When failure adds stimulus and when it adds fatigue
Training to momentary muscular failure means continuing a set until another full repetition is impossible despite maximal effort. Its value appears to depend on the exercise. On isolation movements such as curls, leg extensions, or lateral raises, the fatigue cost is localized and pushing to failure can squeeze out additional recruitment of the target muscle.
On heavy multi-joint lifts such as squats and deadlifts, reaching true failure generates large systemic and technical fatigue, often before any extra growth stimulus is gained, and it raises the risk of form breakdown under heavy load.
Using failure selectively
Because of this difference, many programs reserve genuine failure for the last set of isolation work or machine exercises, where a missed rep is safe, and keep a rep or two in reserve on heavy compounds to preserve technique and recovery. Research suggests stopping a few reps short can match training to failure for growth while costing less fatigue.
Frequent all-out failure also lengthens recovery time and can erode performance on later sets. It is a tool to deploy deliberately, not a default for every set. This is general educational information, not coaching or medical advice.