When working out, training to failure has been researched and many benefits have been found to come from it. So what is training until failure?

There are two forms of failure when exercising; mechanical failure and muscle failure.

Mechanical failure is defined as experiencing muscle fatigue until the point when your muscles and nervous system make a weight feel like it is too heavy to complete a full repetition. This is due to your muscles and nervous system becoming too overwhelmed and fatigued that they cannot generate enough force to contract against the resistance being caused by the weight. Essentially you would only be able to finish a rep if you lowered the weight being used to a lighter weight or had someone assist you by spotting your lift.

On the other hand metabolic failure involves performing a movement until your neurological system and muscles feel a burning sensation due to a build up of acid in the muscle group you are training. 

When training it has been shown that training until metabolic failure allows you to build muscle mass while continuously practicing progressive overload. During your workouts you should be aiming to train until failure as long as you’re safely doing so. This means metabolic failure should be the goal and not necessarily mechanical failure. 

An example of metabolic failure would be squatting a weight that allows you to feel your muscles getting pumped and burning due to the acid build up, while mechanical failure would be not having enough force to squat the weight up and either getting stuck at the bottom or having to bail out of the squat. The first option would always be the safest one, that can also get you results.