In healthy adults aged 18–45, two weekly sessions of endurance training matched for total volume and intensity produce similar improvements in maximal oxygen uptake (VO2max) as four weekly sessions, with an average increase of approximately 10% over eight weeks, indicating that training frequency does not significantly influence cardiorespiratory fitness gains when total exercise load is controlled.

When a person does endurance exercise, their muscles adapt to use oxygen more efficiently by making more energy-producing factories called mitochondria, and their blood carries more oxygen because it contains more hemoglobin. These changes let the body take in and use more oxygen during intense activity, which raises the maximum amount of oxygen the body can use, no matter if the exercise is done twice or four times a week, as long as the total amount of exercise is the same.