Dominic Stone
The human body is home to more than 600 muscles, which are responsible for everything from pumping blood around our bodies to helping us lift heavy objects. There are three types of muscles: skeletal, cardiac and smooth muscle. Skeletal muscles are voluntary muscles, meaning we control how and when they move and work. They are attached to our bones by tendons, allowing us to perform a wide range of movements. Cardiac and smooth muscles are involuntary, meaning they work without our conscious thought. Smooth muscles, for example, help break down food and move it through our digestive system.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Types of muscles | Skeletal, cardiac, smooth |
| Skeletal muscle composition | 30% to 40% of total body mass |
| Skeletal muscle attachment | Connects to bones, muscles, or tissues at two or more places |
| Skeletal muscle movement | Voluntary |
| Cardiac and smooth muscle movement | Involuntary |
| Muscle functions | Breathing, speaking, swallowing, digesting food, pumping blood, giving birth, etc. |
| Muscle pain | Occasional aches are normal, but chronic soreness or weakness warrants medical attention |
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What You'll Learn
Bones
Skeletal muscles, which are the most common type of muscle in the body, are attached to bones by tendons. These muscles help hold the skeleton together, give the body its shape, and enable everyday movements. They contract and relax to pull on the joints, allowing us to move. Skeletal muscles make up between 30% and 40% of our total body mass and include shoulder, hamstring, and abdominal muscles. They consist of flexible muscle fibres that can span the length of the muscle, allowing for a wide range of movements and functions.
Overall, the interaction between bones, muscles, tendons, and ligaments is crucial for our body's structure, movement, and stability.
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Tendons
The shape and size of a tendon depend on the muscle it is attached to. Wide and short tendons are connected to muscles for strong, forceful movements, while thin and long tendons facilitate delicate movements. Tendons are also classified according to their function. Positional tendons are responsible for positioning limbs, such as when writing, while energy-storing tendons act as springs to make locomotion more efficient by storing and recovering energy.
At the point where a tendon attaches to a muscle, there is a frequent site of injury called the musculotendinous junction (MTJ). The tendon is then surrounded by a thin, dense connective tissue sheath called the epitenon or epitenonium. The paratenon, a fatty areolar tissue, fills the space between the epitenon and the tendon tissue. Nerve endings are present in the epitenon and paratenon, and nerves are localized to these structures, along with the endotenon. The endotenon is a thin membrane of loose connective tissue that organizes individual tendon fibres into larger bundles, allowing neurovascular penetration and aiding in the gliding movement of the tendon.
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Voluntary movements
Skeletal muscles can be further broken down into three types of muscle fibres: Type I, Type IIa, and Type IIb. Type I fibres are slow-twitch fibres, which have a low rate of fatigue and are best suited for endurance-type contractions such as maintaining posture and marathon running. Type IIa fibres are a mix of fast-twitch and slow-twitch, and they are suited for medium-duration and moderate-movement actions like walking and biking. Type IIb fibres are fast-twitch and are best suited for short-duration, intense movements such as sprinting and weightlifting.
Some muscles, such as those in the head and neck, do not act on joints but on soft tissue. These muscles enable functions such as moving the eyes, chewing, swallowing, and controlling facial expressions. They are also controlled by the central nervous system and operate similarly to those attached to bones.
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Involuntary movements
There are several types of involuntary movements, each with its own unique characteristics:
- Tremors: Rhythmic oscillations caused by intermittent muscle contractions, often seen in processes that damage the cerebellum, such as stroke, tumour, multiple sclerosis, or chronic alcohol abuse.
- Tics: Sudden, repetitive, non-rhythmic movements involving discrete muscle groups, commonly associated with Tourette syndrome in young people and Parkinson's disease in adults.
- Myoclonus: Sudden, brief involuntary twitching or jerking of a muscle or group of muscles, often associated with CNS pathology, hypoxic damage, neurodegenerative disorders, and encephalopathy. It can be triggered by voluntary movement or a person's surroundings.
- Chorea: Rapid, dance-like movements that often approximate a purpose, such as adjusting clothes or checking a watch.
- Athetosis: Slow, writhing movements, mostly affecting the arms and hands.
- Dystonia: Sustained or repetitive muscular contractions resulting in twisting movements or abnormal postures.
- Hemiballismus: Wild, large-amplitude flinging movements on one side of the body, commonly affecting the proximal limb muscles.
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Types of muscles
There are over 600 muscles in the human body, which can be categorised into three types: skeletal, smooth, and cardiac. Skeletal muscles are the most common type and are attached to bones through tendons, allowing for a wide range of movements. These muscles are under conscious control, meaning an individual can decide how and when they work. They make up 30% to 40% of an individual's total body mass and include shoulder, hamstring, and abdominal muscles.
Smooth and cardiac muscles, on the other hand, are involuntary muscles controlled by the autonomic nervous system. They contract without conscious thought and are responsible for essential bodily functions such as digestion and waste removal. Cardiac muscles, found in the walls of the heart, contract involuntarily, strongly, and rhythmically to pump blood throughout the body. Smooth muscles, including cardiac muscles, have a single central nucleus and are striated.
Skeletal muscles can be further classified based on their shape, size, location, origin, insertion, and function. For example, the deltoid muscles have a triangular shape, while the serratus muscles have a serrated or saw-like shape. The naming of skeletal muscles often reflects their anatomical location, such as the rectus abdominis and transverse abdominis in the abdominal region. Additionally, muscles with two, three, or four origins are named biceps, triceps, and quadriceps, respectively.
The different types of skeletal muscles also have varying fibre compositions, which determine their contraction speed, force, and endurance capabilities. For example, sprinters require a high percentage of type IIx fibres that contract quickly and produce substantial force for short bursts of speed. In contrast, long-distance runners benefit from a high percentage of type I fibres that contract slowly with low force but can sustain contractions for extended periods without fatigue.
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Frequently asked questions
The pelvic floor is a musculoskeletal structure that involves a complex system of muscles and connective tissues. The pelvic diaphragm, for instance, is made up of the perineal media aponeurosis or fascia, including the deep transverse muscle of the perineum.
The multifidus muscle is a thin muscle that sits adjacent to the lumbar spine. The paraspinal muscles, also known as the erector spinae, run the length of the lumbar spine, with fibres attaching to various parts of the spine. The latissimus dorsi, or "lats", are the largest muscles in the upper half of the body, extending from the shoulder blades to the lower back. The rhomboid muscles connect the shoulder blades to the spine.
The shoulder is a ball-and-socket joint that contains about eight muscles. These muscles include the deltoid, the supraspinatus, the infraspinatus, the teres minor, the subscapularis, the pectoralis major, the coracobrachialis, the biceps brachii, and the latissimus dorsi.
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