There are a few factors that suggest it could.

Firstly, the amount of energy required for cardio can be quite high, which could potentially limit resources for building muscle, a known energy-costly activity. Secondly, there's something called the "interference effect," where the pathways required for muscle building can be disrupted by cardio.

This concept of combining two types of training simultaneously, such as lifting and cardiovascular training (e.g., running), is referred to as concurrent training.

It was first reported in research by Hickerson in 1980, who concluded that excessive cardio combined with strength training significantly reduced the body's ability to build muscle and increase strength. By 2012, sufficient research on the topic had emerged, allowing the expression "interference effect" to gain traction. Ever since, coaches like me have often advised that if you're trying to build muscle, you probably want to keep cardio separate from your weight training.
However, in 2021, a follow-up meta-analysis of more recent research concluded that there was no significant difference in either trained or untrained lifters when using concurrent training. This highlights how research can sometimes go in completely opposite directions. What's truly important is to look at the potential mechanisms rather than just the conclusion, and delve into the details to understand what it truly means.

As I mentioned, we know that building muscle requires energy, and similarly, long or intense cardiovascular sessions also require energy. So, it's logical to assume the two would compete. But it's also logical to ask: what would happen if you simply adjusted your energy intake to match the output of cardiovascular training, as well as ensuring enough calories are left over for recovery from strength training? In the 2021 meta-analysis, several recent papers did exactly that, and in many cases, equating energy was sufficient by itself to mitigate the effect. However, this wasn't true for every single research paper included. This is an important consideration.

When we think of the expression "cardio," we probably can't just lump all cardio together and say that all cardio is the same. We have cycling versus running, short-duration sprints versus long endurance, and the timing – when you're do it before strength training, after strength training, or later that day. Until more recently, we haven't actually had the answers to these questions. So, let's break it down step by step.

Firstly, not all cardio is equal. Running is very unique in this sense; it has what's called a very high neuromuscular demand due to landing mechanics, and therefore, impact and ground force reaction. Every time you strike the floor, there's an equal and opposite reaction coming back up at you. Because of this impact, running almost becomes its own category of cardio. In contrast, things like swimming, cycling, rowing, and elliptical training have significantly different recovery demands.

That being said, even within running itself, you have to consider: are you an experienced runner? If you're an experienced runner who has adapted to the stresses of running, it won't be as energy-costly for you. What's the body weight and body composition of the runner? A lighter, leaner runner is less likely to experience issues with recovery. And, of course, the distances people are doing, as well as the intensity and hills, are also going to impact this. So, it's really important to think about this, because you could quite simply just do strength training and switch to a format of cardiovascular training that has a lower neuromuscular demand, like I mentioned – swimming or cycling.

This also raises a question about mixed-modality circuits that are popular these days, where people are engaging in activities like jumping and lunging. Due to the variability of exercises that can be used in these sessions, it's challenging to determine the exact amount of recovery needed. However, exercises like squats, lunges, and jumps all have a lengthening or landing phase, so if you did a session with that type of exercise, you can guarantee it will have a fairly high recovery demand. This is in contrast to exercises like battling ropes, a sledge push, or a rower, which won't have as large a recovery demand as exercises with a significant lengthening phase. As I mentioned, if you want to do concurrent training, you can simply choose a method of lower-recovery cardio that won't cause issues with the interference effect.

Next, the length and duration of the session will obviously matter. If you're concerned about this and you're only doing one 5 K run per week, it is really not something you need to worry about. It's more suitable for people who are doing two to three sessions, plus, and those sessions are both long in duration (let's say over an hour) and physically demanding due to hills and sprints. However, simply spacing these sessions out, so you either do them on different days or later in the day, is a great way to mitigate any issues that could arise from the interference effect.

Let's say you do have to do your sessions on the same day. Ideally, you would look to do the lifting first, simply because it's more technical. And if you're doing cardio and strength training together, that means your workout duration ends up being 60 minutes or more. It's probably sensible to have some sort of intra-workout nutrition, such as a carbohydrate drink and electrolytes. Simply because the session duration is so long, and both strength training and cardio will place demands on glycogen in the body, making sure you have some sort of nutrition in place that can bridge that gap is a great way to work around it.

To wrap up, the interference effect from concurrent training is real, but the reality is that it is very unlikely to hold someone back. You can simply just consume enough energy to meet the demands of both of these activities. Potentially, you can simply change the modality of your cardio so that you're not always doing types of cardio that require a lot of recovery, such as running or those mixed-modality circuits. You can simply space your week out to have the most taxing cardio sessions away from your lifting sessions. And if you do have to do it on the same day and it lasts longer, all you need to do is ensure you have some sort of nutrition in place.

And of course, if you have a lot of training coming into your week and you are doing these runs, you can simply adjust the intensity of the run by doing things like Zone 2 or recovery-based sessions. And with your strength training or lifting, you can perform more technical lifts, especially if you're doing big compound lifts or speed-based work, where you are not approaching maximum muscle failure. So, there are plenty of ways, if you're smart with your training, that you can actually do a high frequency of strength training and a high frequency of cardio, and have literally no negative effect on your ability to build muscle.

So, this is one of those things I find extremely interesting. It's super easy with a little planning to manage, ensuring you maximise both your cardio and strength training at the same time. And, of course, striking a balance between cardio and strength training can be a bit tricky. If you would like help within your own training sessions, I am available for online one-on-one training.