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Bones, muscles and joints

8-minute read

What are bones, muscles and joints?

Bones, muscles and joints make up the musculoskeletal system, along with cartilage, tendons, ligaments and connective tissue. This system gives your body its structure and support and lets you move around. The parts of the musculoskeletal system grow and change throughout life. Injuries and various illnesses can damage bones, muscles and joints.

Parts of the musculoskeletal system

  • Skeleton — this is the framework of the body. The adult human skeleton is made up of 206 bones. There are 5 main shapes of bones: long (such as the upper arm), short (such as the hand), flat (such as the ribs), irregular (such as the vertebrae) and sesamoid (such as the kneecap).
  • Joints — an area where 2 or more bones come together.
  • Cartilage — provides cushioning inside joints (such as in the knee joint), or connects one bone to another (as in cartilaginous joints).
  • Ligaments — tough bands of tissue that join bones to other bones to strengthen joints. For example, the knee joint has 4 ligaments that help to stabilise it — the 2 collateral ligaments on the inner and outer sides of the knee and the 2 cruciate ligaments inside the knee joint
  • Muscles — there are around 600 muscles in the human body. They help the body move.
  • Tendons — these are made of strong fibrous connective tissue and they connect muscles to bone. They appear as the long thin ends of the muscles. The Achilles tendon is the largest tendon in the body — it joins the calf muscle to the heel bone.

What do bones do?

Bones give people shape. They hold the body upright, and also protect organs like the heart and the liver. They store the minerals calcium and phosphorus, and also contain bone marrow, where new blood cells are made.

Much of the skeleton is made up of non-living material (including the minerals calcium and phosphorus), but nevertheless bones contain living tissue which is constantly remodelling, replacing old tissue with new tissue. The centre of bones contains the bone marrow. This produces new blood cells.

The spine or vertebral column is the central support of your body, helping it walk, move and twist. It has 33 bones called vertebrae, separated by discs. The spinal cord runs down the centre of the spine, carrying all the nerve signals from the brain to the rest of the body and also carrying sensory input from the body back to the brain. The

What do muscles do?

There are different types of muscles, each with different functions, but they all work to produce movement of the body or to stabilise the body. Skeletal muscles are also responsible for generating heat in the body to maintain body temperature and help regulate blood sugar levels.

Skeletal muscle

Skeletal muscle (voluntary muscle or striated muscle) is muscle that you can consciously control. Skeletal muscles run from one bone to another, usually passing at least one joint. Each muscle is comprised of muscle tissue, blood vessels, nerves and tendons. Skeletal muscles are usually attached to the bone by tendons.

When your brain tells a muscle to contract, it shortens, pulling one bone towards another across a joint. Muscles work in pairs — when one shortens, a corresponding muscle lengthens. For example, when you contract your bicep on the front of your upper arm, your tricep on the back of your upper arm lengthens. Physical activity maintains or increases the strength of skeletal muscles.

Skeletal muscle plays an important part in regulating blood sugar (glucose) levels, by taking up glucose from the blood to use as fuel or to store for later.

Smooth muscle

Smooth muscle is found inside blood vessels and organs like the intestines. You can’t consciously control smooth muscle. It contracts to move substances through the organ, and so helps regulate your blood pressure, airways and digestion.

Cardiac muscle

The heart is made of special muscle called cardiac muscle. You can’t control it consciously. It contracts to make your heart beat under the control of the heart’s inbuilt pacemaker — the sinoatrial node.

What do joints do?

Joints connect bones. They provide stability to the skeleton, and allow movement. There are different types of joints.

Synovial joints

Joints in the arms and legs are synovial joints. The ends of the bones are covered with cartilage and separated by the joint cavity which is filled with a thick gel called synovial fluid. Synovial fluid helps to lubricate the cartilage and provides nourishment to it. Ligaments stretch across the joint, connecting one bone to another and help to stabilise the joint so it can only move in certain directions.

Cartilaginous joints

Joints in the spine and pelvis and the joints between the ribs and the sternum are cartilaginous joints — they provide more stability but not as much movement. The bones are connected by cartilage in this type of joint.

Fibrous joints

Fibrous joints allow no movement — just stability. They are held together by fibrous connective tissue. You have fibrous joints in your skull.

Conditions and injuries affecting the bones

Many different conditions and injuries can affect the bones, such as:

  • fractures — where a bone is broken
  • osteopenia and osteoporosis — conditions where bone density is reduced and fractures become more likely
  • Paget's disease — a disease that weakens and deforms bones
  • bone cancer — either cancer that starts in the bones (primary bone cancer) or cancer that spreads to the bones from somewhere else in the body (secondary bone cancer)
  • rickets — a bone disease affecting children, caused by low vitamin D levels
  • bone infection (osteomyelitis) — usually caused by bacteria

They all have different forms of treatment. The best way to have healthy bones and prevent illness and injury to the bones is to eat a healthy diet that includes calcium-rich foods, limit soft drinks, caffeinated drinks and alcohol, be as active as you can, do weight bearing and high impact activities if you can, get enough sunshine and keep to a healthy weight.

Disorders affecting the joints

Many conditions can affect the joints. Arthritis, which is characterised by joint pain and stiffness, is one of the most common. Different types of arthritis have different causes.

Some conditions that can affect the joints are:

  • osteoarthritis — this type of arthritis is more common with age and most often affects the knees, hips, finger joints and big toe joint
  • rheumatoid arthritis — an autoimmune disease where the immune system attacks the lining of the joints
  • septic arthritis — a type of arthritis caused by an infection (usually bacterial)
  • psoriatic arthritis — a type of inflammatory arthritis which affects people who have psoriasis
  • Gout — a painful condition where small crystals of uric acid form in the joints, causing pain, redness and inflammation
  • ankylosing spondylitis — a condition affecting the joints of the neck, spine and pelvis, causing back pain
  • sprains — where the ligaments that connect and stabilise the bones in a joint are stretched or torn

Disorders affecting the muscles

Muscle injuries and disorders can cause weakness, pain or paralysis. Sports injuries are a common way that muscles can be damaged. Conditions affecting the muscles include:

  • strains — where the muscle is over-stretched or contracted too quickly, leading to a partial or complete tear of the muscle fibres or the tendon
  • muscle cramps — these sudden contractions of a muscle can be very painful
  • tendonitis — inflammation or irritation of a tendon, the fibrous cord that attaches a muscle to the bone
  • fibromyalgia — a condition that causes pain and stiffness of the muscles, extreme fatigue and poor sleep, as well as other symptoms
  • muscular dystrophies — these are genetic (inherited) disorders that cause loss of muscle mass and progressive weakness
  • sarcopenia — the age-related loss of muscle mass and quality
  • myositis — inflammation of muscle tissue due to an ongoing autoimmune reaction

Who can help?

If you are having problems with any part of your musculoskeletal system, your doctor (GP) is a good place to start. Other healthcare professionals who are involved in diagnosing and treating musculoskeletal problems are physiotherapists and specialists such as rheumatologists or sports medicine physicians.

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Last reviewed: September 2021


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