Strength and endurance are often seen as two opposite ends of the spectrum of fitness adaptations.

As such, the optimal training and programming for each looks very different.

Many people notice, though, that lifting weights increases heart rate and can leave you with a feeling of “breathlessness” after a hard set.

This leads some to think that such an activity will lead to an cardiovascular/aerobic/endurance adaptation.

This is misguided, and making this extrapolation displays a misunderstanding of what conditions need to be created to elicit any sort of cardiovascular adaptation.

So, in this article, I plan to explain why this is, and to touch on some of the conditions that need to be met for strength and endurance based goals.

Basic exercise physiology

You won’t be able to understand why lifting weights does not lead to endurance improvements without understanding the basics of strength or endurance training physiology; and how they differ.

So, let’s start with that

The fundamental difference between strength training and endurance training is what the limiting factor is for continued movement.

In strength training, the limiting factor is the structural capability of your muscles.

If your quadriceps, glutes, and back cannot produce enough force to lift a load while squatting, then the load will not move (at least, not in the direction you want).

In endurance training, the limiting factor is not the structural capability of your body.

Instead, it is the ability of your body to get oxygen out of the air, into your working muscles, and then use that oxygen to turn carbs or fats into fuel.

A common measurement of this is VO2max, which stands for the Maximal Volume of Oxygen that your body can use for exercise.

The details of VO2max are explained in more detail here.

Fundamentally, if you want to train your strength or endurance, then you need to do things where your strength or endurance are the limiting factors of movement, respectively.

To hammer this home, that is the key point of this whole article.

Whatever limits further movement is what gets trained in any exercise.

Recent controversies

Recently, a paper from Hong et al. [1] has been making the rounds on social media.

The researchers set out to test what the cardiorespiratory and oxygen demand is from doing multiple sets of high repetition squats.

Here’s what they did in a nutshell:

• They recruited 22 strength-trained men (20-39 years old) for the study.

• The subjects reported to the lab for 3 different visits.

• On their first visit, they tested their VO2max.

• Subjects came in for a second visit to test their 1-repetition maximum (1RM) on squat.

• Then, on the third visit, the researchers had the subjects perform 5 sets of 10 reps on squat with 65% of their 1RM squat and 3 minutes of rest between sets.

• Throughout the entire workout, including rest periods, the researchers were measuring the VO2 use of the subjects

Here’s what they found in a nutshell:

• There were small peaks of VO2 use above VO2max during a few moments of most, but not all, sets

• During the rest periods, VO2 use was near resting levels

It shouldn’t be surprising that 5 sets of high rep squats is going to have some spikes of oxygen use at some point.

It wouldn’t take a rocket scientist to realize that something like can make you huff-and-puff a bit.

The REAL question is whether or not this would lead to improvements in endurance or the aerobic system at large.

The answer to this is no. In fact, this study is a good example of why strength training DOES NOT count as cardio.

I will explain why in the next section.

Why strength training won’t build cardio or endurance

As we just saw, high volume, high repetition strength training does involve peaks of VO2 use.

In other words, there are moments in time where the cardiovascular system is stressed.

However, even though strength training may spike your heart rate or leave you out of breath at points in a workout, this does NOT mean you will experience endurance or cardiovascular adaptations.

The primary difference is how much time is spent at these high oxygen using intensities.

A good endurance program will typically have the following three components:

• A low volume of high intensity training (~85-100% maximum heart rate)

• A moderate volume of moderate intensity training (~75-85% maximum heart rate)

• A high volume of low intensity training (~60-75% maximum heart rate)

  • The exact heart rate percentages vary by source. Those numbers are not set in stone. I just put them there to give you the idea.

The whole story of why this is and how exactly to go about this is beyond the scope of the article, but two of the main points are as follows:

• The adaptations you receive from each intensity are slightly unique from one another

• This allows you to achieve the highest total VO2 consumption across your training

To be specific, that distribution typically allows for the highest VO2 consumption at each specific intensity.

For instance, the reason you should only do a low volume of high intensity training is because that’s all that a human body can handle (assuming it’s actually high intensity training).

Your body can handle a moderate volume of moderate intensity training, but not an infinite amount.

Finally, your body can handle a very high volume of low heart rate training, which is why it is useful to milk out as much progress as you can.

In regard to high heart rate training (85-100% max heart rate), typically the goal is to accumulate ~10-15 minutes at this intensity in a single session.

In the study mentioned previously [1] the subjects may have reached their VO2max and had high VO2 peaks, but they spent very little time there.

According to figure 4 of the paper, the subjects spent MAYBE 90 seconds at this level of VO2 intake and use.

Furthermore, this training protocol used would have been very stressful on the system. Therefore, if these 5 sets of 10 on squats were part of a real program, the remainder of the strength training would surely be lower intensity; no matter how much you measure it.

So, the 90 seconds they would have spent at this VO2 level would be the totality of aerobic stimulus they’d receive; which is abysmal.

In other words, there’s practically ZERO stimulus to promote aerobic adaptation.

One point worth bringing up is WHY the subjects were not able to sustain more than ~90 seconds near VO2max using this protocol.

The reason: the aerobic system was not the limiting factor. The force production of the muscles and anaerobic energy systems were.

Again, what limits further movement is what gets trained.

Take-home message

If you want to build strength, then you should do strength training.

If you want to build endurance, then you should do endurance training.

If you want to build real fitness, then you should do both.

Just understand that a good exercise only trains one variable of fitness at a time.

Strength training won’t build your endurance the same way that endurance training won’t build your strength.

How to develop both your strength and endurance at the same time is beyond the scope of this article.

Our best resource on doing that can be found here.