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Our bones, muscles, and joints form our musculoskeletal system, which enables us to perform everyday physical activities. There are over 600 skeletal muscles in the human body, and they are all attached to the skeleton. When a skeletal muscle contracts, it pulls on the bone, causing it to move. This movement is made possible through joints, the places where two bones meet. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles, meaning we can control how and when they work. Smooth and cardiac muscles, on the other hand, are involuntary and are controlled by the nervous system.
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What You'll Learn
Skeletal muscles are attached to bones by tendons
Skeletal muscles work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. When the movement is completed, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint. For example, the biceps muscle in the front of the upper arm is a flexor, and the triceps at the back of the upper arm is an extensor. When you bend your elbow, the biceps contract. Then the biceps relax and the triceps contract to straighten the elbow.
Skeletal muscles attach to bones in two or more places. 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 the kneecap.
Agonists, antagonists, synergists, and stabilizers work together to generate any given muscle action. This movement is made possible through joints, the places where bones connect to each other. Synovial joints like the shoulder and knee are typical examples that show how muscles move bones. Synovial joints are fluid-filled joints within a fibrous capsule. The fibrous outer capsule protects the joint and prevents extreme movement, while synovial fluid helps reduce friction between bones as they move.
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![Applied Anatomy and Kinesiology, the Mechanism of Muscular Movement. ... 2D Ed., Thoroughly Rev., Illustrated with L97 Engravings. 1919 [Leather Bound]](https://m.media-amazon.com/images/I/617DLHXyzlL._AC_UY218_.jpg)
Muscles contract to move bones
The human body's musculoskeletal system is made up of bones, muscles, and joints. Bones give the body shape, protect organs, and store minerals like calcium. The joints, where bones connect, allow the body to move. Muscles, which are attached to the skeleton, help control these movements.
Skeletal muscles are the most common type of muscle in the body. They are attached to the bones by tendons and consist of flexible muscle fibres that contract (tighten) and relax, enabling the body to perform various movements. Each muscle can contain thousands of fibres, and these fibres are bundled together and wrapped in connective tissue. The connective tissue supports and protects the muscle cells and allows them to withstand the forces of contraction. It also provides pathways for nerves and blood vessels to reach the muscles.
When a person wants to move, their brain tells a muscle to contract. The muscle then shortens, pulling one bone towards another across a joint. Skeletal muscles work in pairs, with one muscle shortening and the other lengthening. They cannot actively extend or lengthen, so to move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps muscles of the upper arm work in opposition to bend and extend the arm at the elbow.
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Synovial joints allow for the most movement
The human body's musculoskeletal system is made up of bones, muscles, and joints, which enable us to perform everyday physical activities. Bones and muscles are connected by tendons, which are made of collagen. When muscles contract, they pull on the tendons, which in turn pull on the bones, allowing us to move.
There are three types of joints in the body: fibrous, cartilaginous, and synovial joints. Fibrous joints are where bones are bound together by tough, fibrous connective tissue, and they are not designed to move. Cartilaginous joints are where bones are connected by cartilage, and they allow very little movement.
Synovial joints, on the other hand, are the most common type of joint in the body, and they allow for the most movement. These joints have a space, called a synovial cavity, between the adjoining bones. This cavity is filled with synovial fluid, which lubricates the joint and reduces friction between the bones. The articulating surfaces of the bones in synovial joints are covered with smooth articular cartilage, which allows the bones to move smoothly against each other. This gives synovial joints the ability to perform many types of movements, including gliding, angular, rotational, and special movements like abduction (moving a limb away from the body), adduction (moving a limb towards the body), extension (opening a joint), flexion (closing a joint), and rotation. The shoulder, knee, elbow, and hip joints are all examples of synovial joints.
While synovial joints allow for the most movement, they are also the weakest type of joint. This is because the more mobile a joint is, the weaker it tends to be. Synovial joints are also susceptible to arthritis, an autoimmune disease where the immune system attacks the body's joints, causing inflammation and pain.
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Antagonists, synergists, and stabilisers work together to move bones
The human body is home to over 600 muscles, and skeletal muscles are the most common type. These muscles are attached to bones in two or more places, with their ends called origins and insertions. The origin is where the muscle is attached to an immobile bone, and the insertion is where the muscle is attached to a mobile bone.
Skeletal muscles do not work alone; they work in pairs with other muscles, and with bones and joints, to generate movement. Agonists, antagonists, synergists, and stabilisers all work together to make movement possible.
The agonist, or prime mover, is the muscle that provides the primary force driving the action. The antagonist provides resistance and/or reverses the movement. It maintains the body or limb position and controls rapid movement. For example, when extending the leg at the knee, the quadriceps femoris is the agonist, and the hamstrings are the antagonist. When the movement is reversed, so are the roles.
Synergists are muscles that assist the agonist or prime mover in its role. For example, when bending the elbow, the brachialis assists the brachioradialis. A synergist can also be a fixator that stabilises the bone that is the attachment for the prime mover's origin.
Stabilisers help keep bones immobile when needed. For example, the back muscles are stabilisers when they are keeping your posture sturdy.
The musculoskeletal system works in a similar way to a hammer. The bones act as stiff levers, and the articular endings of the bones, encased in synovial joints, act as fulcrums. The effort applied to the system is the pulling or pushing on the handle to remove the nail, which is the load, or “resistance” to the movement of the handle in the system.
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Smooth muscles are controlled by the nervous system
Smooth muscles, unlike skeletal muscles, are involuntary muscles that we cannot consciously control. Instead, they are controlled by the autonomic nervous system, which is responsible for regulating smooth muscle contraction. This is why they are also referred to as involuntary muscles.
The autonomic nervous system uses hormones, neurotransmitters, and other receptors to control smooth muscle activity spontaneously. Smooth muscles serve a variety of vital functions throughout the body, including the stomach, intestines, urinary system, and cardiovascular system. For example, in the digestive system, smooth muscles help break down food and move it through the intestines. In the cardiovascular system, smooth muscles regulate blood flow and pressure by controlling the diameter of blood vessels.
The sympathetic, parasympathetic, and enteric nervous systems all work together to contract smooth muscles. The sympathetic nervous system receives stimulation from the spinal levels T1 to L2 of the spine, which then routes autonomic nervous supply to organs and tissues throughout the body. The parasympathetic nervous system, on the other hand, functions through the cranial nerves, vagus nerve, and pelvic splanchnic nerves, each regulating specific portions of the body.
The unique property of single-unit smooth muscles allows for synchronous contraction, while multi-unit smooth muscles have much finer control as each cell receives its own synaptic input. Smooth muscles are essential for many treatments as they can modify the signaling pathways that affect smooth muscle function. For example, bronchodilators are used to relax airway smooth muscles in asthmatic patients, and medications like metoclopramide stimulate smooth muscle signaling to promote gastric emptying.
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Frequently asked questions
Muscles are attached to the skeleton and cause bones to move when they contract. Skeletal muscles are the most common type of muscle in the body and are attached to the bone by tendons.
Skeletal muscles are voluntary muscles, meaning you control how and when they work. They are located between bones and consist of flexible muscle fibres that contract (tighten) and allow the muscles to move bones.
A joint is a point where two bones meet. Synovial joints, found in arms and legs, are the most common type of joint and allow the greatest freedom of movement. Synovial fluid helps reduce friction between bones as they move.
Muscles can only contract, they cannot extend or lengthen. To move bones in opposite directions, pairs of muscles must work in opposition. For example, the biceps and triceps in the upper arm work in opposition to bend and extend the arm at the elbow.
