What is the 'interference effect' when combining strength training and running?

The "interference effect" refers to the potential for endurance training (like running) to lessen some of the gains you might typically see from strength training (especially in power and possibly muscle growth) if you were only doing strength training.

This happens because endurance and strength training activate different, sometimes opposing, signaling pathways at a molecular level within your muscles. For example, the pathway activated by endurance training (AMPK, for mitochondrial growth) can potentially inhibit the pathway stimulated by strength training (mTOR, for muscle protein synthesis).

Power development (explosive strength) seems most susceptible to this interference. However, for runners, the primary goal of strength training is usually to enhance running performance (often via better running economy), not necessarily to maximize absolute strength or muscle size. So, some minor blunting of maximal strength potential might be an acceptable trade-off if running-specific benefits are achieved.