Many individuals that engage in regular physical activity only train half of the body’s systems. They choose a specific focus such as running, cycling, resistance training, or CrossFit, and neglect other important areas like endurance or flexibility. Over time, they will reach a plateau. The body will adapt to the unidimensional stress imposed on it and performance will likely decrease after three to four years.
Two Energy Systems, One Body
Your body doesn’t run on a single engine. It runs on two, and they operate under completely different conditions.
The first engine, aerobic metabolism powers everything sustained – a long run, a 45-minute bike ride, a brisk walk. It uses oxygen to break down fats and carbohydrates into ATP over time. It’s efficient, steady, and can run almost indefinitely if fueled properly. The cardiovascular and mitochondrial systems are the main beneficiaries here: the heart gets stronger, capillary density increases, and the cells actually produce more mitochondria through a process called mitochondrial biogenesis.
The other engine, anaerobic glycolysis kicks in when the effort is too intense for oxygen delivery to keep pace. Sprints, heavy sets of squats, max-effort pulls rely on rapidly breaking down carbohydrates without oxygen to produce energy in short, explosive bursts. The tradeoff is that this pathway burns out fast. You get 20-30 seconds of high output before fatigue starts cutting in. But the adaptations it produces are different and important: muscle fiber recruitment, myofibrillar hypertrophy, neural efficiency, and significant post-exercise calorie burn through EPOC.
A body trained in both systems develops what exercise physiologists call metabolic flexibility – the ability to shift efficiently between burning fat at lower intensities and burning carbohydrates at higher ones. Train only one pathway and the other deteriorates. That deterioration has real consequences. Understanding the balance between aerobic vs anaerobic exercise on a weekly basis is where most people’s exercise regime either succeeds or fails at the structural level.
What Happens When You Only Do Cardio
Steady-state cardio really gets your heart rate up, in more ways than one. Let’s establish that steady-state cardio isn’t evil. It has its place, its benefits, and has proven for decades that it can constantly and measurably improve cardiovascular fitness and endurance.
It strengthens the heart’s ability to pump blood and deliver oxygen, lowering resting heart rate in the process. It increases energy expenditure through both the actual training session and subsequent elevation of metabolism, a phenomenon known as EPOC (post-exercise oxygen consumption). It’s been shown to improve mood and symptoms of depression. And obviously, it extends aerobic capacity, which has implications outside the gym for many different pursuits.
So why would someone dislike it? One reason is that done to an extreme, it can sufficiently wear you down so that walking out the door for the next bout of activity feels like a Herculean effort. Another is that, in excess, prolonged steady-state cardio training can be counterproductive for body composition and health, especially when not balanced by resistance training.
What Happens When You Only Lift
The dangers on the flip side are no less real.
An exclusively load-bearing exercise program makes muscle stronger, makes bones denser, raises resting metabolic rate. These are all good things, and it’s hard to overstate how valuable they are. But if you’re not doing any cardio, the part of the system that carries oxygen and nutrients to working muscle and sucks waste away from it – it’s not reaching its potential.
In practical terms, this manifests in something weightlifters are all too familiar with: long, agonizing waits between sets. And sometimes you’re not waiting because you’re still catching your breath after those squats. You’re waiting because your cardiovascular system hasn’t yet dealt with the byproducts of those squats in your still-burning muscle cells. A higher VO2 max – your max oxygen uptake during strenuous exercise – means a quicker return to equilibrium because your heart and lungs are dealing with the aftermath more efficiently. Lifters who space out their light aerobic work throughout the week (a light jog here, some swimming there, maybe an easy bike ride on the weekend) find that their overall work capacity improves. More work with the bar in less time under the bar.
Over time, it means fewer missed reps, fewer missed workouts, and many fewer injuries. And injuries are absolutely on the menu when all you do is load the same skeleton in the same range of motion three days a week every week for years on end. Adding multidirectional movement and varied cardio builds the structural balance and joint resilience that heavy barbell work alone doesn’t provide.
The Hormonal Case For Doing Both
Working out affects hormones and this impact can accumulate and influence you positively over time. If done properly, it can have long-lasting beneficial effects.
