What is the Activity System?

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  • Written By: Mary McMahon
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 12 September 2019
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The activity system is the interconnected network of muscles, tendons, ligaments, and bones that permits organisms like humans to move. It is also known as the locomotor or musculoskeletal system. Disorders affecting range of motion can include connective tissue disease, arthritis, and neurological conditions that limit muscle control. This system starts to emerge very early in fetal development, with bones appearing around 13 weeks after conception.

Musculoskeletal anatomy varies between organisms, depending on the types of tasks their bodies need to perform and their size. The activity system includes a number of critical long bones, such as the femur, along with smaller bones to strengthen joints and form structures like the hands and feet. Tendons and ligaments connect the bones and allow them to articulate, determining the range of motion allowed depending on their position, length, and flexibility. This prevents situations where people injure themselves by extending a joint too far.

The muscles also play a critical role in the activity system. Variations in the size of the muscles can determine strength and stamina; big muscles, for example, can support very heavy loads and allow people to do things like lift heavy weights. Leaner muscles may be more common in runners and people who need endurance, where the need to lift weight is not as necessary, and large muscles can actually slow people down by adding weight to the body.


Disorders of the activity system are most often treated by an orthopedic surgeon. These medical specialists can evaluate bone and joint problems as well as disorders of the related soft tissue, like strained or snapped tendons. They may perform surgical repairs or make recommendations like wearing braces and casts to stabilize bones and muscles for healing. Physical therapists are also familiar with the activity system, as they must help patients rebuild strength and dexterity after accidents and surgeries.

Some patients are born with genetic conditions involving the activity system and may experience problems like lack of coordination, muscle weakness, or joint contractures. Other people acquire disorders as a result of disease or injury and need supportive treatments like surgery, medication, or physical therapy. Problems with the musculoskeletal system are very common and many people experience injury or disease related to this area of the body at some point during their lives, ranging from arthritis in older adults due to wear and tear on the joints to broken bones in young athletes.


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