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Introduction Skeletal muscle deconditioning is a hallmark of heart failure (HF) and is linked to reduced exercise tolerance and adverse clinical outcomes. While HF with preserved ejection fraction (HFpEF) and reduced ejection fraction (HFrEF) have distinct pathophysiology, the relationship between skeletal muscle strength and exercise capacity (EC) across both endotypes remains poorly elucidated.Methods The ISARC-HF study (Imaging assessment of sarcopenia across the HF spectrum) was a cross-sectional single-centre study, examining HFpEF and HFrEF patients diagnosed according to the ESC criteria. Quadricep strength was acquired using the K-Pull device (K-Invent, France), and handgrip strength (HGS) using the Jamar dynamometer. We assessed the association between distance walked in the six-minute walk test (6MWT) as a surrogate measure for EC, and quadricep strength (both unadjusted and weight-corrected), using univariate and multivariable regression analysis adjusted for age, sex and ethnicity.Results We analysed 27 HFpEF patients (median age 76 years, 44.4% female, median LVEF 55%, median BMI 34.7 kg/m 2) and 17 HFrEF patients (mean age 73.5 years, predominantly male, median LVEF 29%, median BMI 28 kg/m2).In HFpEF, HGS showed no significant association with 6MWT distance in univariate (b= 2.56, 95% CI -0.4 to 5.5, p = 0.082) or adjusted analysis. Unadjusted quadricep strength demonstrated a borderline association (p=0.060) with 6MWT distance that did not remain significant after covariate adjustment (p=0.478). However, weight-corrected quadricep strength was significantly associated with EC in both univariate (b=34.99m per N/kg, 95% CI 11.83-58.14, p =0.003) and multivariable analysis (b=28.59m per N/kg, 95% CI 8.41-48.77, p = 0.005). Increasing age (b=-2.81m per year, p = 0.023) and non-white ethnicity (b=-115.18m, p = 0.010) were independently associated with reduced EC in HFPEF.In HFrEF, unadjusted quadricep strength was associated with 6MWT distance in univariate analysis (b=0.30, p=0.010) but not after adjustment (p=0.177). Weight-corrected quadricep strength in HFrEF showed significant univariate association (β=32.94m per N/kg, p=0.013) but borderline significance after adjustment (p=0.088).Conclusion This study represents the first use of portable K-Pull dynamometry to characterise the relationship between quadricep strength and functional capacity in HF. Weight-corrected quadricep strength is independently associated with EC in HFpEF patients, even after accounting for age, sex and ethnicity. This suggests that relative muscle strength is a key determinant of functional capacity in HFpEF. The association appears less robust HFrEF, potentially reflecting different underlying pathophysiological mechanisms or sample size limitations. Our finding supports the potential role of targeted resistance-training interventions to improve functional outcomes in HFpEF.