Researchers wanted to know if doing both strength training and cardio exercise together would hurt your ability to build muscle. They had 19 young men do either strength training alone or a combination of strength training plus high-intensity cardio for 16 weeks. By measuring muscle tissue samples, they found that both groups built muscle at similar rates. The group doing both types of exercise got stronger in different ways—the cardio group had better heart fitness, while the strength-only group got more powerful. This suggests that doing both types of exercise together doesn’t actually prevent muscle growth like scientists once thought it might.

The Quick Take

  • What they studied: Whether doing cardio exercise at the same time as strength training prevents your muscles from growing bigger
  • Who participated: 19 young men who had never done structured exercise before, split into two groups for 16 weeks of training
  • Key finding: Both groups showed similar muscle growth at the cellular level, suggesting that combining cardio and strength training doesn’t block muscle development like previous research suggested it might
  • What it means for you: If you want to build muscle and improve your heart health at the same time, you may not have to choose between them. However, if pure strength gains are your only goal, doing strength training alone might give you slightly better results

The Research Details

This was a controlled experiment where researchers randomly divided 19 untrained young men into two groups. One group did only resistance training (lifting weights), while the other group did resistance training combined with high-intensity interval cardio workouts. Both groups trained for 16 weeks. The researchers took small tissue samples from the participants’ muscles at the beginning, after 4 weeks, and after 16 weeks to measure what was happening inside the muscle cells. They also tested how strong each person got and how well their heart could work by the end of the study.

The muscle samples were analyzed using advanced techniques to measure how much new muscle protein was being made, how many special muscle-building cells were present, and how the genes controlling muscle growth were behaving. This allowed the scientists to understand the biological mechanisms behind muscle growth, not just whether muscles got bigger.

Understanding the actual biological processes helps scientists and athletes know whether combining different types of exercise is actually harmful or just feels that way. Previous studies suggested that doing cardio and strength training together might interfere with each other, but this research looks at the actual cellular mechanisms to see if that’s really true. This approach is more reliable than just measuring final results.

This study was published in a respected scientific journal and used rigorous methods including muscle biopsies and genetic analysis. The researchers randomly assigned participants to groups, which reduces bias. However, the sample size of 19 people is relatively small, and all participants were young, untrained men, so results may not apply to women, older adults, or experienced athletes. The study lasted 16 weeks, which is a reasonable timeframe for seeing muscle changes.

What the Results Show

The main finding was that muscle protein synthesis—the process of building new muscle—increased similarly in both groups 48 hours after exercise at both the 4-week and 16-week checkpoints. This means the bodies of people doing combined training were building muscle at the same rate as those doing strength training alone.

Special muscle-building cells called satellite cells increased over time, particularly in the fast-twitch muscle fibers (the ones used for powerful movements). The number of nuclei in muscle cells (which are needed for muscle growth) increased in both types of fibers, with the combined training group showing slightly higher numbers in slow-twitch fibers.

Genes that control muscle growth showed expected patterns, with some increasing over the 16 weeks and others responding specifically to the combined training approach. Type II muscle fibers (fast-twitch) grew larger in both groups, with no significant difference between them.

Strength improvements were greater in the resistance-training-only group, which makes sense since that group focused entirely on lifting. However, the combined training group showed much better improvements in aerobic capacity (heart and lung fitness), which was expected since they included cardio workouts. This suggests that the two types of training produce different benefits: pure strength training is better for getting stronger, while combined training improves overall fitness.

Earlier research suggested that doing cardio and strength training together created an ‘interference effect’ that blocked muscle growth. This study challenges that idea by showing that at the cellular level, muscle-building processes work similarly in both groups. The difference may be that previous studies looked only at final muscle size, while this research examined the actual biological mechanisms, revealing that the process of building muscle isn’t actually blocked—it just might be distributed differently between strength and endurance adaptations.

The study only included 19 young men who had never trained before, so results may not apply to women, older adults, or people who already exercise regularly. The study lasted 16 weeks, which is relatively short for seeing all possible long-term effects. The researchers didn’t measure actual muscle size gains directly in all analyses, relying partly on cellular measurements. Additionally, the specific type of cardio used (long-interval high-intensity training) may produce different results than other cardio approaches like steady-state running or moderate-intensity exercise.

The Bottom Line

If you’re a young, untrained person wanting to build muscle and improve fitness, combining strength training with high-intensity cardio appears to be safe and effective. You’ll likely see good muscle growth and excellent cardiovascular improvements. If your only goal is maximum strength gains, doing strength training alone may give you slightly better results. (Moderate confidence—based on a small study of young men only)

Young people starting an exercise program who want both muscle growth and better heart health should find this encouraging. Athletes who worry that cardio will ruin their muscle gains can feel more confident about combining training types. However, older adults, women, and experienced athletes should be cautious about applying these results directly to themselves, as the study didn’t include these groups.

Muscle growth typically becomes noticeable after 4-6 weeks of consistent training, with more significant changes by 8-12 weeks. Cardiovascular improvements can happen even faster, sometimes within 2-3 weeks. By 16 weeks of consistent training, you should see clear differences in both muscle size and fitness level.

Want to Apply This Research?

  • Log both strength training sessions (exercises, weights, reps) and cardio workouts (type, duration, intensity) separately. Track weekly progress in one key strength exercise (like bench press or squat) and one cardio metric (like distance covered or average heart rate during intervals). Compare progress every 4 weeks.
  • Set up a weekly schedule with 3 strength training days and 2-3 cardio days, allowing rest days between intense sessions. Use the app to send reminders for each workout type and track completion. This helps ensure you’re actually doing both types of exercise consistently rather than skipping one.
  • Every 4 weeks, take measurements of muscle size (using a tape measure on arms, chest, legs) and test cardiovascular fitness (time a standard run or measure heart rate recovery). Log these in the app to visualize progress over 16 weeks. This mirrors the study’s approach of checking progress at regular intervals.

This research applies specifically to young, previously untrained men and may not reflect results for women, older adults, or experienced athletes. Always consult with a healthcare provider or certified fitness professional before starting a new exercise program, especially if you have any existing health conditions. This study shows what happened in a controlled research setting; individual results will vary based on genetics, nutrition, sleep, and training consistency. The findings suggest combined training is safe, but they don’t replace personalized medical advice.