Does HIIT Build Muscle?

Compared to lifting weights, HIIT is not a particularly effective way to build muscle. But that’s not to say it builds no muscle at all.

Take someone who’s unfit and out of shape, put them on a HIIT routine for several weeks, and you’ll likely see some degree of muscle growth.

That’s because any stimulus placing high tensile stress on muscle tissue represents an unusual challenge to that muscle. And one of the ways it adapts to that challenge is to grow in size.

While HIIT isn’t going to deliver the same results in intermediate and advanced lifters, it can serve as a useful supplement to a lower body workout when it’s used in the right way.

The Benefits of HIIT

One of the main benefits of interval training, high-intensity interval training in particular, is its ability to stimulate large gains in cardiovascular fitness with a relatively small amount of training.

In fact, there is research to show that just 10 minutes of sprint interval training, done three times a week, delivers many of the same benefits in terms of VO2peak and heart health as 50 minutes of traditional endurance training.

HIIT is also promoted as a faster way to lose body fat than steady-state cardio, mainly on the basis that it burns lots of calories in the hours after exercise.

However, studies show that the post-exercise calorie burn (known as excess post-exercise oxygen consumption, or EPOC for short) following a bout of high-intensity exercise is much smaller than was once believed.

Studies done using a metabolic chamber, one of the most accurate ways to estimate changes in post-exercise metabolism, show that the afterburn effect lasts for hours, rather than days [1, 2].

In fact, most of the calories burned after a high-intensity workout come in the first hour.

What’s more, studies to compare high-intensity intervals with steady-state cardio don’t show a great deal of difference in the overall rate of fat loss between the two exercise protocols.

In truth, HIIT alone isn’t going to produce radical changes in your physique. For that, you’re going to need to lift some weights and get your diet in order.

However, while some of the reported benefits of HIIT, particularly its effects on fat burning, are overblown, I do want to take a closer look at how you can use HIIT to augment your weight training and assist in muscle growth.

Does HIIT Build Muscle?

If you take someone that’s completely unfit and out of shape, and get them to do HIIT workouts several times a week, chances are you’re going to see some kind of increase in muscle size.

University of North Carolina researchers, for example, rounded up a group of overweight and obese men and women, and got them to perform nine HIIT workouts on an exercise bike [4].

At the end of the study, vastus lateralis – a muscle found in the front of your thigh – had increased in size by 14%.

Research published in the journal Medicine and Science in Sports and Exercise reports similar findings [5].

This time, researchers tracked three groups of sedentary men who did either steady-state cardio, HIIT on an exercise bike, or weight training for six weeks.

As you might expect, the weight training group saw the biggest gains in lean mass, a reasonable proxy for muscle mass. But subjects doing the HIIT workouts still made some progress, posting half the gains seen in the weight training group.

Interestingly enough, there are multiple studies where cycling alone, in some cases just regular steady-state cycling, has been shown to stimulate muscle growth in the legs [7, 8, 9].

Cycling is a form of low-intensity resistance exercise, and the resistance-like loading stimulus has been shown to increase the size of the slow-twitch type I muscle fibers [10].

Granted, many of these studies were done in untrained beginners. In novices, virtually any stimulus represents an unusual challenge to muscle tissue, which will adapt to that challenge, in part at least, by getting bigger.

If you’ve got a few years of training behind you, your muscles will already have adapted to such a low level of stress, and cardio is unlikely to provide much of a stimulus for growth.

What’s more, some of the increase in size will come from something other than contractile proteins, most likely an expansion of the fluid part of a muscle cell, known as the sarcoplasm.

The sarcoplasm is filled with stuff – water, glycogen, mitochondria and so on – that doesn’t contribute directly to the production of muscle force.

Four months of interval training on a bike has been shown to increase fat-free mass, mainly via an increase in the water content of muscle rather than the addition of new muscle protein [10].

The Benefits of HIIT After Lifting

Doing HIIT after lifting can serve as an effective supplement to your leg training, helping to improve your cardiovascular fitness without putting the brakes on (and potentially even contributing to) muscle growth.

And by HIIT after lifting, I’m talking about bike sprints.

By themselves, bike sprints aren’t a direct one-for-one replacement for resistance training. It’s not like you can ditch exercises like the squat, leg press or leg extension, start doing sprint intervals on a bike, and expect to gain the same amount of muscle.

However, I do think that high-intensity interval training can form part of an effective approach to training your thighs when it’s programmed alongside a weight training program.

Done immediately after training your legs, sprint interval cycling is like a supplement to your regular strength training program. Think of it like a “finisher” for your quads.

What exactly do I mean by that?

Let’s say you go to the gym and train your legs with an exercise like the squat or leg press. Each rep lasts 3-4 seconds, and you do 10-12 reps.

You’re expending a high level of effort for between 30 and 48 seconds. Then you rest for a couple of minutes. You do the same thing again, usually for somewhere between 3-5 sets.

Now, let’s look at what happens when you do sprint intervals on a bike.

You cycle as hard as you can for 30-60 seconds. Then you cycle at a slow speed for a couple of minutes or so, before repeating the process 3-5 times.

In other words, both forms of exercise involve a brief but high level of physical effort where you get your heart rate up, followed by a period of rest, repeated multiple times.

Physiologically, while the two types of exercise aren’t exactly the same, they are similar [6].

In other words, bike sprints aren’t better than exercises like the squat, leg press or leg extension, but they can certainly make a contribution to gains in muscle size when used to supplement your lower body workouts.