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Bones, muscles, and joints form our musculoskeletal system, enabling us to perform everyday physical activities. Muscles move body parts by contracting and relaxing. They pull bones and work in pairs of flexors and extensors. For example, the biceps contract to bend the elbow, then relax, and the triceps contract to straighten it. Skeletal muscles are the most common type of muscle in the body, comprising 30-40% of our total body mass. They are attached to bones by tendons and help generate heat, protect organs, and maintain posture. Smooth muscles, found in blood vessels and organs like the intestines, are involuntary and help with digestion and blood pressure regulation. The heart is made of cardiac muscle. Synovial joints, like the shoulder and knee, are fluid-filled joints that allow for the most movement in the body.
| Characteristics | Values |
|---|---|
| Definition | Muscles move body parts by contracting and then relaxing. |
| Types | There are three types of muscle: skeletal, smooth, and cardiac. |
| Number of skeletal muscles | There are more than 600 skeletal muscles in the human body. |
| Skeletal muscle composition | Skeletal muscles comprise 30% to 40% of total body mass. They consist of flexible muscle fibres that range from less than half an inch to just over 3 inches in diameter. |
| Muscle attachments | Muscles attach to bone at two or more places. The attachment to an immobile bone is called the "origin," and the attachment to a mobile bone is called the "insertion." |
| Muscle movement | Muscles work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint, then relaxes, allowing the extensor to contract and straighten the limb. |
| Muscle functions | Muscles help with breathing, chewing food, moving food through the digestive system, and regulating blood pressure. They also help generate heat in the body, protect organs, and maintain posture. |
| Muscle-bone interaction | The mechanical relationship between muscles and bones is well-established. Muscles attach directly to bones and generate motion through active contraction, exposing bones to various mechanical stimuli. |
| Synovial joints | Synovial joints are fluid-filled joints within fibrous capsules that allow for the most movement. They reduce friction between bones and make muscle actions possible. |
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Skeletal muscle
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscle. The focus here is on skeletal muscle.
The tissue of a skeletal muscle is striated, having a striped appearance due to the arrangement of the sarcomeres. Skeletal muscle cells are the individual contractile cells within a muscle, often referred to as muscle fibers. These muscle fibers are composed of myofibrils, which are made up of actin and myosin filaments called myofilaments. These filaments are repeated in units called sarcomeres, which are the basic functional, contractile units of the muscle fiber necessary for muscle contraction.
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Muscle contraction
There are three types of muscles in the human body: skeletal, cardiac, and smooth muscles. Skeletal muscles are attached to bones through tendons and facilitate voluntary movements, such as lifting a heavy box. They work in pairs, with one muscle contracting to bend a limb at a joint, and the other relaxing and extending to straighten the limb. For example, when you bend your elbow, the biceps muscle (flexor) contracts, and when you straighten it, the triceps muscle (extensor) contracts.
The contraction of skeletal muscles is neurogenic, requiring input from motor neurons. When a motor neuron stimulates a skeletal muscle, the protein filaments actin and myosin within the muscle fibres interact to produce a contraction. This process is described by the sliding filament theory, where the filaments slide past each other, resulting in muscle shortening or lengthening.
Cardiac muscle, found in the heart, is responsible for involuntary movements like the heartbeat. Smooth muscle, on the other hand, is found in blood vessels and organs like the intestines. It contracts and relaxes involuntarily to regulate processes such as digestion and blood pressure.
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Types of joints
A joint is any place in the body where two or more bones meet. Joints can be categorised by the type of tissue present (fibrous, cartilaginous, or synovial) or by the degree of movement permitted (synarthrosis, amphiarthrosis, or diarthrosis).
Fibrous Joints
Fibrous joints are where bones are bound by a tough, fibrous tissue. These joints require strength and stability over a range of movement. They are typically not very flexible, with some not moving at all. Fibrous joints usually contain a lot of collagen. There are three types of fibrous joints:
- Sutures: These joints hold the plates of the skull together. They are immovable joints (synarthrosis) and are only found between the flat, plate-like bones of the skull.
- Gomphoses: These joints hold teeth in place in the jawbones (mandibles).
- Syndesmoses: These joints hold two closely related bones together. For example, a syndesmosis joint keeps the tibia (shinbone) connected to the fibula (calf bone).
Cartilaginous Joints
Cartilaginous joints are partly movable joints comprising symphysis or synchondrosis joints. These joints occur only in regions where the connection between the articulating bones is made of cartilage. Synchondroses are temporary cartilaginous joints present in young children until the end of puberty. Symphysis joints, on the other hand, are permanent. Examples of cartilaginous joints include the spinal column, the rib cage, and the pubic symphysis.
