Abstract Body (Do not enter title and authors here): Introduction: Exercise intolerance (EI) and early fatigue limit daily activities in patients with heart failure with preserved ejection fraction (HFpEF). Impaired skeletal muscle (SM) energy metabolism is associated with EI in HFpEF but the metabolic pathways that contribute to SM ATP production have not been studied. Given the importance of SM to whole-body glucose utilization, and the prevalence of insulin resistance and obesity in HFpEF, we examined glycolytic ATP production and its potential contribution to EI in HFpEF.

Hypothesis: Previously unmeasured SM glycolysis limits ATP energy metabolism during exercise and is related to EI in HFpEF patients.

Methods: We exploited non-invasive ³¹P magnetic resonance spectroscopy to assess SM energy metabolism and glycolytic rates and their impact on EI in 62 HFpEF patients. Dynamic ³¹P spectra of exercising calf muscle were acquired during graded plantar flexion exercise (PFE) until fatigue to measure high-energy phosphates, phosphocreatine (PCr) and ATP, and inorganic phosphate for determination of pH changes to quantify total glycolytic ATP during exercise.

Results: Subjects were a mean age of 65 years, 53% were women, mean BMI of 39 kg/m2, 48% had diabetes and 48% were NYHA class III. Greater leg SM total glycolysis during exercise was closely associated with longer sustained exercise assessed by six-minute walk distance (Spearman r = 0.35; p=0.004), peak CPET VO₂ (Spearman r = 0.51; p<0.001), and PFE time to fatigue (Spearman r = 0.42; p<0.001). In contrast, mitochondrial oxidative ATP capacity was not related to exercise capacity. The rate of PCr decline was slower in those with longer six-minute walk (Pearson r = -0.42; p<0.001) and PFE (Spearman r = -0.95; p<0.001) durations and was closely related to greater glycolytic energy production (Spearman r = -0.3; p=0.017). This suggests that greater glycolytic ATP production during exercise in HFpEF patients is associated with slower SM high-energy phosphate decline and is itself closely associated with better physical performance inside and outside the MRI.

Conclusion: Reduced SM glycolytic flux during exercise in HFpEF patients is associated with rapid SM high-energy phosphate decline during exercise and, importantly, closely tied to severe exercise intolerance in HFpEF measured by six-minute walk distance and peak CPET VO₂. Impaired SM glycolytic capacity may offer a new metabolic target to treat HFpEF-associated disability.