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In the context of fitness, cardio is often associated with aerobic exercise that increases blood flow and improves muscle endurance. However, the term 'cardio' originates from the scientific name for the heart muscle, cardiac muscle. This muscle is one of three major muscle types found in vertebrates and constitutes the main tissue of the heart wall. The heart's contractile force and frequency of activation are critical factors in determining its pumping performance, and the electrical properties of cardiac muscle cells enable the heart to contract and pump blood efficiently. While the benefits of cardio exercises for muscle building are debated, incorporating light cardio into a training routine can improve cardiovascular performance and facilitate muscle growth by increasing blood flow and reducing muscle fat.
| Characteristics | Values |
|---|---|
| Definition | Cardiac muscle tissue or myocardium forms the bulk of the heart |
| Location | The heart wall |
| Structure | Three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium |
| Composition | Individual cardiac muscle cells or cardiomyocytes joined by intercalated discs, encased by collagen fibers and other substances that form the extracellular matrix |
| Function | Contracts to pump blood, regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker |
| Contraction Mechanism | Electrical stimulation triggers the release of calcium, causing the cell's myofilaments to slide past each other in a process called excitation-contraction coupling |
| Blood Supply | Coronary arteries bring blood to the myocardium, and it is drained away by the coronary veins into the right atrium |
| Pathology | Diseases of the heart muscle are called cardiomyopathies and can be genetic or acquired, leading to cardiac dysfunction |
| Treatment | Myocarditis, an inflammatory disease of the myocardium, can be treated with interventions to improve cardiac muscle performance and prevent myocardial maladaptations |
| Aerobic Activity | Aerobic activity increases blood flow, improves muscle endurance, and increases muscle protein synthesis, potentially leading to muscle gains |
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What You'll Learn
Cardiac muscle structure
Cardiac muscle, also called myocardium, is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of the heart. The heart wall is a three-layered structure with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (also known as the visceral pericardium). The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels that connect to the heart. The outer epicardium forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
Cardiac muscle cells (also called cardiomyocytes) are the contractile myocytes of the cardiac muscle. They are surrounded by an extracellular matrix produced by supporting fibroblast cells. Cardiomyocytes are the individual cells that make up the cardiac muscle. The primary function of cardiomyocytes is to contract, which generates the pressure needed to pump blood through the circulatory system. Cardiomyocytes are joined together at their ends by intercalated discs to create a syncytium of cardiac cells. Intercalated discs connect cardiac muscle cells. Under light microscopy, intercalated discs appear as thin, typically dark-staining lines dividing adjacent cardiac muscle cells. The intercalated discs run perpendicular to the direction of muscle fibres.
Cardiac muscle cells contain mitochondria, which convert oxygen and glucose into energy in the form of adenosine triphosphate (ATP). Mitochondria are organelles that refer to structures within a cell with multiple roles. Cardiac muscle cells appear striated or striped under a microscope. These stripes occur due to alternating filaments that comprise myosin and actin proteins. The dark stripes indicate thick filaments that comprise myosin proteins, while the thin, lighter filaments contain actin. When a cardiac muscle cell contracts, the myosin filament pulls the actin filaments toward each other, causing the cell to shrink. The cell uses ATP to power this contraction.
Cardiac muscle contracts in a similar manner to skeletal muscle, although with some important differences. Electrical stimulation in the form of a cardiac action potential triggers the release of calcium from the cell's internal calcium store, the sarcoplasmic reticulum. T-tubules regulate the cardiac ECC by concentrating voltage-gated L-type calcium channels and positioning them in close proximity to calcium sense and release channels, ryanodine receptors (RyRs), at the junctional membrane of the sarcoplasmic reticulum. The thick (myosin) and thin (actin, troponin, and tropomyosin) protein filaments are arranged into contractile units, with the sarcomere extending from Z line to Z line, that have a characteristic cross-striated pattern similar to that seen in skeletal muscle.
