Why Sprinting Improves Endurance

Jens Bangsbo of the University of Copenhagen has shown that if you want to run, cycle or swim faster at any distance, you have to train at a pace that is almost as fast as you can move (Journal of Applied Physiology, November 2009). He asked competitive distance runners to reduce their mileage by 25 percent, and to run 8 to 12 30-second sprints 2-3 times a week, with some additional 0.6-0.8 mile sprints 1 or 2 times per week, for 6 to 9 weeks. The control group of runners continued their regular training program, and showed no improvement. The sprint group improved both their 3K (1.8 mile) and 10K (6 mile) race times by more than three percent (more than a minute in the 10-K race). Half of them ran their best times ever, even though many had been racing for more than five years.

Two years ago, Dr. Bangsbo did ground-breaking research supporting the leading theory that exhaustion of the sodium- potassium pump is the major cause of muscle fatigue during exercise (Acta Physiologica, November 2007). In this new study, he shows how sprint training improves a muscle's capacity to pump potassium back inside muscle cells during exercise, which helps all athletes run or cycle faster in competition, even in endurance events such as marathons and multi-day bicycle races.

A muscle can contract only if it has an electrical charge across the muscle cell membrane. This electrical charge comes mainly from having sodium primarily outside the cell and potassium primarily inside the cell. This higher concentration of sodium outside the cell and higher concentration of potassium inside the cell is maintained by sodium-potassium pumps in the cell membranes. The pumps get their energy from an enzyme called ATPase.

When the brain sends electrical signals along nerves leading to each muscle fiber, sodium moves rapidly into muscle cells followed by an equivalent movement of potassium out of the cells, causing the muscle fibers to contract. However, the sodium- potassium pump cannot pump potassium back into the cells as fast as the rapidly-contracting muscle cells move potassium out.

Dr. Bangsbo showed that during rapid contractions, muscle cells lose potassium so fast that there is a doubling of the potassium outside cells in less than a minute. The electrical charge between the inside and outside of muscle cells is reduced, and they contract with much less force until finally they cannot contract at all. During continuous contractions of muscles, the loss of force from a muscle contraction is directly proportional to the amount of potassium that goes outside the cells.

Over time, repeated muscle contractions themselves will markedly increase the ability of the sodium-potassium pump to pump potassium into cells. The greater the force on a muscle during training, the more effectively the potassium pump can pump potassium back into muscles, resulting in greater endurance for the athlete. So intense training is necessary for endurance, and any training strategy that increases the number of intense workouts will give the athlete greater endurance.

You can also increase the effectiveness of the sodium potassium pumps by being excited before a race (which increases adrenalin), and by eating before and during races (which raises insulin levels). Hormones known to strengthen the sodium- potassium pump, and therefore to increase endurance, include adrenalin, insulin, insulin-like growth factor I, calcitonins, amylin, thyroid, testosterone and cortisones.

How to apply this information to your training program:

You cannot gain maximum endurance just with continuous exercise. To improve your potassium-sodium pumps, you have to put maximum force on your muscles. This requires some form of interval training. (CAUTION: Intense exercise can kill a person with blocked arteries to the heart; check with your doctor before increasing the intensity of your program.)

Intervals are classified as short intervals that take fewer than 30 seconds and do not generate significant amounts of lactic acid; and long intervals that take more than two minutes and generate large amounts of lactic acid. The longest you can exercise with maximal force on muscles is about 30 seconds. All competitive athletes should do some sort of 30-second interval. Nobody knows how often you have to do this, but most runners and cyclists do short intervals once or twice a seek. You probably should do long intervals also. However, applying near-maximal force on muscles for more than 30 seconds causes considerable muscle damage, so you have to allow muscles to recover by doing slow training for one or two days afterwards.

Since short intervals do not accumulate much lactic acid, you can do a large number of repetitions during a single workout. Long intervals cause a tremendous amount of muscle damage, so you can only do a few long intervals during a workout. A sound endurance program should include a lot of slow miles, one or two workouts with many short intervals, and probably at least one workout that includes a few long intervals each week.
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