Concurrent activation potentiation (CAP) is a long-winded term that describes an interesting phenomenon. Stronger muscle contractions occur when other muscles activate at the same time other than the prime movers or synergists.
This technique may help you to conquer a final tough rep, but I argue it should come about naturally and without any thought.
Synergy
We already know that muscles work even better alongside other muscles. This occurs due to the structure of our anatomy.
If you cross your arm toward your sternum, or the flat bone between each chest muscle, you will notice that this extends your elbow slightly. This concept applies to many pairings throughout the body. The hamstrings and glutes can extend the knees while extending the hips. The upper back muscles can bend the elbow while extending the shoulders. The large thigh and hip muscles can assist with plantarflexion at both ankles.
The big muscles assist the small muscles in performing their functions. This happens during compound movements only. Exercises such as the bench press, squat, and row benefit from this synergy, not skull crushers, leg extensions, and biceps curls.
Stability
We also know that muscles act more efficiently and contract harder when they have a stable platform to express their strength.
Pulling back your shoulder blades and expanding your chest enough on the bench press will make you more compact against the pad. This allows more of your force to direct efficiently upwards instead of wasting it with side-to-side motion.
This not only seems to improve efficiency though. The nervous system will hold back the muscles if it senses a lack of stability. Perhaps otherwise could harm the joints.
Performance will improve on every exercise with better stability, but exercises involving more muscle will give more opportunities to promote this tightness.
Hormone Levels
Working more muscles at once leads to a greater hormonal response.
CAP Rationale
So we know that more muscles working than just the primary ones can enhance synergy, stability, and hormone levels. Could this also work due to any other reasons?
Dental research has shown that jaw clenching causes other muscles throughout the body to tense up. Recent research in exercise science added to this by showing higher performances during jumping and bench pressing by squeezing the fists tightly or clenching the jaw as well.
It seems a simultaneous yet remote contraction may improve performance through other means, perhaps by priming the nervous system or some other unknown reason.
Conclusions
CAP likely occurs more often than we realize. Holding your breath, for example, helps support the spine anteriorly by providing a column of air. It also tightens up the core, which could lead to CAP. Most also find that using a tight grip boosts performance, and thick bar training that prevents this will decrease the weights possible for any exercise.
I would suggest to avoid limiting your natural instincts.
Tendencies such as clenching the law, squeezing the fists, holding your breath, or gripping tightly tend to occur autonomously, unless you prevent them. By simply having the intention to work as hard as possible on an exercise, I believe that CAP will occur without a conscious effort.
Some experts will teach you to relax your face and neck. They argue that any seemingly extraneous contractions only serve to waste energy. In light of these points, this is poor advice.
I would not recommend forcing CAP or inventing ways to initiate it though, such as by tensing your calves in the bench press. This could serve as a distraction or perhaps just serve no purpose and give you one more thing to think about. The best exercises should feel simple not complex. This allows you to focus on doing your best.
Concurrent Activation Potentiation Should Occur Naturally
We may not know exactly why CAP helps. Understanding exactly why though does not change anything. Like many things, it likely comes from a combination of reasons.
We can still act on the information though. Let concurrent activation potentiation occur naturally. Any tightening that occurs in the jaw, fists, or elsewhere can boost your performance but should just happen and not be forced.