Eight weeks of matched-volume endurance training, performed either two or four times per week, increases skeletal muscle oxidative capacity by approximately 23% and increases hemoglobin mass by approximately 2% in healthy adults, with no difference in the magnitude of adaptation between training frequencies.

Repeated endurance exercise increases calcium and energy signals in muscle cells, which turn on genes that build more mitochondria and improve fat burning. This lets muscles use oxygen more efficiently and reduces fatigue. At the same time, exercise causes blood volume to expand and signals the body to make more red blood cells, which carry more oxygen to the muscles. These changes happen the same way whether exercise is done twice or four times a week, as long as the total amount of exercise is the same.