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The human body has more than 600 muscles, which make up roughly half of a person's body weight. These muscles are responsible for movement, posture, and body position. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to bones and are responsible for most voluntary movements, such as reaching for a book or sprinting around a track. They work in groups to produce precise movements and are controlled by the somatic nervous system. Smooth and cardiac muscles are involuntary and controlled by the autonomic nervous system, helping to keep the body functioning properly by moving substances inside the body, such as blood or food.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscles | Skeletal, smooth, cardiac |
| Types of muscle movement | Voluntary, involuntary |
| Types of muscle contractions | Flexion, extension, abduction, adduction, supination, pronation, elevation, depression |
| Types of muscle fibres | Red, white |
| Types of muscle shapes | Spindle, flat, triangular, circular |
| Muscle tissue composition | Myofibrils, myoglobin, motor neurones |
| Muscle system | Neuromuscular system, musculoskeletal system |
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What You'll Learn
Types of muscle contractions
Muscle contractions are fundamental to our ability to perform different movements. There are three primary types of muscle contractions: isotonic, isometric, and isokinetic.
Isotonic contractions are those in which the muscle changes length without altering the resistance. This type of contraction can be either concentric or eccentric. During a concentric contraction, the muscle shortens while generating force and overcoming resistance. For example, when lifting a heavy weight, a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder. Conversely, an eccentric contraction results in the elongation of a muscle while it is still generating force. This can be both voluntary and involuntary. For instance, a voluntary eccentric contraction would be the controlled lowering of a heavy weight, while an involuntary eccentric contraction may occur when a weight is too great for a muscle to bear, and it is slowly lowered while under tension.
Isometric contractions occur when there is no change in muscle length during contraction. The muscle is under tension but neither shortens nor lengthens.
Isokinetic contractions are similar to isotonic contractions in that the muscle changes length during contraction. However, the key difference is that isokinetic contractions produce movements of a constant speed. An example of this type of contraction is the breaststroke in swimming, where the water provides a constant, even resistance to the movement of adduction.
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Types of muscle
The human body has over 600 muscles, which make up about half of a person's body weight. These muscles are responsible for movement, posture, and body position. They can be grouped by location, such as chest, leg, or back muscles, or by the type of movement they perform, such as abductors, flexors, or extensors.
There are three types of muscle tissue: visceral, cardiac, and skeletal. Visceral muscle is the weakest type of muscle tissue and is found inside organs like the stomach, intestines, and blood vessels. It contracts to move substances through the organ and is controlled by the unconscious part of the brain, making it an involuntary muscle. Cardiac muscle is primarily responsible for transporting substances like blood and food throughout the body. Skeletal muscle, on the other hand, is attached to the bones of the skeletal system and is responsible for most of the body's movements.
Skeletal muscles rarely work alone and often work in groups to produce precise movements. The muscle that produces a particular movement is called the agonist or prime mover. It pairs with an antagonist muscle that produces the opposite effect. For example, the biceps brachii muscle flexes the arm at the elbow, while its antagonist muscle extends the elbow. Synergist muscles assist the agonist and help stabilize the movement, while fixator muscles hold the origin stable.
Muscles perform both voluntary and involuntary movements. Voluntary movements are actions that we consciously control, such as flicking a thumb or sprinting. Involuntary movements happen automatically without conscious thought, such as the beating of the heart or breathing.
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Types of muscle fibre
Muscle fibres are classified based on two criteria: the speed of contraction and how they regenerate adenosine triphosphate (ATP), which provides the energy for muscle contraction.
There are three main types of muscle fibres: slow oxidative (SO or Type I), fast oxidative (FO or Type IIA), and fast glycolytic (FG or Type IIX). Most skeletal muscles contain all three types, but in varying proportions. Slow oxidative fibres contract slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They contain a large number of mitochondria and are capable of sustained contractions over an extended period without fatiguing. On the other hand, fast oxidative fibres have relatively fast contractions and primarily use aerobic respiration to generate ATP. Lastly, fast glycolytic fibres contract rapidly and produce powerful, high-tension contractions but fatigue quickly. They rely on anaerobic metabolism and do not possess a substantial number of mitochondria, resulting in a white colour.
The diversity in muscle fibres allows skeletal muscles to perform a wide range of movements. Muscle fibres also exhibit plasticity, meaning they can change in size or convert to a different fibre type to adapt to new functions. Physical therapy interventions can also affect muscle fibre types, leading to improvements in muscle performance. For example, endurance training can modify slow fibres to make them more efficient by increasing the number of mitochondria and improving aerobic metabolism and ATP production.
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Skeletal muscle
In conclusion, skeletal muscles play a crucial role in human movement and have various specialized functions. Their ability to contract in response to stimuli allows for the generation of force and power, making them essential for our daily physical activities.
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Involuntary vs voluntary movement
Human movements can be broadly classified into two types: voluntary and involuntary. Voluntary movements are self-generated, willed actions performed as a result of cognitive processes. They are conscious and intentional movements that we perform every day, such as running, dancing, or choosing to move our neck or leg muscles. These movements are mediated by several descending motor pathways and are controlled by the motor cortex, a zone of the cerebrum located behind the frontal lobe.
On the other hand, involuntary movements are non-intentional and occur without our conscious control. They are automatic muscle responses to particular stimuli and are crucial for everyday life. Reflexes, for example, are a type of involuntary movement that helps maintain balance and safety. These movements occur rapidly and without the involvement of the brain or conscious attention. Instead, they depend on circuits of neurons located near the spinal cord.
While voluntary movements give us a sense of control and agency, involuntary movements are essential for our survival. Involuntary muscles control our blood vessels and vital organs, such as the heart, lungs, and digestive system. The heart muscle, or myocardium, is composed of specialized muscle cells called cardiac muscle, which contract automatically and rhythmically due to the autonomic nervous system and its own pacemaker cells.
In some cases, the line between voluntary and involuntary movements can become blurred, resulting in semi-voluntary movements. These occur when there is dysfunction in certain brain areas, leading to a disordered sense of will or agency. For example, tics, functional movement disorders, and compulsions can fall into this category.
Additionally, abnormal oscillations can lead to abnormal involuntary movements that can be harmful to a patient's daily life and require treatment. These movements can be classified into four major groups: tremor, myoclonus, chorea/ballism, and dystonia/athetosis.
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Frequently asked questions
There are two types of muscle contractions: voluntary and involuntary. Voluntary movements are actions that you consciously control, such as sprinting or flicking your thumb. Involuntary movements happen automatically without conscious thought, such as your heart beating or breathing.
There are three main types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that you can control, such as the muscles in your neck, arm, and shoulder. Smooth and cardiac muscles are involuntary and controlled by the autonomic nervous system, such as muscles in your digestive tract or heart.
Muscle movements can be described using terms such as flexion, extension, abduction, and adduction. Flexion refers to decreasing the angle between two bones (bending), while extension involves increasing the angle (straightening). Abduction is moving away from the body's midline, and adduction is moving towards it. Other terms include supination and pronation, which refer to rotating the forearm to change the palm's orientation.
