How do muscles work?

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Hint: From pouring blood all over your body to helping you lift your heavy backpack, they do everything. In your body, you've got more than 600 muscles. You control some of your muscles, while others do their function like your heart without you thinking about them at all.

Complete answer:
Muscles are all made of the same substance (sort of like the material in a rubber band), a form of elastic tissue. Each muscle comprises thousands, or even tens of thousands of small fibres.
In your body, you have three distinct muscle types: smooth muscle, cardiac muscle, and skeletal muscle.
Muscles are attached by tendons to bones and assist them to move.
It gets shorter when a muscle contracts (bunches up), and thus pulls on the bone to which it is connected. It goes back to its usual size when a muscle relaxes.
Just the muscles can pull and can't drive. Therefore to move a joint, muscles have to work in pairs. One muscle contracts and pulls a joint one way, while another muscle contracts and pulls the other one.

Muscles are made of muscle fibres that are composed of myofibrils, which are the muscles' contractile units. These fibres activate during a workout: the muscle shortens, producing a contraction of the muscle, allowing the ends to move closer together and thereby causing motion. You strain your muscles with extra loads of body weight while you weight train in order to improve your strength, stamina, or muscle mass.
Muscles atrophy without gravity, also known as muscle wasting.
Conversely, they hypertrophy when muscles are exposed to great stress. This has been referred to as muscular hypertrophy.

Note: There is an outer covering called the perimysium in each muscle (mainly skeletal). It covers the muscle fibres and preserves them. The fibrous layer of epimysium that covers the entire muscle lies across the entire structure. Each fibre bundle is known as a 'fascicle'. And the endomysium covers each fibre.