Introduction

It’s well-documented in exercise sciences that doing a lot of cardio can interfere with muscle and strength adaptations. This is often referred to as the “interference effect.” The first study that found an interference between cardio and strength training adaptations dates all the way back to the 1980’s. This study found that when the participants did only 5 days of strength training, they gained more strength than when they added 6 cardio sessions to their weekly routine. Since this landmark study, several studies have supported the finding that doing a lot of cardio can interfere with muscle and strength gain. But, as you can imagine, the interference effect is not the same with every type of cardio you do. By programming your cardio intelligently, you can still incorporate cardiovascular training into your fitness plan without significantly affecting muscle and strength gain. Since most people do some combination of cardio and strength training, in this article we will look into how you can minimize the interference effect of cardio on muscle and strength gains. But first, I’d like to look into how exactly cardio interferes with strength training.

How Cardio Affects Strength Adaptations

Cardiovascular exercise refers to any type of exercise that keeps your heart rate elevated for a continuous length of time and provides mainly endurance adaptations. On the other hand, strength training is a form of exercise that involves your muscles working against resistance to promote improvements in muscular strength, size, and power. If we compare the two forms of training, we see that they directly oppose each other and provide divergent adaptations.

Long-Term Adaptations

The body can’t adapt optimally in two opposite directions. Simple example: You can’t be the best marathon runner and strength athlete at the same time. If you try to optimize both cardio and strength training adaptations, you’ll likely end up somewhere in the middle. A study by the Pennsylvania State University portrays this quite nicely. When trained males only performed 4 strength training sessions in a week, they gained more muscle and strength than when they did 4 strength and 4 cardio sessions in a week. The opposite held true as well. When the trained males only focused on endurance training, endurance performance improved more than when they also had a lot of strength training. So in line with the “Specificity” training principle, if you want to become better at something, you keep your training specific to that goal. If the goal is optimizing muscle/strength gain, do primarily strength training. If the goal is optimizing endurance, do primarily endurance training.

Short-Term Interference

Next to the long-term divergent adaptations, doing cardio can also have a short-term negative effect on strength training by influencing recovery. There is only a limited amount of training you can recover from. Doing a lot of cardio will affect the amount and quality of strength training you can handle. This is because cardio training generates fatigue just like strength training does. If doing strenuous cardio reduces the amount of quality strength training you can handle, it makes sense that your potential to gain muscle and strength also decreases. A 2009 study provides a good example. When the participants performed strenuous sprint training, their lower body strength performance was suppressed for the next 2-3 days. This likely was because sprinting is physically demanding and requires recovery, just like you need to recover from a leg day. So to sum up, excessive cardio can interfere with strength training in 2 main ways:

1. The aerobic adaptations “compete” with strength adaptations
2. The fatigue from cardio inhibits you from maximizing strength training performance.

But as I’ve mentioned at the beginning of this article, the interference effects we discussed above do not have to occur if you somewhat limit your cardio and program intelligently. In the next paragraph, we’ll look into how you can prevent cardio from interfering with muscle and strength gains.

Minimize The Interference Effect

The studies that show an interference effect between cardio and strength training have a few things in common. The commonalities are primarily related to the type, duration, frequency, and timing of the cardio. These are the four factors we need to take into account. I will go over each factor one by one and eventually we’ll reach a conclusion on how to structure your cardio to minimize the interference effect.

Cardio Type: Low-Impact > High-Impact

A 2012 meta-analysis investigated the effects of cardio on muscle and strength adaptations. One interesting finding in this study is that running for cardio resulted in an interference effect, but cycling did not. This likely is because running is high-impact and has a high eccentric component, which causes greater muscle damage and requires more time to fully recover from. Cycling, on the other hand, is low-impact and requires less recovery time. If we think back to the ways cardio can interfere with strength training, you’ll remember that excessive fatigue was an important reason. To minimize the fatiguing effects of cardio (and, therefore, its interference with strength performance) it makes sense to opt for a low-impact cardio modality. A few examples are listed below.

