It's the most stubborn of gym myths, but is there any truth to it? Is treadmill time really the enemy of weight training? We dig in.

Cardio will ruin your 'gains' has been a gym myth since people started going to the gym. But, sports science has moved on a lot since the early days, so what does the latest thinking tell us? Surely in a balanced, healthy training plan there’s room for both? Or, has this myth persevered for so long because, when you boil it down, it’s undeniably true? 

We tasked exercise physiologist and musculoskeletal therapist Ben Dillon with helping us get to the bottom of it. Ready? Then let’s hit the ground running. (Or not, as the case may be).

Why cardio might inhibit your muscle strength and size?

As outlined above, the idea that cardio might inhibit muscle building, does sort of make sense if we’re talking calories in versus calories out. But what does science tell us? 

The jury is out. “There are a few possible mechanisms by which cardio exercise may inhibit muscle strength and size,” Dillon explains. “One is that cardio exercise can lead to muscle protein breakdown, which can impede muscle growth. This is because during cardio exercise, the body uses muscle glycogen (a stored form of carbohydrate) as a primary fuel source.

"This leads to an increase in the breakdown of muscle protein to provide amino acids for energy production. Additionally, the body may also tap into muscle tissue as a source of energy during extended cardio sessions, leading to muscle loss.

“Another mechanism is that cardio exercise can lead to fatigue, which can impede muscle strength and size,” Dillon continues. “This is because cardio exercise can lead to an increase in the production of stress hormones such as cortisol, which can suppress muscle growth. Additionally, cardio exercise can lead to a decrease in the production of muscle-building hormones such as testosterone and growth hormone, which can also impede muscle growth.”

In other words, pretty damning confirmation that there is truth to the myth, but is that the entire story? 

Can cardio ever actually improve your gains? 

While Dillon makes good points, recent science suggests thinking around cardio and strength training is continuing to evolve. In fact, recent data suggests we can get away with some cardio, and that it might even have a beneficial effect. 

A 2018 study review found intense cardio such as HIIT sessions have minimal or negative effect on muscle loss, while a review in the journal Sports Medicine found that lifting weights and running at the same time (not literally, obviously, but as part of an overall plan) found that cardio might have a negative effect on explosive strength, but won’t stop you developing muscle and strength in general. 

“One way cardio can improve muscle gains is by increasing blood flow to the muscles, which can bring more oxygen and nutrients to the muscle tissue,” Dillon adds. “This can help to improve muscle recovery and growth. Additionally, cardio can also improve overall fitness and cardiovascular health, which can make the individual stronger and better able to perform resistance training exercises.”

Another way cardio can improve muscle gains is by increasing muscle endurance, which allows the individual to perform more reps and sets during resistance training, leading to more muscle stimulus, which over time can lead to muscle gains.

Does the order of when you perform cardio alongside weight training matter?

So cardio can play a role in your plans to pack on size. And it turns out that when you perform cardio within your session can help you maximise the effects. A 2015 study suggests that warming up with aerobic exercise prior to your weights session can actually encourage protein metabolism and muscle hypertrophy – findings backed up in a separate 2021 paper.

“It is worth noting that the muscle-building effects of cardio exercise are likely to be most beneficial for individuals who are new to resistance training or who have relatively low levels of muscle mass,” Dillon warns. “For those who are more advanced, doing cardio after weight lifting instead of before can also have a positive effect on muscle gains.”

Which cardio is it best to do alongside weight training? 

The best form of cardio to do alongside weight training will depend on your individual goals, but Dillon explains that both steady-state cardio, such as jogging or cycling, and high-intensity interval training (HIIT), such as sprints or circuit training, can be beneficial.

“Steady-state cardio can help improve cardiovascular endurance and burn calories. This can be a good option if your primary goal is to improve cardiovascular health or lose body fat,” he says.

“But, HIIT, such as sprints or circuit training, can be more effective at improving cardiovascular fitness and burning calories in a shorter amount of time.”

Ultimately, Dillon explains “the best form of cardio to do alongside weight training is the one that you enjoy and can stick to consistently.”

Can weight training impact my cardio?

So far we’ve explored the impact of cardio on weight training, assuming your primary goal is to put on size. But what of the reverse? If your goal is to lose weight or improve your running ability, can weight training actually help improve your cardio abilities? 

“Weight training can actually have a positive impact on cardio,” Dillon enthuses. “When you lift weights, you are working against resistance, which can help improve the strength and endurance of your muscles. This can lead to an increase in cardiovascular fitness, as your body becomes better at using oxygen to fuel your muscles during exercise.”

Weight training can also help increase muscle mass, which can lead to an increase in metabolism, aiding weight loss and overall fitness. What’s more, accessory exercises – like weighted lunges to aid with knee strength for running – can improve your cardio. 

“Finally, weight training can also help improve your posture and balance, which can make it easier to perform cardio exercises such as running, cycling, and swimming,” says Dillon.

Can your nutrition help when combining cardio and weight training? 

We’ve explored how weight training and cardio training work together in the gym, but what about in the kitchen? Can nutrition help balance things out? 

“We have strong evidence that consuming 1.6g protein per kg body weight per day is a good amount for promoting muscle growth and 2.2g is the upper limit,” explains Daniel Clarke, junior sustainable nutrition manager at Huel.

“Doing cardio will mean you burn more calories, so the simple solution is to eat more as a calorie surplus is also important for gaining muscle. If you find you're struggling to put on muscle by eating enough then doing slightly less cardio may help this, alternatively eat more frequently throughout the day.”

Clarke goes on to point out that “While it's true that cardio training will not promote the same gains as resistance training, it doesn't negatively affect muscle growth. In fact, cardio may help, particularly in those prone to muscle loss like older adults”.

Key takeaways 

There’s a lot to take in here, so let’s summarise what we’ve learned:

• Cardio can lead to muscle breakdown, but counteract this by eating enough of the right calories and there’s no reason why you can’t improve your fitness while maintaining a muscle physique.
• Your cardio of choice depends on your age, goals and abilities. But science suggests HIIT sessions have next to no impact on muscle loss.
• Performing some cardio before your weights session can encourage protein metabolism. But, Dillon believes that more experienced gym-goers would get more benefit from performing cardio last – that way you still have energy in the tank for your big lifts.
• Weight training can have a beneficial effect on your cardio goals, improving stability and metabolism. But, weight training isn’t a replacement for a good run or cycle.
• For well-rounded fitness, cardio is key and it won’t hugely affect your gains either.