Isolated quadriceps training increases maximal exercise capacity in chronic heart failure: The role of skeletal muscle convective and diffusive oxygen transport

Objectives: This study sought to elucidate the mechanisms responsible for the benefits of small muscle mass exercise training in patients with chronic heart failure (CHF). Background: How central cardiorespiratory and/or peripheral skeletal muscle factors are altered with small muscle mass training in CHF is unknown. Methods: We studied muscle structure, and oxygen (O₂) transport and metabolism at maximal cycle (whole-body) and knee-extensor exercise (KE) (small muscle mass) in 6 healthy controls and 6 patients with CHF who then performed 8 weeks of KE training (both legs, separately) and repeated these assessments. Results: Pre-training cycling and KE peak leg O₂ uptake (Vo_2peak) were ∼17% and ∼15% lower, respectively, in the patients compared with controls. Structurally, KE training increased quadriceps muscle capillarity and mitochondrial density by ∼21% and ∼25%, respectively. Functionally, despite not altering maximal cardiac output, KE training increased maximal O₂ delivery (∼54%), arterial-venous O₂ difference (∼10%), and muscle O₂ diffusive conductance (D_MO₂) (∼39%) (assessed during KE), thereby increasing single-leg Vo_2peak by ∼53%, to a level exceeding that of the untrained controls. Post-training, during maximal cycling, O₂ delivery (∼40%), arterial-venous O₂ difference (∼15%), and D_MO₂ (∼52%) all increased, yielding an increase in Vo_2peak of ∼40%, matching the controls. Conclusions: In the face of continued central limitations, clear improvements in muscle structure, peripheral convective and diffusive O₂ transport, and subsequently, O₂ utilization support the efficacy of local skeletal muscle training as a powerful approach to combat exercise intolerance in CHF.