Why is Cardiovascular Exercise mainly effective after twenty minutes?
During the first twenty minutes of exercise, your body uses its energy from muscle and liver glycogen (carbohydrates converted to glycogen and stored for later use). After twenty minutes, the glycogen stores are depleted and the body begins to use the fat stored in body tissue (adipose tissue). Moderately intense activity sustained for hours allows increased fat metabolism and caloric expenditure. High intensity activity sustained for a short duration is not long enough to deplete glycogen stores. Glycogen stores need to be depleted before the body starts using fat for energy.
