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

Purpose: 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. Methods: Forty-nine healthy young adult men and women completed body composition analysis, a maximal lower-body strength assessment, a maximal aerobic fitness ($\stackrel{.}{\text{V}}{\text{O}}_{2\max }$) 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. Results: All O₂Hb and StO₂ re-saturation metrics, particularly upslope (r = 0.622 and r = -.613, respectively), were significantly (p < .05) related to $\stackrel{.}{\text{V}}{\text{O}}_{2\max }$. In the strength group, O₂Hb and HHb upslope (r = 0.584; p < .001; r = -.550; p = .001, respectively) and re-saturation max (r = 0.372; p = .036; r = .562, p < .001, respectively) were significantly related to $\stackrel{.}{\text{V}}{\text{O}}_{2\max }$. For the aerobic group, O₂Hb upslope (r = .486; p = .048), re-saturation max (r = 0.535; p = .027), and hypersaturation AUC (r = 0.564; p = .018) were significantly related to $\stackrel{.}{\text{V}}{\text{O}}_{2\max }$. The aerobic group had significantly (p = .011; BF₁₀ = 8.043) greater O₂Hb upslope (1.6 ± .789 vs. 1.1 ± .474 A.U.s^-1) and (p = .027; BF₁₀ = 2.929) hypersaturation AUC (1158.3 ± 545.02 vs. 860.4 ± 365.35 A.U.s^-1) than the strength group. Conclusion: Upslope was the most related to $\stackrel{.}{\text{V}}{\text{O}}_{2\max }$ 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.