Resistance training sets performed to volitional concentric failure enable standardized measurement of muscle hypertrophy over time.

Study: Similar improvements in skeletal muscle oxidative capacity after moderate (10-RM) and high repetition (20-RM) resistance training.

When people lift weights until they can't do another rep, their muscles grow — no matter if they do 10 reps or 20 reps. This study shows muscle growth happens the same way either way, so lifting to failure is a consistent way to measure muscle growth over time.

Study: Resistance training with different repetition duration to failure: effect on hypertrophy, strength and muscle activation

The study found that when people train until their muscles can't do another rep, they gain similar muscle size even if they change how fast they lift, which means measuring muscle growth this way is consistent.

Study: Pectoralis Clavicular and Sternocostal Thicknesses Increase Similarly in Response to One and Three Sets of Pec Deck Resistance Training in Untrained Young Men

The study had people lift weights until they couldn’t do any more, and measured their muscle growth the same way each time. This shows that pushing to failure helps keep muscle measurements consistent over time.

Study: Repetitions to failure versus not to failure during concurrent training in healthy elderly men: A randomized clinical trial

The study shows that lifting until you can't do another rep isn't needed for muscle growth—what matters more is the total amount of lifting. So, training to failure isn't required to measure muscle gains reliably.