VO2 max is a parameter that measures the oxygen consumption by large muscle groups during maximum physical effort, such as running or cycling. According to the classic definition by Hill and Lupton in 1923, this parameter determines the point at which further increase in muscle oxygen consumption is impossible despite increasing the load during exercise due to limited capabilities in delivering more significant amounts of oxygen.
The work of skeletal muscles depends on adequate oxygen delivery, and this process is adjusted to meet metabolic needs. This is known as aerobic, or oxygen-based energy production. Increasing intensity requires more energy, which the body cannot produce in oxygen-based processes. This transitional state of increased energy demand produces energy in anaerobic processes, during which lactic acid is produced. The accumulation of lactic acid is known as lactic acidosis.
The process of transporting oxygen from atmospheric air to mitochondria in the working muscle cells is multi-stage. The primary role of the respiratory system is to deliver oxygen from atmospheric air to the lung alveoli and then absorb it into the blood through tiny capillaries. At the same location, carbon dioxide is removed from the bloodstream and exhaled through the lungs. This entire process is known as gas exchange. This well-oxygenated blood is pumped by the heart to the muscles and delivered to mitochondria, the energy factories in every cell of our body. The described gas exchange at the level of the lung alveoli and capillary vessels in the lungs and muscle capillaries occurs through diffusion, which is the difference in oxygen and carbon dioxide concentrations in respective areas. Oxygen is transported thanks to hemoglobin, a protein found in red blood cells. Since oxygen is mainly transported by hemoglobin, hemoglobin concentration (Hb), along with cardiac output (Q), is another essential factor determining oxygen delivery (DO2). DO2 depends on Q, Hb, and oxygen saturation (SaO2). DO2 = Q × Hb × SaO2 × K (coefficient of hemoglobin’s oxygen-binding capacity, 1.33 ml/g). According to Fick’s principle, VO2max equals Qmax multiplied by the arterial-venous oxygen tension difference, representing oxygen extraction by mitochondria in contracting muscle cells. Consequently, VO2 max depends on the integrated functioning of the lungs and heart and exercising skeletal muscles at maximum intensity. It can range from over 90 ml/kg/min in elite endurance athletes to values below 20 ml/kg/min in older individuals. Aging can gradually affect all organs’ functioning in oxygen delivery and utilization.
Based on: Front Biosci (Landmark Ed) 2018 Mar 1;23(8):1505-151