Synovial Joints
Synovial joints are freely movable joints and are considered the body's main functional joints. They are defined by the presence of a fluid-filled joint cavity contained within a fibrous capsule. The synovial fluid enables all parts of the joint to smoothly move against each other. These joints are further classified into several types, depending on the shape of their articular surfaces and the movements permitted:
- Hinge: These joints allow back-and-forth movement, like a door hinge. Examples include the elbow and knee joints.
- Pivot: These joints allow one bone to swivel in a ring formed by a second bone. An example is the neck joint.
- Condyloid: These joints have two axes that permit up-and-down and side-to-side motions. They can be found at the base of the index finger, carpals of the wrist, elbow, and wrist joints.
- Saddle: This joint allows movement back and forth and side to side, but not rotation. An example is the joint at the base of the thumb.
- Ball-and-socket: These joints feature the rounded head of one bone sitting in the cup of another bone, permitting movement in all directions. Examples include the hip and shoulder joints.
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Muscle attachments
Skeletal muscles are attached to bones in two or more places, either directly or indirectly through tendons or aponeurosis. Tendons are the most common form of attachment, serving to concentrate the pull of the muscle to a small area on the bone. These connective tissues allow the tension created by the contractile component of the muscle to be transmitted to the associated bones so that joint movement can occur. Tendons are made of strong fibrous connective tissue.
There are two types of attachments: origin and insertion. A muscle's origin is where it attaches to an immobile bone, and an insertion is where the muscle attaches to the bone that moves during the muscle action. For example, the rectus femoris has two origins, one at the pubis and one at the ilium, and its insertion is at the patella, or kneecap.
An important tool for predicting the action of a given muscle is to picture its line of pull. You can approximate the line of pull of a muscle by drawing an imaginary double-headed arrow with its base at each attachment and pointing toward the centre of the muscle.
The triceps brachii has three bellies with varying origins (scapula and humerus) and one insertion (ulna). It is a prime mover of elbow extension. The biceps brachii is one of the main muscles of the upper arm which acts on both the shoulder joint and the elbow joint.
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Muscle actions
The human body has more than 600 muscles that help us move, breathe, swallow, and stay alive. Bones, muscles, and joints form our musculoskeletal system, which enables us to perform everyday physical activities. Muscles move body parts by contracting and then relaxing. They pull bones but cannot push them back to their original position, so they work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint, and when the movement is completed, the flexor relaxes and the extensor contracts to straighten the limb at the same joint. For example, the biceps muscle, located in the front of the upper arm, is a flexor, and the triceps, located at the back of the upper arm, is an extensor. When you bend your elbow, the bicep contracts, then relaxes, and the tricep contracts to straighten the elbow.
There are three types of muscle tissue in the body: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles, meaning we can consciously control their movement. They help generate heat in the body, protect organs, and maintain posture. They are attached to bones by tendons. When we want to move, the brain signals a muscle to contract, and it shortens, pulling one bone toward another across a joint. Skeletal muscles work in pairs—when one shortens, a corresponding muscle lengthens.
Smooth muscle is found inside blood vessels and organs like the intestines. We cannot consciously control smooth muscle, and it contracts and relaxes without us realizing it. For example, smooth muscles contract to move food and stool through the digestive system and in arteries and veins to regulate blood pressure.
The heart is made of a special type of muscle called cardiac muscle. We cannot consciously control the cardiac muscle, either. It contracts to make the heart beat and is controlled by the heart's inbuilt pacemaker—the sinoatrial node.
There are two types of muscle attachments: origin and insertion. A muscle's origin is where it attaches to an immobile bone, and an insertion is where the muscle attaches to the bone that moves during the muscle action. For example, the rectus femoris has two origins: one at the pubis and one at the ilium. Its insertion is at the kneecap.
Prime movers, sometimes called agonists, are muscles that provide the primary force driving an action. Antagonists are in opposition to prime movers and provide resistance and/or reverse a given movement. Synergists are muscles that assist prime movers, and stabilizers keep bones immobile when needed.
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Frequently asked questions
Muscles contract and relax to move bones. They pull on the joints, allowing us to move.
The origin is where a muscle attaches to an immobile bone, and the insertion is where the muscle attaches to the bone that moves during an action.
Shoulder muscles, hamstring muscles, and abdominal muscles are all examples of skeletal muscles.
Bones, muscles, and joints form the musculoskeletal system, which enables us to do everyday physical activities. Bones provide structure and support, while muscles contract and relax to create movement.
Muscle movements include flexion (bending) and extension (straightening), abduction (moving away from the body's midline), and adduction (moving towards the body's midline).