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Cardiomyopathies
Cardiac muscle, or myocardium, is the involuntary, striated muscle tissue that forms the bulk of the heart. It constitutes the main tissue of the wall of the heart and is composed of individual cardiac muscle cells, or cardiomyocytes, joined by intercalated discs. The intercalated discs enable the rapid transmission of electrical impulses, allowing the syncytium to act in a coordinated contraction of the myocardium.
There are several types of cardiomyopathies, which differ based on their cause and how they affect the heart's structure and function. All types weaken the heart muscle, and certain types are temporary and get better with time. Some people have overlapping cardiomyopathies. For example, a person may have both dilated cardiomyopathy and arrhythmogenic cardiomyopathy. Dilated cardiomyopathy is a common type that occurs when the heart's ventricles (usually the left ventricle) become enlarged and weaker, making it harder for the heart to pump blood. This can lead to heart failure.
Hypertrophic cardiomyopathy occurs when the heart muscle becomes larger and thicker than normal, which can block the ventricles and also make it harder for the heart to pump blood. Arrhythmogenic cardiomyopathy is a rare condition that develops when fatty or scarred tissue replaces the normal muscle tissue in the right ventricle, causing an irregular heartbeat (arrhythmia). Restrictive cardiomyopathy is another rare type of cardiomyopathy that causes the ventricles to stiffen, preventing them from relaxing and filling with enough blood to pump to the rest of the body.
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Benefits of cardio for muscle building
Cardio, or cardiovascular exercise, is a type of physical activity that increases heart and blood flow rate, improving the efficiency of the cardiovascular system in supplying oxygenated blood to working muscles and enhancing the removal of waste products such as carbon dioxide. Cardio exercises include running, swimming, cycling, and rowing, all of which have benefits for muscle building.
When you engage in cardio training, your body undergoes various adaptations that can directly and indirectly support muscle growth. One of the key benefits is an increased capillary density in the muscles. Capillaries are the tiny blood vessels that connect the larger blood vessels to the muscle cells, providing a direct pathway for oxygen and nutrient delivery. With regular cardio exercise, the number of capillaries per muscle fiber increases, improving the overall blood flow to the muscles. This enhanced blood flow ensures that your muscles receive an ample supply of oxygen, nutrients, and growth-promoting hormones, creating an optimal environment for muscle growth and repair.
Additionally, cardio exercises can stimulate the release of anabolic hormones, such as human growth hormone (HGH). HGH is crucial for muscle development as it promotes protein synthesis, increases nitrogen retention, and stimulates the growth of new muscle cells. By elevating HGH levels, cardio exercises indirectly support muscle building processes, helping you achieve greater muscle growth over time.
Cardio also plays an important role in managing body composition. When done in moderation and combined with proper nutrition, cardio exercises can help reduce body fat levels while preserving muscle mass. This is particularly beneficial if you're aiming for a lean, muscular physique. By maintaining a consistent calorie deficit through cardio and a balanced diet, you can effectively burn fat while minimizing muscle loss. This results in a more defined and toned appearance, showcasing the muscle gains you've achieved.
Furthermore, incorporating cardio into your training regimen can enhance your overall exercise performance. Cardio exercises improve your endurance capacity, allowing you to engage in more intense and sustained muscle-building workouts. With improved endurance, you can push harder during resistance training sessions, leading to greater muscle stimulation and subsequent growth. Additionally, the increased blood flow and oxygen delivery to your muscles from cardio exercises can help reduce muscle fatigue, enabling you to perform more reps and sets with better form and intensity.
Finally, cardio exercises provide important recovery benefits for muscle building. They increase blood flow to the muscles, aiding in the removal of waste products and delivering nutrients essential for repair and growth. This enhanced recovery process helps reduce muscle soreness, allowing you to return to your strength training routine sooner and with greater intensity. Additionally, the improved circulation from cardio exercises can promote a better range of motion and flexibility, reducing the risk of injury and enabling you to train more consistently over time.
