Assuming that bodyfat% remains low (6-15%), how does having a lot of muscle mass affect performance in sprinting-type events (50m-400m), middle distance runs (800m-5K) and endurance runs (10K-100K)?
I have read quite some articles stating that strength training helps improve running performance, since more strength means that taking a step requires less %force output of your 1RM it is now possible to run longer and/or faster. It's also not true that strength training only works your type II muscle fibers (which aren't used that much in endurance runs). It actually strengthens your type I fibers aswell.
I've also read that neuromuscular efficiency tends to reach near-maximal state fairly soon in a resistance program (beginner stage, around 140kg 1rm proper depth squat according to stronglifts.) In order to see further reasonable strength gains, it is said that you have to increase your muscle mass. Therefore, assuming that one is fairly proficient with the big compound lifts, the most important factor in strength differences should be muscle mass.
So since muscle mass is very heavily related to strength, and strength gains improve running performance, does muscle mass really slow me down in running?