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The human body is an intricate system, with over 600 muscles that help us move, breathe, and even speak. These muscles are made up of thousands of small fibres that work together to produce movement. When it comes to forming the letter 'U', several muscles are at play. The specific muscles involved depend on whether we are writing or speaking the letter. For example, when writing, the muscles in our forearms and hands contract to form the shape of the letter 'U'. On the other hand, when speaking, the muscles involved in producing the 'uh' sound include those in the vocal cords and those that control our breath.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of elastic fibres bundled tightly together |
| Types of muscles | Skeletal, Smooth, Cardiac |
| Skeletal muscle shapes | Spindle, Flat, Triangular, Circular |
| Skeletal muscle functions | Movement, Posture, Balance |
| Smooth muscle functions | Digestion, Urination, Vision, Childbirth |
| Cardiac muscle functions | Pumping blood, Generating body heat |
| Muscle contractions | Isometric, Isotonic, Twitch, Temporal summation |
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What You'll Learn
Forearm muscles
The forearm, or lower arm, is the portion of the arm located between the elbow and the wrist. There are twenty muscles in the forearm, divided between the anterior and posterior compartments. These muscles enable the arm to flex and extend, while also coordinating the movement of the wrist and hand.
The anterior compartment of the forearm contains muscles that engage in the flexion of the arm and hand. There are two layers to the anterior compartment: the superficial compartment, located closer to the surface of the skin, and the deep compartment, found closer to the arm bones. The superficial compartment contains five muscles involved in the flexion of the arm, while the deep compartment contains three muscles.
The posterior compartment of the forearm contains muscles that engage in the extension of the arm and its associated structures. Like the anterior compartment, the posterior compartment is also divided into two layers: the superficial and deep compartments. The superficial layer contains seven muscles used for extension, while the deep layer contains five muscles.
Some specific muscles that move the forearm include the pronator teres, pronator quadratus, anconeus, brachioradialis, and supinator. The extensor carpi ulnaris is another important extensor and adductor of the wrist, while the flexor digitorum profundus, flexor carpi ulnaris, flexor radialis longus, and palmaris longus help to flex the wrist.
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Leg muscles
The leg muscles are organised into three groups: anterior, lateral, and posterior compartments. The anterior muscles in the front of the lower leg help lift and lower the foot and extend the toes. The tibialis anterior, located on the outside of the shin bone, is the main muscle used to raise the front of the foot upwards. The lateral group consists of muscles that produce an eversion of the foot at the subtalar joint, which helps maintain balance while standing on one leg or walking on rough surfaces.
The posterior muscles are in the back of the lower leg. They help with flexing and pointing the toes, jumping, running, pushing off into a sprint, locking and unlocking the knee, and maintaining good posture. The calf muscles, the gastrocnemius and soleus, link into the Achilles tendon, the body's strongest tendon, which can withstand a force up to 10 times your body weight. The gastrocnemius works with the hamstrings to bend the knee and propel us forward when walking, running, and jumping. The soleus controls plantar flexion of the foot and is important for good posture to prevent us from falling forward. The hamstrings, in the back of the leg, bend the knees, move the hips backward, and are important for decelerating when running.
The upper leg muscles are very strong and support the body's weight. They include the anterior muscles, which help stabilise the body and maintain balance, as well as allowing the body to bend and extend the knees, flex the thigh at the hip joints, and rotate the legs at the hips. The medial muscles help with hip adduction, as well as allowing the thigh to flex, extend, and rotate. The gluteus maximus, located at the back of the hip, is the largest muscle in the upper leg and one of the strongest muscles in the body. It controls motion at the pelvis and plays a role in maintaining posture.
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Agonist and antagonist muscles
The human body has over 600 muscles, which help us perform a variety of functions, from pumping blood around the body to allowing us to lift heavy objects. When muscles contract, they bring two bones closer together, creating movement. However, muscles cannot push bones back to their original position, so they work in pairs of flexors and extensors. One muscle in the pair contracts to move a body part, and the other muscle in the pair then contracts to return the body part to its original position. These are known as antagonistic pairs.
