Does Cardio Cause Muscle Loss? Debunking The Myth!

By Kathit Sondhi

Cardio has always been a hot topic in the fitness world. While it's great for heart health, endurance, and fat loss, many people fear that too much cardio can lead to muscle loss. But is that really the case?

In this post, we’ll break down the science behind muscle retention during cardio, explore how different cardio types affect gains, and share real-world applications from athletes and research alike. We will study the science behind cardio and its impact on muscle mass, and how you can strike the perfect balance between endurance and strength.

Understanding the terms - cardio and muscle loss

Cardio, or aerobic exercise, refers to any activity that increases heart rate and improves the efficiency of the cardiovascular and respiratory systems. It primarily uses oxygen to generate energy and enhances endurance over time.

Muscle loss, or muscle atrophy, occurs when the rate of muscle protein breakdown (MPB) exceeds the rate of muscle protein synthesis (MPS).

Several factors contribute to muscle loss, including energy deficits, hormonal imbalances and aging.

Done right, cardio helps with fat loss and endurance without sacrificing muscle.

Short (15-30 minutes) to moderate (30-45 minutes) duration sessions of low to moderate-intensity cardio primarily burn fat for fuel, not muscle.

Steady-state cardio of typically 60+ minutes per session in a caloric deficit or 90+ minutes for endurance athletes without proper fueling, can lead to muscle breakdown.

The key is balancing intensity, recovery, and nutrition to preserve muscle while improving aerobic fitness.

Steady-state cardio isn’t bad, it just needs to be balanced with strength training and proper nutrition.

HIIT

HIIT is also a great way to burn fat while preserving muscle.

• Shorter duration but more intense, reducing muscle breakdown.

  
  

• Boosts post-exercise calorie burn (EPOC), representing the calories burned post exercise as your body restores itself to pre-workout levels. Higher-intensity workouts lead to a greater EPOC effect, keeping your metabolism elevated for hours post-exercise.

  
  

• Boosts muscle-building hormones like growth hormone and testosterone.

  
  

The best methods for doing cardio without losing muscle

Concurrent training is the answer!

Concurrent training refers to simultaneously training for both endurance (cardio) and strength (resistance training). This approach is commonly used by athletes who require both power and endurance. Think about MMA fighters, soccer players, and even CrossFit athletes.

But even for everyday people, concurrent training is a smart, time-efficient way to build strength, support heart health, manage weight, and improve overall fitness. Whether you're chasing after your kids, sitting at a desk all day, or just trying to stay healthy and active.

Cardio is a tool not a muscle killer!

The idea that cardio automatically makes you lose muscle comes from the interference effect. It is the belief that too much endurance training can hurt your strength and muscle growth (hypertrophy). But research shows it’s not that simple! Whether cardio gets in the way depends on how you structure your training. If you manage things like intensity, timing, and volume properly, you can still build muscle while improving endurance. It’s all about balance!

My coaching experience: Applying the science in real life

As a Strength & Conditioning Coach while working in a gym, I’ve had the chance to train both regular gym-goers and High-Performance athletes. One recurring concern I hear is: won’t cardio make me lose muscle?

To address this, I implemented daily undulating periodization (DUP), alternating between high and low central nervous system (CNS) days. This approach combined strength and endurance training, in a way that matched their recovery capacity.

The results were powerful:

Over the course of two mesocycles (8 weeks training phase), my athletes showed measurable improvements in both strength and endurance. Some of the key performance metrics I tracked included:

Yo-Yo intermittent recovery test scores – improved Yo-Yo test scores, indicating improved aerobic capacity and repeat sprint ability. (YO-YO Test)

Vertical jump height – Improved jump height, explosiveness and power output.

1 RM test- Improvement in maximal strength for both the squat and bench press by 8 to 10%.

What stood out most was that, far from losing muscle, these athletes gained strength, recovered faster between sessions, and improved sport-specific conditioning.

Graph showing the progress of the same athlete who improved his Maximal Strength

The takeaway: Cardio doesn’t kill gains, but poor programming does.

Muscle growth through cardio: Fact or fiction?

One study looked at a group of older men, around 68 years old on average, who followed a structured cardio routine for six months. They weren’t lifting heavy weights or doing intense resistance training, just regular walking or running. But here’s the crazy part: by the end of the program, their thigh muscles grew by 9% in cross-sectional area!

How did they do it? They exercised five days a week, gradually increasing both the duration and intensity of their workouts. By the later stages of the program, they were hitting 85% of their heart rate reserve (which is pretty intense) and sustaining it for 45 minutes per session. This shows that if done strategically, even traditional cardio can stimulate muscle growth, especially in the legs.

