Identification of Individuals with Lower Versus Higher Exercising Limb Oxygen Delivery: Does Fatigue Progression Differ?
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Background: In exercising skeletal muscle, oxygen delivery (O2Del) increases in proportion to oxygen cost (O2) to support aerobic production of the cell’s energy currency, ATP, and sustained exercise efforts. When O2Del/demand matching is acutely reduced within an individual, even if VO2 is maintained, greater fatigue occurs. Whether functionally significant differences in O2Del/demand matching exist between individuals, and whether these also impact fatigue, remains unknown. Purpose: 1. Identify inter-individual differences in exercising leg O2Del across a range of work rates (WR). 2. Determine whether those with relatively lower delivery exhibit increased fatigue progression. Methods: Ten healthy men completed multiple trials of progressive isometric knee extensor exercise (IKEE) to exhaustion to quantify individual O2Del/WR relationships. O2Del was determined as leg blood flow via ultrasound x arterial oxygen content (blood sampling). Higher and lower responder identification was based on O2Del/WR responses. A submaximal exercise fatigue test (SEFT) at 70% of peak, with intermittent maximal effort contractions, was used to track fatigue. Results: Higher versus lower O2Del/WR response individuals (p < 0.001) were identified and grouped accordingly. Differences in vasodilation accounted for O2Del/WR differences (p < 0.05). However, there was no between group difference in fatigue index (FI) (p = 0.43). This may have been because of the large variation in exercise efforts during the SEFT, confounding fatigue assessment. In a follow up study, it was determined if practice would improve repeatability and accuracy of exercise efforts to guide future study design. Participants performed 3 SEFTs on separate days. Initial MVC demonstrated good repeatability between trials (COV < 10%) but this did not improve with practice (p = 0.2). FI repeatability was problematic (COV > 10%), and did not improve with practice (p = 0.258). No significant interaction between the trial number and MVC interval on the average WR efforts (p = 0.673) suggests that the WR performed remained consistent within and between trials. Conclusion: In IKEE, there are inter-individual differences in O2Del/WR, which are attributable to differences in vasodilation. The SEFT did not detect any impact of O2Del/WR on fatigability, perhaps because of poor repeatability of FI, that was not improved with practice.