The muscle that contracts to move a body part is called the agonist, or prime mover, and the muscle that relaxes or lengthens to allow the movement is called the antagonist. For example, when you perform a bicep curl, the biceps contract to produce the movement and are therefore the agonist, while the triceps relax to allow the movement and are therefore the antagonist. Conversely, when we are talking about active elbow extension, the triceps contract to extend the arm and are the agonist, while the biceps relax and are the antagonist. This shows that a muscle can be the agonist for one movement and the antagonist for another.
While skeletal muscles are the prime movers of a joint, other muscles also play a role in producing movement. Synergist muscles help the agonist perform the same movement, and stabiliser muscles keep everything else in place while the agonist and synergist muscles are working.
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Voluntary and involuntary movements
The human body has more than 600 muscles, which help us do everything from pumping blood around the body to helping us lift something heavy. The muscular system is responsible for permitting the movement of the body, maintaining the body's posture, and the circulation of blood cells.
Voluntary movements are actions that we control. We choose to perform an action and our muscles move our body to make it happen. We use our nervous system to control these movements. For example, flicking your thumb to scroll through an article on your phone and sprinting around a track are both voluntary movements.
Voluntary muscles are long, multinucleated cells, containing sarcomeres arranged into bundles. These muscles are composed of cylindrical fibres and are usually attached to bones and the skin. They play an important role in allowing the body to move by contracting and relaxing and their actions are mainly under the control of the somatosensory nervous system. These voluntary muscles include skeletal muscles.
Involuntary movements happen automatically without us thinking about them. Our muscles use a combination of voluntary and involuntary movements to work with nearly all of our body's systems and functions. The muscles in and around our organs move involuntarily to keep our bodies working properly. For example, the beating of our heart and the muscles in our chest and back moving our ribs when we breathe are involuntary movements.
Involuntary muscles are striated and branched in the case of cardiac muscle. The actions of involuntary muscles are mainly controlled by the autonomic nervous system in the body. These involuntary muscles include smooth muscles and cardiac muscles. These muscles are found in the internal walls of the intestines, stomach, uterus, and blood vessels. They help push food molecules along the length of the alimentary canal, control the internal diameter of the blood vessels, and contract the uterus during labour and childbirth.
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Types of muscle tissue
Muscle is a soft tissue and one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Each muscle type has unique cellular components, physiology, specific functions, and pathology.
Skeletal muscle is the most common and widely distributed muscle tissue in the body, making up around 40% of the body's total mass. It is attached to bones and allows movement. Together, skeletal muscles and bones are called the musculoskeletal system or locomotor system. Skeletal muscles are under our conscious control, which is why they are also known as voluntary muscles. They are striated in appearance, with a striped, or striated, pattern of light and dark regions. This striated appearance is caused by the regular arrangement of actin and myosin proteins within the cells into structures known as myofibrils. Myofibrils are responsible for the skeletal muscles' great strength and ability to pull with incredible force and propel the body. Skeletal muscle is broadly classified into two fiber types: type I (slow-twitch) and type II (fast-twitch).
Cardiac muscle, or myocardium, is an involuntary, striated muscle that forms the middle layers of the heart. It is made up of individual cardiomyocytes, which are similar in structure to skeletal muscle. Each cardiomyocyte contains cytoskeletal and contractile elements, all of which are connected through intercalated discs. These discs allow the cardiac muscle cells to receive rapid electrical transmission and contract as a single unit. The cells of cardiac muscle tissue are shorter than skeletal muscle tissue and form a network of many branches between the cells.
Smooth muscle is non-striated and involuntary. It is found within the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin. Smooth muscle is arranged in layered sheets that contract in waves along the length of the structure. Smooth muscle is also known as visceral muscle due to its presence in the organs, blood vessels, and bronchioles of the body.
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