Can Cardio Actually Improve Muscle Hypertrophy?

For decades, lifters have repeated one lazy line like gospel: cardio kills gains. The idea sounds logical—endurance training competes with strength, recovery suffers, and muscle growth dies. Except modern science doesn’t support that fear.

In fact, updated research shows combining cardio with resistance training does not meaningfully impair muscle or strength gains in most cases. But here’s the thought almost nobody considers: what if cardio doesn’t just fail to hurt hypertrophy—what if it actually enhances long-term muscle growth?

Emerging evidence suggests that under specific conditions, aerobic training may improve capillary density, recovery capacity, and even muscle fiber hypertrophy, making it a potential ally rather than an enemy for building muscle.

The Interference Effect: Why “Cardio Kills Gains” Is Mostly a Myth

The interference effect is the main reason lifters fear cardio. The theory says endurance training and resistance training compete for adaptations, meaning cardio supposedly blunts muscle hypertrophy and strength gains. This idea came from older research, outdated protocols, and extreme training setups that don’t reflect how most people actually train.

Modern evidence paints a very different picture.

Recent meta-analyses and updated reviews show that combining cardio with resistance training does not significantly impair muscle growth or strength for the majority of lifters. In real-world training conditions—reasonable volumes, proper recovery, and smart programming—hypertrophy is largely unaffected.

Why the myth survives:

• Early studies used excessive cardio volume
• Poor calorie intake and recovery were common
• Training intensity was mismanaged
• Results were exaggerated and generalized

When these variables are controlled, the so-called interference effect becomes small, inconsistent, or nonexistent.

More importantly, most discussions stop here. They only ask whether cardio hurts gains. The more interesting and underexplored question is whether cardio can support the physiological systems that allow muscle hypertrophy to continue long term. That’s where the science starts getting uncomfortable for the anti-cardio crowd.

Cardio doesn’t just burn calories. It drives adaptations in:

• Capillary density
• Aerobic recovery capacity
• Mitochondrial function

These adaptations don’t oppose hypertrophy by default. Under certain conditions, they may actually enable greater muscle growth over time, especially by improving recovery between sets, sessions, and training blocks.

The interference effect isn’t the villain people think it is. In most cases, it’s either irrelevant—or misunderstood.

Cardio Before Lifting: Evidence That Aerobic Training Can Prime Muscle Growth

Most people assume cardio only matters when done alongside weights. A lesser-known idea is that aerobic training performed before a hypertrophy phase may actually enhance later muscle growth. A controlled Canadian study tested exactly this—and the results caught many lifters off guard.

Researchers recruited untrained individuals and used a clever design. For six weeks, only one leg performed cycling, three times per week at a moderate intensity. The other leg did nothing at all. No weights. No strength work. Just basic aerobic training for one limb.

After those six weeks, both legs trained bilaterally for ten weeks using traditional hypertrophy exercises:

• Squats
• Leg press
• Leg extensions
• Hamstring curls
• Calf raises

By the end of the study, the leg that had cycled during the first six weeks showed greater muscle hypertrophy than the leg that remained inactive. This advantage was seen in both slow-twitch and fast-twitch muscle fibers.

In simple terms, doing cardio first made the muscle more responsive to later resistance training.

The aerobic phase itself did not build large amounts of muscle. Instead, it appeared to prepare the muscle environment for growth. This finding challenges the idea that time spent on cardio is wasted for hypertrophy. Under the right conditions, it may act as a primer, improving how muscle adapts once heavy lifting begins.

This study alone doesn’t prove cardio is magic for muscle growth—but it opens the door to a mechanism most lifters never consider.

Capillary Density: The Hidden Factor That Makes Muscles Grow Better

The key reason cardio improved hypertrophy in that study wasn’t magic—it was capillary density.

