We aim to study adolescents with Fontan including all ventricular morphologies and matched cardio typical controls comprehensively evaluating the following skeletal muscle (SkM) system elements: 1) muscle strength, 2) muscle size/quality, 3) mitochondrial oxidative capacity, and 4) motor unit (neural) activation. Our collaborative, multi-center research team will be the first to comprehensively evaluate the SkM system in the Fontan population. Our central hypothesis is that multiple elements of Fontan SkM health will be diminished compared to controls and will be independently related to worsened measures of fitness, cardiac function, and frailty.