You want to get stronger and also improve your conditioning, but someone already warned you that mixing lifting with running or cycling is a waste of effort. The idea that cardio "eats" your muscle gains circulates so much in gyms that plenty of people just cut cardio out of their routine, or do it half-heartedly, afraid of sabotaging their strength training. This phenomenon even has a name in training literature: the interference effect. The problem is that the way it gets repeated gym to gym has little to do with what the research actually shows.
What the interference effect actually is
Strength training and aerobic training trigger different signaling pathways inside the muscle cell. Resistance exercise mainly activates the mTOR pathway, which drives protein synthesis and muscle growth. Aerobic exercise activates AMPK more strongly, a pathway tied to energy production and mitochondrial biogenesis. The theory behind the interference effect is that heavily activated AMPK can blunt mTOR signaling, competing for the same cellular resources in the window right after training.
That theory turned into a blanket rule in gym culture: anyone who wants to get big avoids the treadmill, anyone training for a marathon avoids heavy squats, as if the two worlds were simply incompatible. In practice, the size of the effect depends on how much cardio you do, at what intensity, in which modality, and how it fits into your week — variables the blanket rule ignores completely.
The fear of the interference effect usually weighs more on your mind than the effect actually does in practice.
What the numbers actually show
A meta-analysis pooling 43 studies and over a thousand participants compared people who trained strength alone against people who combined strength and endurance work. Overall, it found no significant difference in strength or hypertrophy gains between the two groups. The one area where combined training consistently came up short was explosive power, which matters most for athletes who rely on speed and jumping, and matters far less if your goal is simply to get stronger and bigger.
That doesn't mean the effect never shows up. Short-term studies, running just a few weeks, have recorded up to 31% less hypertrophy and 18% less strength gain in people combining both types of training, compared to lifting alone. That gap tends to shrink over time, as the body adapts to the total volume and training gets better distributed across the week — which suggests a lot of the problem sits in programming, not in the combination itself.