Mechanical Tension Is What Matters Most Muscle growth primarily comes from mechanical tension. You can create this with: Slow, controlled reps High reps to near-failure or failure Constant tension and perfect form Training Close to Failure Light weights like 20 lbs are effective if you take your sets close to failure (0–2 RIR). That means the last few reps should feel brutally hard. Smart Exercise Selection With dumbbells, you can train nearly every muscle group with intelligent programming.