The Old Fear: The Interference Effect
For years, a concept known as the "interference effect" has dominated fitness conversations. The theory, which dates back to a foundational study in 1980, suggested that combining cardio and strength training in the same program could be counterproductive.
The idea was that the two types of exercise send competing signals to your muscles. Aerobic exercise signals the body to build endurance, while resistance training signals it to build size and strength. The fear was that these competing signals would blunt the body's ability to adapt fully to either, essentially 'killing your gains'. This led many serious lifters to treat cardio as the enemy, while endurance athletes often avoided the weight room for fear of gaining unwanted bulk. But as with many things in exercise science, the reality is far more nuanced.
What the Latest Science Reveals
Recent large-scale studies and meta-analyses are rewriting the rulebook on concurrent training. A landmark study from the British Journal of Sports Medicine, which followed over 147,000 adults for up to three decades, found that combining strength and aerobic exercise delivered the strongest overall survival benefits. Participants who mixed both types of training had a significantly lower risk of death from any cause compared to those who did only one or neither. Specifically, the sweet spot for strength training appeared to be around 90-120 minutes per week, which, when paired with sufficient aerobic activity, yielded the most significant health rewards. Another comprehensive 2022 meta-analysis of 43 studies concluded that for the average person, concurrent training did not meaningfully reduce gains in overall muscle size or maximal strength. The old fears, it seems, were largely overstated for most people.
Does the Order of Exercise Matter?
This is the million-dollar question for many. A 2025 study provided a direct comparison: one group did cardio before weights, while the other lifted first. After 12 weeks, both groups improved. However, the group that lifted weights first saw substantially greater gains in maximum strength, explosive power, lean body mass, and muscle endurance. The reason? Cardio can create central fatigue, which may compromise your ability to lift with maximum intensity afterward. Lifting weights first generally does not have the same negative impact on the subsequent cardio session. For this reason, if your primary goal is building muscle and strength, the evidence strongly suggests you should hit the weights first. If your main goal is improving cardiovascular endurance, doing cardio first is fine, but be aware it may slightly reduce your strength performance in that session.
The Benefits Go Beyond the Gym
The advantages of mixing exercise aren't just for athletes. Research from Iowa State University showed that a workout routine split evenly between aerobic and resistance training reduced cardiovascular disease risk factors just as much as an aerobic-only plan. This is great news for those who find endless cardio boring or have joint issues that make running difficult. A combined routine offers more variety and the unique benefit of improving muscle strength, which is crucial for metabolic health and daily function. Furthermore, for individuals with heart failure, a 2026 meta-analysis found that adding strength training to an aerobic rehab program led to significantly greater improvements in cardiorespiratory fitness and functional capacity than aerobics alone.
Making It Work for You
So, how can you apply this? For general health and fitness, the consensus is clear: do both. A good starting point is the recommended 150 minutes of moderate aerobic activity per week, plus at least two full-body strength training sessions. If you're short on time, you can combine them. Lift weights for 30 minutes, then hop on a bike or treadmill for 30 minutes. If you have more time or are focused on performance, separating your sessions can be ideal. You could lift in the morning and run in the evening, or alternate days. Research suggests separating intense sessions by at least six hours can help minimize any potential interference. Ultimately, the best routine is the one you can stick with consistently. The science shows that for most people, the question isn't whether to combine cardio and strength, but how to best fit both into your life.
