This is Your Body in a Sprint

Ready? Set? It’s all systems GO when you put the pedal down. – By Selene Yeager

body-in-a-sprint-bicycling

They say a high tide raises all boats. Well, the same could be said for blazing fast sprints. When you launch into a full-throttle attack, your body’s energy systems spring into action, forcing you to get fitter fast, especially when you do them regularly, says exercise physiologist Paul Laursen, PhD, of the training service lab PlewsandProf.com.“Sprinting requires a high degree of physical effort, fatigue and acute discomfort, but when you do it consistently, with adequate recovery of course, it elicits relatively rapid improvements in high-intensity endurance performance over a relatively short, two-to-four week period of time.” Here’s what’s happening down to the metabolic level.

1. You Go Anaerobic

Every pedal stroke you take requires ATP—high-energy molecules that power everything you do. During most of your ride, you make ATP by blasting fat with oxygen in your cell’s energy producing mitochondria. As you ramp up your intensity and force more muscle fibres to work harder, your muscles scream for more energy than you can produce with just fat and oxygen. So you switch over to more carbohydrate pathways and dip into your glycogen (stored carbohydrate) reserves to produce ATP in a process called glycolysis. That process yields pyruvate, which is then either driven into the mitochondria for more aerobic energy, or broken down into lactate for even more immediate energy.

But you better hit that finish line quickly, because once you start using up your finite carbohydrate stores, and/or producing more lactate than you can clear, you will come to a grinding halt. This is why even sprinters benefit from building a strong aerobic base. “A well developed aerobic base means that high intensity Type 2 dominant fibers can be spared for the all important sprint,” says Laursen. Aerobic base training also builds mitochondria in your Type 1 endurance fibers, and those mitochondria not only let you burn more fat, but also spare your limited carbohydrate reserves, so you have a greater amount of total energy at your disposal.

2. Your Heart Rate Soars

If you wear a heart rate monitor, you know those numbers shoot toward your ceiling as you sprint. Your systolic blood pressure—the top number, which measures the pressure exerted on your vessels while your heart is beating to push blood out—also rises as high as 200 mmHg during the effort. Both heart rate and blood pressure should decline steadily and relatively quickly after you’re done. Within 30 seconds, your heart rate should be back down to about 50 to 65 per cent of your max. “If you do repeated 30 second sprints with just 30 seconds of rest, your heart rate will remain elevated, however,” says Laursen. That’s why that type of short interval sprint training is an effective way to boost overall fitness.

3. You Smoke the Skinny Climbers

Tired of looking at the scrawny backsides of your mountain goat friends as they dance away from you up yet another climb? Challenge them to a town line sprint. Unlike willowy climbers, sprinters tend to be heavier—a good 6 to 10kg heavier if you compare top tour climbers like Contador with missiles like Cavendish. That’s because sprinters need absolute power, rather than fretting about power-to-weight ratios, because they really only need to overcome air and rolling resistance, and not so much gravity (unless it’s an uphill sprint). Bigger riders tend to have more muscle, and therefore more absolute power.

4. You Build More Muscle

Sprinting helps you stay in front of Father Time (at least for a while) by keeping your muscles strong. Research shows that high-intensity sprint training recruits those all important fast-twitch muscle fibers, increases your testosterone and dihydrotestosterone levels and stimulates protein muscle synthesis, so you make more muscle and slow down aging.

5. You Make More Mitochondria

When you sprint you send the message to your muscles that you need energy and lots of it. That activates mitochondrial biogenesis—science speak for stimulating the production of more mitochondria in your muscle cells. More mitochondria means more energy and power available to do more work.

6. You Burn Fat Like Crazy

You may not be using a lot of aerobic energy during your sprint, but the minute you start coasting, your body’s oxygen consumption and fat burning soars. In one study when cyclists performed a session of four 30-second sprints, their rate of fat oxidation was 75% higher for two hours afterward. Over time, sprint training also helps you be a better fat burner during high intensity exercise, says Laursen, who recently found that well-trained runners burned three-times as much fat during high-intensity interval sessions than their less fit peers. That means they have far more energy to burn and can keep going harder, longer.

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