8% and 7 g, 11 g and 15 g, respectively). We also examined the within-subject variation of TEx over three trials of a CL test (15 g) within one week (when variability in body weight is minimal). The rats' body density was maintained during growth (mean (SD) 1.031 (0.026) g/ml - 1.026 (0.005) g/ml) despite their significant increase in body weight (mean(SD) 109.05(13.8))) g - 442.92(29.39) g). Thus, the absolute loads in longitudinal %BW tests increased gradually, causing a decrease in TEx under all workloads. The CV confidence limits for TEx in CL tests showed high within subjects variation (17.1-111%) compared to the body weight variation (0.4-2.8%). We conclude that load adjustment based on %BW does not adequately equate to the workload between rats of different sizes. The methodology also showed high within-subject variation between trials (not related to body mass changes.) that compromises the significance of small effects.