Range of motion is the angular travel of a joint through its movement arc. Active ROM (under muscle control) and passive ROM (externally moved) often differ.
Mechanism — Tissues adapt to the ranges you use under load. Train in shortened range, lose end-range capacity. Train through full ROM, build strength AND mobility together.
Use case — Default to full ROM unless injury or programming dictates otherwise. Deep squats, full ROM presses, lengthened-position eccentrics for hypertrophy and tendon adaptation.
Caveats — Hyper-mobility under load is a risk. Match ROM to the trainee's joint structure and stability — not everyone needs ass-to-grass squats. The 'no half reps' rule is dogmatic; pin squats and partial-ROM lockouts have a place.