Athena Vale
Muscles are an essential part of the human body, with around 600 muscles in the average human body. They work by contracting and relaxing to cause movement, whether that's conscious movement like jumping in the air or unconscious movement like keeping our hearts pumping. Muscles are part of the neuromuscular system, which includes the brain, nerves and skeletal muscles. Skeletal muscles are attached to bones by tendons and work with them to support our weight and move us. They are fuelled by glucose from carbohydrates in our diet, as well as minerals, electrolytes and other dietary substances.
| Characteristics | Values |
|---|---|
| Number of Origins | Some muscles have more than one origin, connecting to more than one bone or to more than one place on a bone |
| Shape | Spindle, flat, triangular, circular |
| Size | Used to distinguish between two muscles found in the same region |
| Type | Skeletal, smooth, cardiac |
| Function | Pumping blood, supporting movement, lifting heavy weights, giving birth |
| Control | Voluntary, involuntary |
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What You'll Learn
Types of muscle: skeletal, smooth and cardiac
There are three main types of muscle in the human body: skeletal, smooth and cardiac.
Skeletal muscles are voluntary muscles that move when you think about moving that part of your body. They can contract quickly and use short bursts of energy (fast-twitch muscles) or move slowly, like back muscles that help with posture. They come in four main shapes: spindle, flat, triangular and circular. Skeletal muscles are the most common and widely distributed muscle tissue in the body, making up around 40% of the body's total mass.
Cardiac muscle (myocardium) makes up the middle layers of the heart and doesn't exist anywhere else in the body. It is involuntary and squeezes and relaxes to pump blood through the cardiovascular system. Cardiac muscle, like skeletal muscle, is arranged in bundles, so it appears striated, or striped.
Smooth muscles are involuntary muscles that line the inside of some organs. They are usually in sheets or layers, with one layer of muscle behind the other. They do essential jobs like moving waste through the intestines and helping the lungs expand when you breathe. Smooth muscle fibres are located in the walls of hollow visceral organs (such as the liver, pancreas and intestines), except the heart, and appear spindle-shaped.
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Muscle shape: spindle, flat, triangular, circular
Muscles can be classified into four basic shapes: fusiform, triangular, rhomboidal, and pennate. However, they can also be rectangular, radiate, and circular.
Fusiform muscles, such as the brachioradialis, have fibres that run parallel to one another. These muscles are long and strap-like, providing a large range of motion. Triangular muscles, such as the gluteus medius, have expansive proximal attachments that converge to a small distal attachment. The large proximal attachments provide a well-stabilized base for generating force.
Muscle spindles, which are not a basic muscle shape, can be imagined as threads spiralled around muscle fibres near the muscle belly. As the muscle lengthens or stretches, it pulls on the spindle, causing it to lose its spiral shape and stretch. This signals the muscle to contract, protecting it from being overstretched.
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Muscle size: biceps, triceps, quadriceps
The number of origins a muscle has is determined by the number of bones or places on a bone to which it connects. A muscle with two origins is called a biceps, and it is spindle-shaped, being wider in the middle and tapering at both ends. The biceps are found on the front of the upper arm. A triceps muscle has three origins, and a quadriceps muscle has four origins. The deltoid muscles of the shoulder are triangular, wider at the bottom and tapered at the top. The size of a muscle can be used to distinguish between two muscles found in the same region.
There are about 600 muscles in the human body, and they can be categorised into three main types: skeletal, smooth, and cardiac. Skeletal muscles are voluntary, meaning they move when you consciously think about moving that part of the body. Some skeletal muscles contract quickly and use short bursts of energy (fast-twitch muscles), while others move slowly, like the back muscles that help with posture. Cardiac muscle, or myocardium, makes up the middle layers of the heart and is not found anywhere else in the body. It contracts and relaxes to pump blood through the cardiovascular system. Smooth muscles are involuntary and line the inside of some organs, performing essential functions like moving waste through the intestines and helping the lungs expand during breathing.
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Muscle movement: voluntary and involuntary
Skeletal muscles are voluntary muscles that move when you think about moving that part of your body. They make up 40% of your body weight and consist of 50% to 75% of your total body's proteins. They can convert chemical energy into mechanical energy to cause voluntary muscle contraction and movement. They attach to bones and regulate movement of the body.
Voluntary muscles can contract quickly and use short bursts of energy (fast-twitch muscles), or they can move slowly, like your back muscles that help with posture.
Involuntary muscles, on the other hand, are not under conscious control. They contract and relax automatically and receive signals from the autonomic nervous system, which regulates your internal bodily functions. Most involuntary muscles are made up of smooth muscle.
The muscles in and around your organs move involuntarily to keep your body working properly. Your heart beating and muscles in your chest and back moving your ribs when you breathe are examples of involuntary movements. Your muscles use a combination of voluntary and involuntary movements to work with nearly all your body's systems and functions.
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Muscle function: pumping blood, supporting movement, lifting weights, giving birth
The human body contains about 600 muscles, which have a range of functions, from pumping blood to supporting movement, lifting heavy weights, and giving birth.
Pumping blood
The heart is a muscle that pumps oxygen-rich blood around the body. The skeletal muscle pump or musculovenous pump is a collection of skeletal muscles that aid the heart in the circulation of blood. Veins embedded within a muscle are compressed during contraction of that muscle, causing an increase in blood pressure due to the presence of one-way valves within the veins. This increase in pressure drives the blood towards the heart. The skeletal muscles of the legs are particularly important skeletal muscle pumps as they prevent pooling of the blood in the feet and calves due to gravity.
Supporting movement
Muscles work by either contracting or relaxing to cause movement. This movement may be voluntary (made consciously) or done without our conscious awareness (involuntary). The brain, nerves and skeletal muscles work together to cause movement. This is collectively known as the neuromuscular system. A typical muscle is serviced by anywhere between 50 and 200 (or more) branches of specialised nerve cells called motor neurones. These plug directly into the skeletal muscle. The brain sends a message to the motor neurones. This triggers the release of the chemical acetylcholine from the presynaptic terminals. The muscle responds to acetylcholine by contracting. For example, when you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.
Lifting weights
When you lift weights, your muscles work together, and concentric and eccentric muscle contractions happen at the same time. A concentric muscle contraction is when the muscle shortens and generates a force greater than that of the weight. By producing a greater force than the weight, this enables you to lift the weight. If the concentric muscle contraction does not produce enough force, you will not be able to lift the weight. Lifting a weight in your hand and pulling it up toward your shoulder (for example, doing an arm curl) causes a concentric contraction in your bicep. The opposite occurs during an eccentric contraction. Due to a force stronger than the muscle can produce, the muscle lengthens. An example of this is when you are lowering the weight during an arm curl by slowly straightening your arm and bringing the weight away from your shoulder and back down toward your side.
Giving birth
The uterus is a huge muscle like a pear. During pregnancy, the cervix is thick and closed. As you approach the time of birth, your contractions draw the cervix up into the body of the uterus, and it becomes thinner (called effacement) and opens (called dilation). When the cervix is fully dilated (about ten centimeters), contractions help the baby begin to move from the uterus into the vagina. The bones and muscles of the pelvis provide support for the growing uterus and baby, and provide a passage through which your baby emerges during birth.
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Frequently asked questions
Muscles contract and relax to generate movement. They are connected to bones with a special kind of tissue called a tendon.
The foundation for muscle contraction is the sarcomere, found in all muscle cells. Sarcomeres contain a motor protein called myosin, which powers the muscle to contract by “grabbing” onto another protein called actin and “flexing”.
Muscles need signals from our brains and energy from our food to contract and move.