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Cardio exercises
The Physical Activity Guidelines for Americans recommend getting at least 150 minutes per week of moderate-intensity aerobic activity or 75 minutes per week of vigorous aerobic activity, or a combination of both. This can be achieved through various forms of cardio exercises, some of which include:
- Walking: A great starting point for beginners, as it is equipment-free and can be done anywhere. Even a 10-minute walk can improve heart health.
- Running: A popular cardio option that can be done outdoors or on a treadmill.
- Biking: Another popular choice that can help improve heart health.
- Jumping Jacks: A beginner-friendly cardiovascular exercise that can be performed almost anywhere. It involves starting with the feet hip-width apart and arms down, then raising the arms straight in the air while jumping out with the feet apart. The intensity can be adjusted by jumping higher or slower.
- Air Jump Rope: This exercise requires swinging an imaginary jump rope in the air, providing an alternative to jogging in place and serving as a warm-up routine.
- Dancing: An enjoyable way to get your cardio fix, dancing can be done anywhere to improve your mood and increase your happiness.
- Marching in Place: This exercise elevates the heart rate and works the abdominal muscles, making it suitable for a warm-up or a standalone cardio activity. The intensity can be increased by marching faster or raising the knees higher.
These are just a few examples of cardio exercises that can be performed with minimal or no equipment, making them accessible for people of all fitness levels.
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Cardiac muscle vs. skeletal muscle
There are three types of muscle tissue in the body: cardiac, smooth, and skeletal. The cardiac muscle, also known as myocardium, is a special type of muscle tissue found only in the heart. It constitutes the main tissue of the heart wall and forms a thick middle layer between the outer layer of the heart wall (the pericardium) and the inner layer (the endocardium). The cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, joined by intercalated discs and encased by collagen fibres and other substances that form the extracellular matrix.
Cardiac muscle is similar to skeletal muscle in that both possess contractile units known as sarcomeres. However, cardiac muscle differs from skeletal muscle in several ways. Firstly, cardiac muscle exhibits rhythmic contractions that are not under voluntary control, while skeletal muscle is under voluntary control. The rhythmic contractions of the cardiac muscle are regulated by the sinoatrial node of the heart, which serves as the heart's pacemaker. Additionally, cardiac muscle has an intermediate speed of contraction and energy requirement, while skeletal muscle has a high speed of contraction and energy requirement.
Skeletal muscles are attached to bones all over the body and are responsible for performing voluntary muscular movements. They are striated in appearance, meaning they appear striped or banded under a microscope. Skeletal muscles are often grouped by their location, such as chest, leg, or back muscles, or by the type of movement they perform, such as abductors, flexors, or extensors.
Cardiac muscle cells form a highly branched cellular network in the heart. They are connected end-to-end by intercalated discs and are organised into layers of myocardial tissue that wrap around the chambers of the heart. The contraction of individual cardiac muscle cells produces force and shortening in these bands of muscle, resulting in a decrease in heart chamber size and the consequent ejection of blood into the pulmonary and systemic vessels. The rate at which the heart contracts and the synchronization of atrial and ventricular contractions are dependent on the electrical properties of the cardiac muscle cells and the conduction of electrical information within the heart.
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Frequently asked questions
Cardio muscle, or cardiac muscle, is one of three major muscle types in vertebrates, found only in the heart. It is an involuntary, striated muscle that constitutes the main tissue of the heart wall.
Doing cardio can help those looking to build muscle by improving the cardiovascular system and the quality of weight training workouts. It also increases blood flow, which ensures the muscles receive all the nutrients they need for growth and recovery.
No, cardio does not hinder muscle gains. In fact, it can result in gains of its own, as aerobic activity helps increase blood flow, improve muscle endurance, and increase muscle protein synthesis. However, it is important to vary the types of cardio and intensity to avoid interference with strength training.