So, what’s happening under the hood here? What are the actual physiological changes taking place during cardio that can lead to muscle hypertrophy?

Mitochondrial biogenesis: More energy, more endurance

Every cell in your body has tiny powerhouses called mitochondria, responsible for producing energy. When you consistently do aerobic exercise at the appropriate intensity, your body responds by creating more mitochondria within muscle cells, a process called mitochondrial biogenesis. More mitochondria means:

Better energy production, so your muscles have more fuel to keep going.

Improved endurance, helping you sustain activity for longer.

Enhanced muscle function, which supports growth and prevents fatigue-related muscle breakdown.

Protein metabolism: Building more, breaking down less

Muscles grow when your body builds more protein than it breaks down. Aerobic exercise actually helps with this by boosting protein synthesis and slowing muscle breakdown. It does this by improving mitochondria (your cells’ powerhouses), increasing blood flow and nutrient delivery, and triggering Mechanistic target of rapamycin (mTOR), a key pathway that tells your body to build and repair muscle. It’s like the master switch that controls muscle growth and recovery by regulating how your cells use nutrients to repair and grow tissue. When mTOR is activated, your muscles get the signal to grow and adapt, making them more efficient over time. Plus, aerobic training at the right intensity helps keep inflammation and stress levels in check, so your muscles recover faster.

Reduction in catabolic pathways: Preserving muscle mass

Muscle breakdown (catabolism) is a natural process, but too much of it can lead to muscle loss. Certain genes in your body control how much muscle protein gets broken down, and aerobic exercise can suppress the expression of these genes, helping to preserve muscle mass. In other words, doing cardio the right way doesn’t just build muscle, it also protects the muscle you already have from breaking down too quickly.

The above diagram suggests that Aerobic exercise training can contribute to skeletal muscle hypertrophy through various metabolic, vascular, and signaling adaptations

Combining strength and cardio works

 A 2014 study by Rønnestad & Mujika puts this theory to the test, focusing on endurance (cyclists) who are often thought to be at high risk of muscle loss due to all the cardio they do.

What they did

Researchers split trained cyclists into two groups:

1) Cycling + Strength Training (Concurrent Training)

2) Cycling Only (Endurance Training Alone)

Both groups stuck to their usual high-volume cardio routines, but one group also hit the weights multiple times per week.

What they found

The cyclists who combined strength training with cardio:

• Gained leg muscle strength

• Boosted power output (which is HUGE for performance)

• Did not lose muscle mass, even with all that cycling!

  
  

Meanwhile, the endurance-only group? They maintained their endurance but saw no improvements in strength or power.

What this means for You

If elite cyclists, who do hours of cardio every week, can keep and even build muscle with proper training, you’re not going to lose your gains just by adding some cardio!

So next time someone says “Cardio kills gains,” hit them with this study!

Final thoughts

Cardio doesn't automatically lead to muscle loss. Too much steady-state cardio in a calorie deficit might cause muscle breakdown, but if cardio is planned well with strength training, it can help retain or even grow muscle. HIIT is a good option to burn fat while keeping muscle. Even endurance training can help with muscle growth if done right. And yes, bodyweight training works too, especially for beginners or those restarting. The main thing is to balance cardio with strength work to perform better and maintain muscle.

References

1. Tipton, K. D., & Wolfe, R. R. (2001). Exercise, protein metabolism, and muscle growth. International Journal of Sport Nutrition and Exercise Metabolism.

2. Konopka, A.R. and Harber, M.P., 2014. Skeletal muscle hypertrophy after aerobic exercise training. Exercise and sport sciences reviews, 42(2), pp.53-61.

3. Phillips, S. M., et al. (2009). The role of muscle protein synthesis in recovery and adaptation to exercise. Journal of Sports Science.

4. McCarthy, J. P., et al. (2002). Compatibility of concurrent strength and endurance training. Medicine & Science in Sports & Exercise.

5. Schwartz RS, Shuman WP, Larson V, Cain KC, Fellingham GW, Beard JC, Kahn SE, Stratton JR, Cerqueira MD, Abrass IB. The effect of intensive endurance exercise training on body fat distribution in young and older men. Metabolism. 1991;40(5):545–51. doi: 10.1016/0026-0495(91)90239-s.

6. Rønnestad, B. R., & Mujika, I. (2014). "Optimizing Strength Training for Running and Cycling Endurance Performance: A Review." Scandinavian Journal of Medicine & Science in Sports.

About the author

Kathit Sondhi is a Gold medalist in international combat sports and a coach at Netrin, specializing in heart rate-based training. With elite-level athletic experience and deep expertise in performance optimization, he is passionate about helping individuals train smarter, maximize results, and unlock their full potential through data-driven fitness strategies.