Skeletal Muscle Resaturation Relates to Aerobic Fitness in Adults Participating in Strength and Aerobic Exercises

<b>Purpose</b>: This study examined differences in metrics of skeletal muscle re-saturation between strength and aerobically oriented individuals and potential relations between re-saturation and aerobic fitness. <b>Methods</b>: Forty-nine healthy young adult men and women completed body composition analysis, a maximal lower-body strength assessment, a maximal aerobic fitness (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mtext>V</mml:mtext><mml:mo>.</mml:mo></mml:mover><mml:msub><mml:mtext>O</mml:mtext><mml:mrow><mml:mn>2</mml:mn><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>) test, and a near-infrared spectroscopy vascular occlusion test (NIRS+VOT). Skeletal muscle tissue oxygenation (StO₂), oxygenated hemoglobin (O₂Hb), and deoxygenated (HHb) hemoglobin were collected from a NIRS device attached to the vastus lateralis. Re-saturation measures (e.g. upslope, re-saturation max, and hypersaturation area under the curve (AUC)) were derived from the reperfusion phase of the NIRS+VOT. <b>Results</b>: All O₂Hb and StO₂ re-saturation metrics, particularly upslope (<i>r</i> = 0.622 and <i>r</i> = -.613, respectively), were significantly (<i>p</i> < .05) related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mtext>V</mml:mtext><mml:mo>.</mml:mo></mml:mover><mml:msub><mml:mtext>O</mml:mtext><mml:mrow><mml:mn>2</mml:mn><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>. In the strength group, O₂Hb and HHb upslope (<i>r</i> = 0.584; <i>p</i> < .001; <i>r</i> = -.550; <i>p</i> = .001, respectively) and re-saturation max (<i>r</i> = 0.372; <i>p</i> = .036; <i>r</i> = .562, <i>p</i> < .001, respectively) were significantly related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mtext>V</mml:mtext><mml:mo>.</mml:mo></mml:mover><mml:msub><mml:mtext>O</mml:mtext><mml:mrow><mml:mn>2</mml:mn><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>. For the aerobic group, O₂Hb upslope (<i>r</i> = .486; <i>p</i> = .048), re-saturation max (<i>r</i> = 0.535; <i>p</i> = .027), and hypersaturation AUC (<i>r</i> = 0.564; <i>p</i> = .018) were significantly related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mtext>V</mml:mtext><mml:mo>.</mml:mo></mml:mover><mml:msub><mml:mtext>O</mml:mtext><mml:mrow><mml:mn>2</mml:mn><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>. The aerobic group had significantly (<i>p</i> = .011; BF₁₀ = 8.043) greater O₂Hb upslope (1.6 ± .789 vs. 1.1 ± .474 A.U.s^-1) and (<i>p</i> = .027; BF₁₀ = 2.929) hypersaturation AUC (1158.3 ± 545.02 vs. 860.4 ± 365.35 A.U.s^-1) than the strength group. <b>Conclusion</b>: Upslope was the most related to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mover><mml:mtext>V</mml:mtext><mml:mo>.</mml:mo></mml:mover><mml:msub><mml:mtext>O</mml:mtext><mml:mrow><mml:mn>2</mml:mn><mml:mi>max</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math> in strength and aerobically oriented adults. Interestingly, O₂Hb re-saturation max may not be sensitive to differences between routine strength and aerobic exercise and may reflect shared underlying physiological mechanisms of the predominant fitness orientation.