 Objectives: 1 Determine the effect of 4-weeks of high intensity interval training HIIT and free acid form β-Hydroxy-β-methylbutyrate HMB-FA on VO2peak, lactate threshold, critical power, anaerobic working capacity and neuromuscular economy in a population of endurance trained individuals. 2 Determine the effect of 4-weeks of HIIT and HMB-FA on changes in skeletal muscle physiological cross-sectional area PCSAVolumeFascicle length and muscle quality MQecho intensity in the rectus femoris and vastus lateralis. 