Training to muscular failure impairs strength progress by increasing fatigue without providing meaningful neural adaptation or mechanical benefit.

Study: Influence of Resistance Training Proximity-to-Failure, Determined by Repetitions-in-Reserve, on Neuromuscular Fatigue in Resistance-Trained Males and Females

Training muscles until you can't do another rep makes you much more tired and slows you down the next day, compared to stopping a few reps short. Being less tired means you can recover better and train more effectively over time.

Study: The Effect of Resistance Training Proximity to Failure on Muscular Adaptations and Longitudinal Fatigue in Trained Men

When people lift weights until they can't do another rep, they actually gain less strength than when they stop a few reps short. The study found that stopping before total failure led to bigger strength gains, even though both groups worked hard.

Study: Acute and Delayed Effects of a Resistance Training Session Leading to Muscular Failure on Mechanical, Metabolic, and Perceptual Responses.

This study found that lifting weights until you can't do another rep leaves you really tired for days, and your muscles and nerves don't work as well afterward — without making you stronger faster. So, going to failure might hurt more than help.