Patients palliated with Total Cavopulmonary Connection have a lower muscle mass and a lower exercise capacity. We assessed calf muscle oxidative metabolism during and after heel raise exercise to exhaustion in young patients with TCPC compared to healthy peers. Near-infrared spectroscopy was used for measuring oxygen metabolism in the medial portion of the gastrocnemius muscle. Forty-three patients with TCPC, aged 6â€“18 years, were compared with 43 age and sex-matched healthy control subjects. Subgroups were formed to include children (6â€“12 years) and adolescents (13â€“18 years) to determine if these age groups influenced the results. During exercise, for the patients compared to controls there was a lower increase in deoxygenated hemoglobin (oxygen extraction) (5.13â€‰Â±â€‰2.99au vs. 7.75â€‰Â±â€‰4.15au, pâ€‰=â€‰0.001) and a slower rate of change in total hemoglobin (blood volume) (0.004â€‰Â±â€‰0.015au vs 0.016â€‰Â±â€‰0.01au, pâ€‰=â€‰0.001). Following exercise, patients exhibited a slower initial increase in tissue oxygenation saturation index (0.144â€‰Â±â€‰0.11au vs 0.249â€‰Â±â€‰0.226au, pâ€‰=â€‰0.007) and a longer half-time to maximum hyperemia (23.7â€‰Â±â€‰11.4 s vs 16.8â€‰Â±â€‰7.5 s, pâ€‰=â€‰0.001). On the subgroup level, the adolescents differed compared to healthy peers, whereas the children did not. Young patients with TCPC had impaired oxidative metabolism during exercise and required a longer time to recover. In that the differences were seen in the adolescent group and not in the children group may indicate a declining function with age.