Endurance In Sports - What Does Aerobic Capacity Consist Of?

The cardiorespiratory system

To understand the mechanics of endurance capability, we must first take a look at how the cardiorespiratory system works. 

Cardiorespiratory system is a combination of two separate systems; the circulatory system (heart, veins, arteries, blood) and the respiratory system (lungs, nose & nasal cavity, sinuses, mouth, pharynx, larynx, trachea, diaphragm, bronchial tubes, bronchioles, alveoli, capillaries). Their main purpose is to ensure that your tissues have enough nutrients and oxygen to maintain vital functions, while getting rid of harmful byproducts like carbon dioxide. Both systems are also dependent on each another; without healthy lungs, your body would be unable to breathe in oxygen. Without a strong heart, oxygen-rich blood could not be delivered to the body’s cells and back.

The better your body is able to deliver and utilize oxygen during exercise, the better your endurance capacity is. Thus, the coordinated function of these systems have a tremendous impact on how well your body performs during longer activities.

Lungs

Lungs are the most important part of the respiratory system. Their main function is respiration, which can be divided into two phases; inspiration (breathing in oxygen) and expiration (breathing out carbon dioxide). During inhalation, air enters through the mouth and nose and moves:

• Down the throat into the windpipe (trachea)
• Into the lungs via two main airways (left and right bronchi)
• Into smaller and smaller passageways (bronchioles)
• Into tiny air sacs (alveoli)

Each alveoli is covered by small blood vessels called capillaries. This is also where gas exchange (the process where oxygen and carbon dioxide move between the bloodstream and the lungs) occurs. As deoxygenated blood moves through the capillaries, they pick up oxygen from the alveoli through diffusion (flow of matter from higher concentration to lower concentration)

From there, carbon dioxide is exhaled out of the lungs through the mouth, whereas oxygen-rich blood is sent to the heart and pumped to the entire body.

Heart

The heart is a four-chamber pump responsible for circulating blood throughout the body. It consists of several layers of tough muscular tissue, known as the myocardium. Myocardium is covered by a thin layer of tissue on both sides; endocardium on the inside and pericardium on the outside. 

The heart cavity is divided into the left and right heart, both of which consist of two separate chambers. The upper chamber is called the atrium (auricle), which receives the blood entering the heart. The lower chamber is known as the ventricle, which is responsible for  pumping the blood out of the heart. As the venous blood makes its way to the lungs, it loses carbon dioxide and picks up oxygen before returning to the left atrium. Furthermore, each chamber is separated by a valve. These allow blood to only move in one direction while also maintaining blood pressure. 

Heartbeat is a result of alternating contractions and relaxations of the myocardium which are stimulated by a electrical impulses. During exercise, the heart increases its stroke volume and heart rate to ensure tissues receive the oxygen they need to maintain physical activity. 

Veins & arteries

Arteries are blood vessels responsible for delivering oxygen-rich blood away from the heart to various regions of the body. Veins do the opposite – they return blood back to the heart. These two main blood vessels are a part of two closed systems; pulmonary and systemic systems. They both start and end at the heart.

• Pulmonary: pulmonary arteries transport blood that is low in oxygen from the heart’s right ventricle to the lungs. Pulmonary veins return reoxygenated blood from the lungs back to the heart’s left atrium.
• Systemic: systemic arteries deliver oxygen-rich blood from the heart’s left ventricle to various regions of the body. Systemic veins return blood that is low in oxygen from various tissues back to the heart’s right atrium.

Capillaries are the smallest and most numerous blood vessels in the vascular system. They form a network that connects the arteries and the veins. Their main function is exchanging oxygen, nutrients, and waste products between the blood and tissue cells.

Blood

Blood is a specialized fluid that consists of four components; plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). Blood has several functions that are essential for survival. These include:

• Supplying oxygen and nutrients to cells and tissues.
• Forming blood clots to protect wounds and prevent blood loss.
• Carrying cells, antibodies, and hormones that fight infection and maintain bodily functions.
• Transporting waste products (e.g. carbon dioxide) to the lungs, kidneys, and digestive system. From there, waste products can be filtered or removed from the body.
• Regulating body temperature.

