Reliability of the active drag assessment using an isotonic resisted sprint protocol in human swimming

The purpose of the presents study was to investigate the reliability of the active drag (D_a) assessment using the velocity perturbation method (VPM) with different external resisted forces. Eight male and eight female swimmers performed 25 m sprints with five isotonic loads (1-2-3-4-5 kg for females; 1-3-5-7-9 kg for males), which were repeated twice on different days. The mean velocity and semi-tethered force were computed for each condition, and the free-swimming maximum velocity was estimated with load-velocity profiling. From the obtained variables, D_a at the maximum free-swimming condition was calculated using VPM. Absolute and typical errors and the intra-class correlation (ICC) were calculated to assess test-retest reliability. 95% confidence interval (95% CI) lower bound of ICC was larger than 0.75 in 3, 4 (females only) and 5 kg trials in both sexes (corresponding to 37-60 N additional resistance; all p < 0.001), which also showed small absolute and relative typical errors (≤ 2.7 N and ≤ 4.4%). In both sexes, 1 kg load trial (16-17 N additional resistance) showed the lowest reliability (95% CI of ICC; - 0.25-0.83 in males and 0.07-0.94 in females). These results suggested that a tethered force of 37-60 N should be used to assess D_a using VPM.