Physiological Adaptations: Oxygen Consumption

Maximal oxygen consumption (VO_2max) is defined as the maximal amount of oxygen that can be consumed, transported and utilized within the body during maximal exertion. In the laboratory setting, VO_2max is considered the gold standard measure of aerobic fitness. During exercise, oxygen is required to support energy production via oxidative metabolism. The more oxygen one can consume, transport and utilize the greater energy they can produce to sustain exercise. This is important not only for endurance performance, but also for the recovery between intermittent bouts of high effort sprints. The aerobic system plays a fundamental role in facilitating recovery across repeated sprints.

VO_2max can be directly determined by measuring the expired gas fractions (O₂ and CO₂) at the mouth with a metabolic cart during a maximal graded exercise test. Alternatively, for those of us who do not have access to expensive laboratory equipment, we can indirectly estimate VO_2max with field tests like the Beep Test, Yo-Yo IRT, 12 minute run, etc. Coaches will often use these field tests to track changes in fitness throughout training.

There are myriad of factors that affect oxygen consumption. VO₂ is limited by several factors that can be improved with training. For example, cardiac output is one of the biggest factors the effect oxygen consumption. Cardiac output is equal to the amount of blood ejected from the heart per minute (stroke volume*heart rate). Oxygen carrying capacity of the blood, and the ability of the muscles to take up oxygen are two other important factors effecting oxygen consumption.

Increases in aerobic fitness are largely the result of specific physiological adaptations that occur in response to progressive exercise.  However, it’s important to understand that many of these adaptations take several weeks to several months to take place. There are adaptations that take place at the heart, the blood, the vascular system and skeletal muscle. Below is a brief list of some of the important adaptations that take place in response to endurance or interval training.

Heart –  Increase in left ventricle volume and wall thickness. These changes enable the heart to pump more blood per beat due to increase ventricular filling and greater contractility (stronger contraction).

Blood – Increases in plasma volume decrease blood viscosity and enhance flow while increases in red blood cells enable more oxygen carrying capacity.

Vascular System – Increases in capillary networks surrounding muscle tissue provide more oxygen which is used to create energy. The body also becomes more efficient at directing more oxygenated blood to working muscles and away from other areas.

Skeletal Muscle – Increases in size and content of mitochondria enable greater energy production via increased oxygen uptake at the muscle site. Increases in myoglobin and enzyme content further enhance O2 uptake and utilization at the muscle.

Needless to say, these adaptations do not take place over night. This is why relying on a 1-2 week training camp to get out-of-shape athletes prepared for competition is virtually impossible. A solid base level of fitness throughout the off-season that is further developed in the last month or so of training prior to camp is ideal. An understanding of the physiology of fitness adaptations is critical to effective program design and management. It is our duty as coaches to prepare our athletes safely and effectively.