Intervals essentially teach your body how to go faster. When you increase your pace, your muscles will demand more oxygen to keep functioning. You’ll end up breathing harder to bring more oxygen into your lungs, and your heart will beat faster to deliver that oxygen to your muscles. At a certain pace, however, your muscles won’t have enough oxygen to process energy aerobically (called aerobic cellular respiration), so they’ll switch into anaerobic cellular respiration instead.
The byproduct of anaerobic cellular respiration is lactate which, at a high enough concentration, will limit the capacity of work your muscle can do. Without lactate, you’d basically be able to sprint forever, but because we’re stuck with it, we have to do the next best thing: train our bodies to clear it away efficiently.
When you do a hard interval and recover, you’re teaching your body how to build and clear lactate. You’re also effectively raising the threshold where your cells switch from aerobic respiration to anaerobic respiration (this is called your lactic threshold). This is why I include a lot of in and out work during my plans such as mile repeats followed by a mile ‘float’ or km on km off.
Simply put: if you can run harder without going anaerobic and producing lactate, you can run harder for longer, this meaning intervals are so important for improving regardless of the distance you’re training for.
Nutrition for Intervals
For faster, shorter workouts, the body prefers to have quick energy available, so carbohydrates (both simple and complex) are important on interval days. Bread, rice, and fruit are easy to include with meals, and you might find that a gel or other “energy” food is helpful during or immediately before your workout. This is also useful if you’re training for a marathon and plan to use gels on race day to help train the stomach and find a gel that works well for you both in terms of taste and how it sits on your stomach.
Also, keep in mind that intervals will thoroughly deplete your muscle glycogen, which is the main fuel source your muscles rely on to function properly under stress. It’s important to eat within an hour after every workout to replenish glycogen stores, and this rule is especially critical when it comes to intervals. I’d always recommend having a quick snack immediately post training before getting home to eat a high carbohydrate meal. Don’t negate your hard work with poorly-planned nutrition!
