During muscle contraction, the shortened muscle fibers bulge, squeezing against the surrounding connective tissue and one another. During very vigorous contraction, the blood vessels within the endomysium can be choked off completely. Arterial blood would back up and the blood pressure would rise excessively were it not for the fact that the anastomotic channels permit blood to bypass the nutritive circulation. Muscles that need to generate a lot of force (e.g., muscles used during sprinting) work much of the time without a supply of oxygen, because their nutritive blood supply is closed off while they are contracting. These muscles are specialized to function anaerobically, but in so doing they rapidly use up their energy stores. The so-called oxygen debt is repaid when the muscle stops working and the nutritive arterioles open once again.