For example, resistance training boosts growth hormone production and helps maintain good testosterone levels in men. These changes are far from insignificant, as growth hormone and testosterone both play very important roles in muscle building, fat loss, your mood, sex drive, and your cognitive function. The stress placed on your body by compound strength training (think squats, deadlifts, rows, presses) also stimulates the endocrine system, which helps to a greater extent than machine training or isolation work.
On the other hand, aerobic exercise exerts a different kind of hormonal control. Specifically, moderate-intensity cardio in the 60-70% of max heart rate range is very effective at normalizing elevated cortisol. This seems strange at first considering that exercise itself is a stressor, but low-to-moderate cardio, when performed on a regular basis, can help you chill out and lower your body’s chronic stress response. Regularly adding 20-30 minute aerobic sessions to your lifting-heavy week is a sure-fire way to notice an improvement in your sleep quality and anxiety levels.
Now, put these two together and you get a synergistic effect not had by either in isolation. On the one hand, hormones will be giving your body the signal to grow and adapt, while on the other your stress response system will be well-modulated and controlled. This makes for a killer combination when your body needs to recover in a hurry.
Structuring The Week Without Sabotaging Either Goal
Serious lifters are worried about the interference effect which posits that endurance work can potentially decrease the strength and hypertrophy adaptations when both are performed concurrently. Yes, it’s a real thing, but it’s not as insidious as many insinuate.
The interference effect has the biggest impact when high-volume cardiovascular work directly precedes or follows high-intensity resistance training. Practically, this looks like doing a long run in the AM and a heavy squat session in the PM, or doing maximal effort in both strength and conditioning two days in a row. That’s where the adaptations step on each other’s toes.
But that doesn’t mean you need to hang up your running shoes. It just means you need to think a bit more about how you schedule your training.
The most effective way to mitigate interference would be to separate your conditioning and lifting sessions by at least six hours whenever possible. Or, if your schedule allows, by days. Perform your aerobic work at low-to-moderate intensities during a block of heavier lifting. This means steady-state work in the aerobic zone, lower-intensity cycling, or a few sessions of incline treadmill walking.
Use your separate days to perform more anaerobic conditioning instead. This is where you get in your higher-intensity stuff. Just avoid doing it in the 24 hours before lifting heavy compounds.
The Long-Term Stakes: Aging And Mortality
Here’s where the argument becomes genuinely hard to ignore. A 2022 study published in the British Journal of Sports Medicine found that people who engaged in both 150-300 minutes of aerobic exercise and one to two muscle-strengthening sessions each week had a massive 41% lower risk of all-cause mortality than sedentary people – a much larger risk reduction than those who only did cardio or only lifted.
That gap is the entire argument in one number. One is meaningfully better than none. But the compounding effect of both is not small. It’s huge. We know what the crossover pathways here are. Cardio is how you protect the heart, manage hypertension, and keep metabolic health in check. Muscles are how you fortify yourself against insulin desensitization, maintain your functional mobility, and push back against sarcopenia. Both systems defend different fronts, and you can’t have one work optimally if the other is three steps behind.
The case for VO2 max in relation to longevity is about as clear as it is for resistance training, too. High cardiorespiratory fitness is one of the most powerful overall predictors of long-term survival we’ve got (and, for the record, a way better predictor than almost any blood value you can name), and the only way to get it is through consistent cardio. Meanwhile, grip strength as well as lower-body strength are two of the most consistent independent predictors of whether a senior citizen will be able to feed, bathe, and dress themselves in later decades of life, and the only way to get those is through resistance training.
The Hybrid Approach Isn’t Complicated
The division between “runners vs. lifters” is based on culture, not science. Biologically, your body doesn’t mind which “tribe” you’re in. Your body responds to the stress you put on it and signals you to adapt. A solid program that includes aerobic and anaerobic training and is properly balanced in terms of recovery and progression in both styles will then lead to a much more optimized, resilient, and efficient body overall.
The goal isn’t to be in world record shape for marathon running and world record strength for a powerlifter – just that you can train both energy systems often enough that one doesn’t end up being your limiting factor. That is really what your well-balanced training program should look like.
