Using a single-joint “isolation” movement to failure before performing a heavier multi-joint “compound” movement is performed is called pre-exhaustion training. A practical example would be leg extensions before front squats (for the quadriceps) or cable flyes before the bench press for the chest.
Arnold Schwarzenegger popularized this technique in the movie Pumping Iron. If you watched it, you’ll remember Arnold’s performing leg extensions before squats.
Though a seemingly unusual practice, the idea behind pre-exhaustion training is this: When you fatigue the prime mover muscle with an isolation exercise prior to a heavier compound movement, you will foster greater muscle fiber recruitment because muscular fatigue will set in before neurological fatigue does.
Compound movements require a far greater degree of neuromuscular activity than single joint movements do. Theoretically, you’ll get the best of both worlds by inserting pre-exhaustion training into your repertoire, as you’ll recruit more muscle fibers, which will ultimately lead to much greater muscle growth.
Some prominent coaches and trainers believe pre-exhaustion training is more friendly to the joints, as muscular fatigue sets in prior to training heavy compound movements. Thus muscular fatigue can now be achieved using lighter loads yet still yield hypertrophic benefits.
All of this sounds great! But what does science have to say?
One 2003 study in the Journal of Strength and Conditioning Research conducted with 17 men showed the effect of pre-exhaustion training on lower-extremity muscle activation during the leg press. Prior to performing the leg press exercise, subjects performed a 10-repetition maximum in the leg extension; then a 10-repetition maximum was performed in the leg press.
Muscle activation was measured using electromyography (EMG), which showed that activity of the quadriceps, or target muscle, was significantly less when subjects were pre-exhausted. Judging the muscle-building effect of an exercise requires more than an EMG reading, but the subjects were able to complete more repetitions and use more weight on the leg press when not in a pre-exhausted state.
The conclusion of this study was contrary to most bodybuilders’ belief that pre-exhaustion training is an effective strategy for increasing muscle fiber recruitment.
A 2007 study in Brazil titled “Effects of Exercise Order on Upper-Body Muscle Activation and Exercise Performance,” produced a similar conclusion. The study, which also used EMG, involved performing repetitions on the machine pec deck, prior to the bench press, in a pre-exhaust style.
The study demonstrated that the chest muscles were no more efficiently recruited using a pre-exhaustion technique, as EMG signals confirmed. The only muscle that had a higher EMG signal during the bench press was the triceps, and this was simply because the chest was fatigued, and motor units from the pectoralis region could not be as effectively recruited. This study concluded that if you want to become better at a particular exercise, perform it first in the training session.
Pre-exhaustion training will not lead to greater muscle fiber recruitment or even to greater joint safety for that matter. This is due to fatigue in the muscles that are normally used as prime movers during a compound movement; that alters the motor pattern of the compound movement, resulting in less efficient and even unsafe technical execution of compound lifting movements.
The majority of pre-exhaustion training benefits are simply “bro science” and therefore should be used sparingly (if at all) in a training program.