How does exercise increase myoglobin stores?

Myoglobin is a protein within the muscle tissue which acts as an oxygen carrier.

As a long term effect of exercise. The ability of the muscles to store myoglobin is increased. Because muscles increase their oxidative capacity through regular exercise, the myoglobin stores also increase because they get used to the demands of exercise and work placed upon them so increase stores as they will be needed. The increase in oxidative capacity is achieved by an increase in the number of mitochondria within the muscle cells, an increase in the supply of ATP and an increase in the quantity of enzymes involved in respiration.