The period popularized direct in vivo assessment of cardiovascular and muscular energy dynamics during graded exercise, introducing dye-dilution methods for cardiac output and biopsy-based tracking of glycogen, phosphocreatine, and ATP. These advances linked energy metabolism tightly to performance, endurance, and recovery, and informed rehabilitation, athletic conditioning, and early considerations in spaceflight medicine. Methodological integration across cardiovascular, metabolic, and nutritional domains established a unified research program for exercise physiology.