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 fibres 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
4. You Build More Muscle
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.