• Cycling
• Swimming
• Row machine
• Elliptical

Cardio Duration: Keep It Short

As you can see in the graph below, the longer your cardio, the more cardio tends to interfere with muscle and strength gain. This graph comes from the previously mentioned 2012 meta-analysis on cardio and strength training. Limiting your cardio sessions to only 20-30 minutes will have a minimum interference effect. But as you can imagine, if you only do 20-30 minutes of cardio, the number of calories you can burn is limited. This is why it could be beneficial to perform high-intensity cardio (HIIT). Next to the fact that HIIT allows you to burn significant calories in a short time frame, HIIT cardio also stresses the same energy systems as traditional strength training. So theoretically, HIIT should interfere less with muscular adaptations compared to moderate-intensity cardio. What we should keep in mind here though is that HIIT can be fatiguing. Performing HIIT almost every day will take a toll on your recovery, so it should be limited to only a few times per week. Also, an exception to this rule is low-intensity cardio like walking. When you walk, the intensity is too low to produce significant aerobic adaptations. The fatigue you accumulate from walking is also minimal. So if you want to go for 30+ minute walks as your cardio, that’s fine and most likely won’t interfere with strength training.

Cardio Frequency: Limit Cardio To Max 3x Per Week

Most studies showing an interference effect involve the participants doing 4 or more cardio sessions in a week. If we look at research where participants perform cardio less frequently, we see a minimal interference effect. A good example is a 2009 study. When the researchers made the participants perform two 30-min cycling sessions in a week, no interference with muscle and strength gain was found. This makes sense for two reasons. For one, just a few cardio sessions per week likely won’t fatigue you much for strength training. Also, if we think back to the specificity principle, for maximizing muscle and strength gain you should mostly train like a strength athlete. So at least making sure you’re not doing more cardio than strength training is key for minimizing the interference effect. Considering the current evidence, a practical rule of thumb is to limit your cardio to a maximum of 3 sessions per week if you want to maximize muscular adaptations.

Cardio Timing: Separate Cardio & Strength Training

Doing cardio before strength training impairs strength performance by fatiguing you. Doing cardio after strength training tends to impair post-workout muscle protein synthesis (MPS). MPS is the process through which muscles recover and grow. So if your schedule allows for it, you could benefit from completely separating your cardio and strength training sessions. A recent experimental study supports this by showing that muscle growth increases if cardio and strength training is performed on separate days. Having at least 6 hours of rest between your strength training and cardio sessions is a good rule of thumb. With that said, I understand that most people don’t have the time to go the gym on separate occasions for their cardio and strength training. So we need to find a way in order to perform cardio and lift weights in the same session while minimizing the interference effect. There’s good evidence indicating the interference effect is local. Lower body dominant cardio (e.g. running, cycling) only interferes with lower body muscle growth. So one potentially effective way to program your cardio is by having lower-body dominant cardio if it’s done after upper body training and upper body-dominant cardio after lower body training. This way, you can likely minimize the interference effect while still doing cardio and lifting weights in the same session. When it comes to upper body dominant cardio, think about forms of cardio like rowing, battle ropes, the ergometer etc.

Conclusion: How To Structure Cardio

If we take all factors discussed above into account, we eventually reach a simple conclusion. You can be pretty sure that your cardio won’t interfere with muscle and strength gain if you can tick most of the boxes shown below:

• You use a low-impact form of cardio (e.g. cycling, swimming)
• Your cardio sessions do not exceed 30 min per session (unless it’s walking)
• You limit your cardio to no more than 3 sessions per week
• You separate your cardio and strength training by at least 6 hours (or do lower body cardio after upper body training and vice versa)

An Example: A simple way I structure my own cardio when I am in a fat loss phase and want to increase energy expenditure is by using 2-3 HIIT cycling sessions per week, usually timed after my upper body training. This HIIT session only lasts around 15 min, so it’s time-efficient. A huge benefit I’ve found from HIIT cycling is that there’s very little “pounding” on the lower body and the recovery requirements are relatively low. If you’d like to experiment with HIIT cycling yourself as your form of cardio, you can receive my free HIIT workout if you sign up to my weekly mailing list below.