Unsupported to the oxygen uptake kinetics (V.O2k), the analysis of the physiological responses determining exercise tolerance in severe intensity domain are unable to be characterized, and the specificity of tethered swimming to reproduce similar physiological process inducing muscle to exhaustion cannot be established. Therefore, the current study aimed to compare free vs. tethered swimming with regard to the V.O2k at severe domain to verify the association between the mechanisms of oxidative response. 7 male swimmers performed two incremental tests, in free and tethered swimming. In free swimming, a continuous incremental protocol was designed with 8x100-m at from 60 to 100% of 200-m maximal velocity. In tethered-swimming, a pulley-load system graded workload (WL) was increased 5% every minute from 30 to 100% of maximum tethered force. The V.O2k described oxidative response during free and tethered swimming performed to exhaustion at delta 70%. A Wilcoxon test was used to compared free vs. tethered swimming at delta 70% regarding differences on TD, time constant (tau) and V.O2 amplitude (A1') for primary response, V.O2 slow-component (V.O2SC), V.O2 end-exercise (EEV.O2), and time-limit (tLim). Differences were not significant when comparing free vs. tethered-swimming regarding V.O2peak (54.3±6.0 vs. 52.5±5.0 ml kg-1 min1, p=0.18), neither were free vs. tetheredswimming in V.O2 response (A1': 3665±293.7 vs. 3544.4±579.8 ml kg-1; V.O2SC: 143.6±90.8 vs. 350.0±262.1 ml kg-1; and EEV.O2: 3808.9±314.9 vs. 3894.3±342.2 ml kg-1), or the transients (TD: 12.2±9.5 vs. 9.4±4.4 s; and tau: 36.0±14.6 vs. 25.6±12.7 s) and tolerance (tLim: 322.3±47.3 vs. 334.71±142.8 s) at delta 70% (all with p>0.05). The free vs. tethered-swimming did not differ with respect to the initial muscle target oxidative demand. Also, both conditions showed analogous profiles for V.O2 adjustments during performance (V.O2SC) and final elevation (EEV.O2), demonstrating similarities regarding muscle physiological processes. Free vs. tethered-swimming showed similar temporal profiles of O2 dynamics and from lung-tomuscle in concert with the onset of exercise (i.e., similar TD and tau), which probably even supports the similar tLim. This might be evidence of a similar
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