Cardiovascular Exercise and Strength Gains: Debunking the Myth That "Cardio Kills Strength"

As a sports and exercise specialist chiropractor working with both athletes and recreationally active populations, a common question I am asked is whether cardiovascular exercise impairs strength and power gains that are accrued with resistance training. This concern is not necessarily unfounded, given that many strength athletes fear that the inclusion of cardiovascular training will negatively affect their progress in the gym. However, the claim that “cardio kills strength gains” oversimplifies the relationship between cardiovascular training and strength development. Let’s break down some of the reasons for this and debunk this myth once and for all.

Understanding Energy Systems

To appreciate how cardiovascular and strength training can coexist, we first need to understand the energy systems that fuel human performance. The energy systems described below are used in all forms of exercise, however the systems used in a given moment are dictated by intensity level and the presence or absence of oxygen. These systems are:

The Phosphocreatine (PCr) System

The PCr system is the body’s immediate energy reserve. It primarily powers short, explosive activities that last from 0 to ~10 seconds, such as maximal lifts, sprints, or heavy weightlifting. The energy comes from the breakdown of creatine phosphate stored in the muscles.

The Glycolytic (Anaerobic) System

This system kicks in when activities last between ~10 seconds and around ~2 minutes. It uses glucose from the blood and muscles to produce energy. Though it is primarily associated with higher-intensity activities, it also supports extreme-intensity efforts that exceed the capacity of the PCr system. Think of it as the energy system engaged during a longer sprint or a heavy set of squats.

The Aerobic System

The aerobic system is responsible for energy production during prolonged, low to moderate-intensity activities (lasting ~2 minutes or more). This system uses oxygen to metabolize fat and carbohydrates, and is essential for endurance athletes who perform sustained activities like long-distance running, cycling, or swimming. This is also the system we are using while at rest.

Cardiovascular and Strength Training: The Interaction

The myth that cardiovascular exercise impairs strength training arises from the way at which we learn the energy systems at a rudimentary level. Similar to the descriptions above, we are taught that there are certain thresholds in which the PCr system switches to the glycolytic system which then switches completely and separately to the aerobic system. This would mean that if you were training to improve performance in one, you likely couldn’t be making the best gains in the others. In reality, this is an oversimplification however remains partially true when training at the extremes of high and low intensity.

The systems don’t operate in an all or none type of way. Think of the transition between them more as a volume dial as opposed to an on-off switch. Optimizing all three energy systems via different forms of training will help us remain versatile when environmental or physical demands change during activity and also enhance your ability to recover between exercise bouts, regardless of exercise type. This is what is means to be truly “metabolically flexible”. The ability to tap into and transition between these systems for a given demand is what, in my opinion, differentiates a well-rounded and high-performance athlete from a “one-trick pony.” Obviously however, in the elite sporting world, there are circumstances where it pays to be a one-trick pony, like power lifting or ultra-endurance running.

Cardiovascular Training, Resistance Training, and Energy System Biases

We know that more high-intensity cardiovascular exercise sessions (like sprints or HIIT) can deplete the PCr stores in muscles. Despite this, it is unlikely to significantly impair maximal strength gains in the long run if someone is also resistance training. It may actually improve the efficacy of the PCr system as the exercise stimuli are different, rest and recovery between efforts may vary, and different muscles are being used. What about moderate cardiovascular exercise and strength gains? Well, studies suggest that well-programmed endurance training at a lower to moderate effort level also do not negatively impact the phosphocreatine system’s capacity when it comes to strength efforts. In fact, endurance training can potentially enhance recovery and resilience to training in the weight room by improving mitochondrial density and oxygen delivery to muscle fibres, aiding overall recovery and performance. Anything that increases blood flow, nutrient delivery and utilization and metabolic byproduct clearance (ie: aerobic efficiency) is a positive thing when it comes to overall performance in all domains.

One important bit of nuance to emphasize here is in order to be a great generalist, you’ll likely have to sacrifice your goals of being an ultra-specialist. What I mean by this, is that if you’re training for overall health, performance and longevity, the goal should be to use various forms of exercise to maintain novelty and enjoyment. This should include both strength-based training and endurance training, including sessions that involve maximum efforts, with more consistent “low and slow” efforts to build your aerobic foundation. This is an awesome foundation to building an above-average level of fitness. A good rule of thumb is that the more intense a workout, the less likely it should be performed compared to other low and slow options. In cycling, this is sometimes referenced as the 80-15-5 rule. Eighty percent of your training should be lower intensity, fifteen percent moderate intensity and five percent extremely high intensity.

Where this generalist approach to resistance and cardiovascular training gets tricky is in professional sports. When you’re being paid to excel in your craft, you’re being paid to be a specialist. This means you may sacrifice building strength for maximum effort situations at the benefit of being able to go all day, every day.  A perfect example of this would be again in professional cycling. Obviously, the task itself in the sport of cycling is in and of itself a way to train for everybody, but when it comes to optimizing your performance on the bike, it’s quite intuitive where these athletes will spend most of their time: on the bike. Although perhaps less than in past decades, cyclists can still spend 25-30 hours a week riding while only dedicating 3-5 hours of strength training depending on the team and discipline. Interestingly, you can also see some variability within the sport itself, where more muscular riders often fall into a category of a “sprinter” and often (but not always), suffer slightly more on the long, arduous days of climbing in the mountains.

The Role of Versatility and Physical Longevity

The key takeaway is that a well-rounded healthy exerciser must have the capacity to tap into various energy systems, ensuring their ability to perform a wide variety of tasks—whether it’s lifting heavy weights, sprinting, or competing in endurance sport. Obviously this gets scaled with age. But in a time where being a “hybrid athlete” is the big buzz, I can’t understate the importance of training for versatility. This is essential for all of us who want to maintain physical longevity, avoid injury and be able to do cool, fun things as we age.

The Bottom Line: Cardio Does Not "Kill" Strength

Both cardiovascular and strength training have important roles to play in a balanced, long-term athletic development strategy. When programmed effectively, cardiovascular exercise can complement and enhance strength training, allowing athletes to improve their overall resilience, recovery, and physical capacity.

The key is balance and understanding what your priorities are. Excessive endurance training, particularly in the form of long-duration activities may absolutely interfere with lean mass accumulation, however if you are a professional cyclist or runner whose job is to perform in endurance sport, your training program must be biased towards cardiovascular exercise. Vice versa for an athlete who competes in a strength and power-based sports.

By understanding the science behind energy systems and applying this knowledge to a training plan, athletes can achieve optimal results across all aspects of their training, especially if your focus is to enhance your overall health and longevity.

In short: cardio doesn’t kill strength—it enhances the foundation on which strength is built.

Yours in good health,

Andrew

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