Plasma accounts for approximately 55% of all fluid in the blood. 92% of plasma is water, whereas the remaining 8% consists of glucose, proteins, fats, hormones, minerals, and vitamins. The remaining 45% of the blood consists of red blood cells, white blood cells, and platelets.

The main role of blood plasma is to deliver nutrients, proteins, and hormones to various locations of the body, and transport waste products away from the cells. Red blood cells have a disc-like shape and contain hemoglobin – an iron-containing protein responsible for carrying oxygen from the respiratory organs to the tissues.

White blood cells make up for approximately one percent of the blood. Their main function is to support the immune system and fight against infections. Platelets are the smallest blood components and make up for less than one percent of overall blood volume. They are responsible for forming blood clots to protect the wound and prevent excessive blood loss.

Muscular endurance

Local muscle endurance describes a muscle’s or muscle group’s ability to contract for an extended amount of time. There are several factors that affect this, including muscle fiber type, mitochondria content, the availability of energy substrates, and movement efficiency.

The proportions of type I and type II muscle fibers have a strong effect on how your body performs in long distance activities. On average, people have the same amount of each muscle fiber type, although some elite athletes may have as much as 70-80% of a specific muscle fiber type. This is important because both fiber types have different metabolic properties and training responses; slow-twitch fibers (type I) being more suited for prolonged exercises and fast-twitch fibers (type IIa & type IIb) for activities requiring stronger force production. Type I fibers are also smaller in size and contain a higher number of mitochondria (powerhouse of the cell) and myoglobin (an iron and oxygen-binding protein found in the cardiac and skeletal muscle tissue), leading to better fatigue resistance.

The main types of fuel for energy metabolism are glucose, stored muscle glycogen, and free fatty acids. The availability of these energy substrates and how the muscle uses them can have a significant impact on muscular endurance. Because muscles have finite amounts of stored energy, they must utilize metabolic pathways like substrate-level phosphorylation (aerobic & anaerobic) and oxidative phosphorylation (aerobic) to produce adenosine triphosphate (ATP) from stored energy sources. The latter is dependent on sufficient oxygen delivery to contracting muscles, which allows for the oxidization of glucose and fat for fuel.

Because aerobic respiration occurs in the mitochondria, the amount, size, and function of them have a tremendous impact on the oxidative capacity of the muscle. Additionally, mitochondria can also recycle lactate and use it as a fuel source, resulting in improved lactate buffering at higher intensities.

Lastly, technique and movement efficiency can also have an impact on muscular endurance. More specifically, how many motor units (a motor neuron and all the muscle fibers it innervates) are be activated during exercise, and how efficiently they can be recruited through the central nervous system (see: rate coding & motor unit synchronization). Muscular endurance is muscle specific, which means that muscles or muscle group must be trained separately to have a desired training outcome.

Other factors affecting endurance

In addition to certain genetics traits such as muscle fiber distribution, your age, gender, and training history have a substantial effect on endurance.

Males tend to have a higher maximal oxygen uptake (VO₂max) due to having larger hearts, more muscle mass, more hemoglobin as well as less body fat than females. However, females often have proportionally more fatigue-resistant slow-twitch muscle fibers than males do. Some studies have even hinted that females may be able to burn fat for fuel more efficiently, which may be a result of estrogen’s (female sex hormone) effect on the metabolism.

Studies have shown that your maximal oxygen uptake starts declining around 5-10% per decade after the age of 30 depending on how physically active you are. This leads to lower cardiac output (amount of blood the heart pumps) and reduced maximum heart rate during exercise. Similarly, studies have also shown that muscular strength declines approximately 3-5% every decade after the age of 30. This is due to age-related loss of muscle mass (sarcopenia). Interestingly, this degeneration mostly affects type II fibers, whereas type I remain relatively similar in terms of functionality as you age.