The ability of the muscle to exert maximal effort in a brief duration.

In most cyclic movements, the ability of muscles to quickly relax or reduce tone and muscle strength allows for more efficient expression of agonist muscle strength during certain phases of movement.

Explanation:

Several movements are based on relatively short activities of the corresponding muscle groups (rapid individual movements or postural corrections) or alternating activations of antagonistic muscle groups (walking, running, throwing, kicking). The time required to perform most of these movements is limited and is often equal to the duration of explosive operations.

don't worry, but do consult a doctor.

explanation:

having a high metabolism at age 13 is not abnormal.   however, if you are concerned, do consult a doctor.   the doctor may identify a correctable issue, and then you'll have your answer.   in the meantime, there are a few steps you might consider.

first, reflect on just how much physical activity (work out and dancing) you are getting relative to your caloric intake.   keep in mind that if you are interested in gaining weight, you must have a higher caloric intake than output.   second, if eating robust healthy meals is not providing enough calories, you'll need to include high caloric, yet healthy, snacks.   i warn against using an over the counter high-calorie supplement (drink or food) until you have consulted a doctor.  

the aorta is the large artery leaving the heart branches into smaller arteries, which eventually branch into capillaries.the arteries carry oxygen and nutrients away from your heart, to your body's tissues. the capillaries are small, thin blood vessels that connect the arteries and the veins. the statement that arteries carry oxygen-rich blood to capillaries is true. veins on the other hand are blood vessels that take oxygen-poor blood back to the heart.