Effects of Different Isometric Exercise Modalities on Microvascular and Metabolic Function

Objective: This study compared near-infrared spectroscopy (NIRS)-derived microvascular function, muscle oxygenation (SmO₂) dynamics, and fatigability during sustained isometric forearm flexion under two conditions: a rating of perceived exertion (RPE) level of 3 (RPE-Clamp) versus a constant torque (CT) matched to the torque produced during the first second of RPE-Clamp. Methods: Thirteen healthy males (22.2 ± 4.7 year) completed both tasks on separate days. SmO₂ was measured during exercise, maximal voluntary isometric contractions (MVIC) were performed before and after exercise, and time to task failure (TTF) was recorded. Vascular occlusion tests (VOT) were used to assess microvascular function pre- and post-exercise. Results: CT induced significantly greater performance fatigability than RPE-Clamp (p < 0.001), despite similar TTF (p = 0.171). SmO₂ significantly decreased during early (p = 0.001) and final (p = 0.028) phases of CT, while increasing during mid (p < 0.001) phases in RPE-Clamp. Post-VOT indicated a significantly reduced desaturation rate (slope 1; p < 0.001), a greater extent of tissue hypoxia (minimum SmO₂; p = 0.003), and increased microvascular reactivity (slope 2; p = 0.021) than pre-VOT, with greater peak re-saturation (MaxSmO₂; p = 0.010) observed in CT than RPE-Clamp. Conclusions: Despite greater fatigability and desaturation in CT, both exercise modalities may enhance microvascular reactivity. Prolonged isometric exercise and subsequent hypoxic stress may contribute to a reduced desaturation rate post-exercise.