Real-time quantification of skeletal muscle metabolism in healthy men using hyperpolarised [1-¹³C]pyruvate reveals training-dependent adaptation

Skeletal muscle undergoes rapid and dynamic changes in energy demand during exercise, limiting the physiological relevance of resting-state measurements. Here, we demonstrate the feasibility of combining hyperpolarised ¹³C magnetic resonance spectroscopy (¹³C-MRS) with ³¹P-MRS to non-invasively measure real-time metabolic responses in exercising human skeletal muscle. Sedentary and endurance-trained men perform plantar-flexion exercise inside a magnetic resonance scanner. Dynamic hyperpolarised [1-¹³C]pyruvate spectra from the medial gastrocnemius show that increasing exercise intensity escalates pyruvate-to-lactate conversion relative to mitochondrial pyruvate utilisation. Endurance training is associated with greater bicarbonate labelling, consistent with increased mitochondrial pyruvate utilisation, and reduced lactate production. These metabolic signatures are accompanied by intensity- and training-dependent differences in phosphocreatine degradation and intracellular pH decline measured by ³¹P-MRS. Together, these findings demonstrate the feasibility of combining hyperpolarised ¹³C- and ³¹P-MRS to phenotype skeletal muscle metabolism during exercise, providing a platform for investigating metabolic adaptations in ageing, disease and therapeutic intervention.