After six weeks of cycling, the trained leg showed significant increases in capillary density. When researchers looked deeper, they found something critical: the greater the increase in capillaries, the greater the muscle fiber growth during the following resistance-training phase. This relationship held true for both slow-twitch and fast-twitch fibers.

High-responding muscles had:

• More capillaries
• Better nutrient delivery
• Greater hypertrophy

Low-responding muscles had fewer capillaries and poorer growth.

Capillaries are tiny blood vessels that deliver:

• Oxygen
• Glucose
• Amino acids
• Hormones and growth signals

Muscle hypertrophy isn’t just about mechanical tension. It also depends on how efficiently a muscle is supplied and supported. Without enough capillaries, muscle fibers struggle to sustain growth even when training volume and intensity are adequate.

This isn’t an isolated finding. Other research links higher capillary density to:

• Increased satellite cell activation
• Faster recovery from training
• Improved long-term hypertrophy

As muscles grow larger, nutrients must travel farther within each fiber. If capillary supply doesn’t increase, the muscle hits a biological bottleneck. Either growth slows—or the muscle adapts by building more capillaries.

This is where cardio starts looking less like a threat and more like a support system for continued muscle growth.

Why Bigger Muscles Need Better Blood Supply

As muscle fibers grow, a problem appears that most lifters never think about. The inside of the muscle cell becomes larger, but the distance nutrients and oxygen must travel also increases. This creates a potential bottleneck for energy delivery and recovery.

Greg Nuckols from Stronger By Science offers a clean explanation:
when muscle fibers expand, the cellular machinery inside them gets farther away from the nearest capillary. To avoid an internal energy crisis, the muscle must either stop growing or increase capillary density to keep up with demand.

This makes capillaries a limiting factor for long-term hypertrophy, not just a side detail.

Even heavy resistance training is more aerobic than most people realize. Activities commonly labeled as “anaerobic” still rely heavily on the aerobic system. For example, a 200-meter sprint—often thought of as purely anaerobic—still derives roughly 30% of its energy from aerobic metabolism.

During lifting, the aerobic system plays a major role in:

• Recovery between sets
• Performance on later sets in the same session
• Recovery between training days

When capillary density and aerobic capacity improve, muscles can recover faster and sustain higher-quality work. That means more effective volume, better training sessions, and a greater stimulus for hypertrophy over time.

In this context, cardio isn’t competing with muscle growth. It may be supporting the exact systems that allow muscle to keep growing.

Mitochondria: The Energy Engines Behind Muscle Growth

Capillaries aren’t the only endurance-related adaptation linked to hypertrophy. Mitochondria—the organelles responsible for producing energy inside muscle cells—also appear to play a major role in how much muscle a person can build.

A 2018 study from the United States found a clear pattern: individuals who experienced the greatest hypertrophy had significantly higher mitochondrial volume compared to low responders. In other words, the muscles that grew the most were also the ones with a stronger energy-producing capacity.

This matters because muscle growth is an energy-expensive process. Creating new contractile proteins, repairing tissue damage, and sustaining repeated high-quality training sessions all demand large amounts of ATP. Mitochondria supply that energy.

Greater mitochondrial content may support hypertrophy by:

• Powering protein synthesis
• Improving recovery between sets
• Sustaining performance later in a workout
• Enhancing recovery between sessions

In vitro research also links mitochondrial function to cell growth, further supporting this relationship.

Importantly, this doesn’t mean bodybuilders need endurance-athlete-level conditioning. The goal isn’t maximizing endurance adaptations. Even moderate improvements in mitochondrial volume and function may help muscles tolerate more work and recover faster, creating better conditions for long-term hypertrophy.

Once again, the adaptations driven by cardio don’t inherently oppose muscle growth. Under the right circumstances, they may help fuel it.

Concurrent Cardio and Lifting: When Doing Both May Actually Improve Hypertrophy

The idea that cardio must be separated from lifting to protect gains doesn’t hold up well under scrutiny. While most research shows that combining cardio and resistance training does not reduce muscle growth, some studies go further and show better hypertrophy with concurrent training.

