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Your muscles contain a great number of tiny blood vessels or capillaries. Normally there are some 3000 of them in a cross-section measuring just one square millimeter. When the muscle is at rest, most of the capillaries are closed. As muscles start working. they open wider and wider (Fig 5). The harder the work, the morę blood flows through the capillaries. Morę motor unils are activated, the muscles work with grealer force, and in time they get stronger.

Capillaries

Blood vessels

Fig 5.

Each muscle is connected to the skeleton by an origin and an insertion (in some cases morę than one). (Fig 6).

Muscles have two basie ways of working (Fig 7):

•    Concentrically, when the muscle fibers contract (shorten) so that origin and insertion come closer to each other.

•    Eccentrically, when the muscle fibers lengthen so that origin and insertion move away from each other. When a muscle contracts without changing its length we cali it isometric contraction.

Eccentric work

The muscle at work is called the agonist. Other muscles cooperating with it are known as synergists. On the other side of the joint you find a muscle with the opposite function to the agonist, logically called the antagonist. Agonist and antagonist take turns working: when the agonist contracts, the anlagonist gets longer, and vice versa.

Agonist

Fig 6.

If the muscle work is too hard, the agonist may stay partly contracted afterwards. The muscle is left too short and stiff. As yet, science hasn’t come up with the physiological explanation for this, but you experience it yourself every time you overdo it and get stiff.

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