Why do distance runners often limit muscle-bulking work while still doing heavy strength training?

Answer: Added mass costs energy, while strength can improve without much bulk

Any heavy weight permanently damages a runner’s joints — Heavy strength training does not inevitably damage a runner’s joints. Its effects depend on programming and the athlete, and it can improve running economy rather than automatically impair it.

Added mass costs energy, while strength can improve without much bulk ✓ — Extra mass increases the load carried while running, but strength gains need not require large increases in muscle size. Research finds that well-programmed heavy resistance training can improve running economy, so avoiding bulk is not the same as avoiding heavy weights.

Heavy training inevitably turns endurance fibers into sprint fibers — Heavy lifting does not inevitably convert all endurance-oriented muscle fibers into sprint fibers. Strength and endurance training can be combined, with adaptations depending on training dose and recovery.