Several trials report enhanced growth of slow-twitch muscle fibers when aerobic training is performed alongside resistance training. This is notable because slow-twitch fibers are typically harder to grow than fast-twitch fibers and often receive less direct stimulus from traditional low-rep, high-load lifting.

Cardio may provide that missing stimulus.

Potential benefits of concurrent training include:

• Improved slow-twitch fiber hypertrophy
• Increased capillary density during hypertrophy phases
• Better recovery between sets and sessions
• Higher training quality across the week

These effects don’t mean more cardio is better. Problems usually appear when:

• Cardio volume is excessive
• Recovery and calorie intake are insufficient
• Training intensity is poorly managed

When aerobic work is moderate and intelligently programmed, interference is minimal—and in some individuals, hypertrophy may actually be enhanced.

This suggests that cardio doesn’t have to be an “off-season” tool. For many lifters, strategically combining cardio and resistance training may be superior to avoiding cardio entirely, especially for long-term muscle development.

Using Cardio Phases and Training Breaks to Enhance Long-Term Muscle Growth

Another option that often gets ignored is alternating phases of training instead of forcing everything at once. You don’t need to lift heavy year-round for muscle growth to continue. Short breaks or shifts in focus do not erase gains the way many lifters fear.

A 2013 Japanese study compared two groups:

• One group trained continuously for 24 weeks
• Another group trained for 6 weeks, then took a 3-week break, repeating this cycle

After 24 weeks, muscle size and strength gains were similar between both groups. Although the break group temporarily lost some size and strength, their gains rebounded quickly once training resumed.

This matters because those “off” periods don’t have to be inactive. They can be used for:

• Aerobic training
• Improving work capacity
• Enhancing capillary density and mitochondrial function

Instead of wasting time, these phases may prime the body for better hypertrophy when resistance training resumes.

There’s also evidence that short breaks from lifting may help reset anabolic signaling pathways, potentially improving long-term muscle growth. Adding cardio during these phases could further enhance recovery systems without sacrificing future gains.

For highly trained lifters or those using very high resistance-training volumes, alternating between:

• Concurrent cardio + lifting phases
• Lifting-only phases

may produce better results than sticking rigidly to one approach year-round.

Higher Reps, Endurance Adaptations, and Their Role in Hypertrophy

Cardio isn’t the only way to develop endurance-related adaptations that may support muscle growth. Higher-rep resistance training can also drive improvements in local muscular endurance, capillary density, and mitochondrial function.

Higher reps:

• Still build muscle effectively
• Increase time under tension
• Place greater demand on the aerobic system

Over the long term, these adaptations may help muscles tolerate more volume and recover faster between sets and sessions. This creates a more favorable environment for sustained hypertrophy, especially when training spans many months or years.

It’s worth noting that endurance adaptations are muscle-specific. Cardio primarily benefits the muscles involved in the activity. If someone only performs lower-body cardio, upper-body muscles won’t receive the same endurance stimulus.

In those cases, higher-rep training can help:

• Improve local endurance in untrained muscle groups
• Enhance capillary development within the muscle
• Support hypertrophy without additional cardio volume

This doesn’t mean all training should be high-rep. Instead, a mix of rep ranges may be optimal—heavy loads for mechanical tension, and higher reps to support endurance-related adaptations that allow muscle growth to continue over time.

More research is needed, but the idea is consistent with multiple lines of evidence: muscles that recover better tend to grow better.

References

• Short-term aerobic conditioning prior to resistance training augments muscle hypertrophy and satellite cell content in healthy young men and women
• Muscle fibre capillarization is a critical factor in muscle fibre hypertrophy during resistance exercise training in older men
• Low skeletal muscle capillarization limits muscle adaptation to resistance exercise training in older adults

FAQs (Frequently Asked Questions)