Isometric exercises have been largely ignored by bodybuilders and strength athletes alike because they don't seem to help a lot with the main goal of building muscle. However, certain isometric exercises have been shown to strengthen connective tissues - ligaments, tendons, and fascia - thereby improving structural integrity of the skeletal system and mitigating the risk of injury. Added to this benefit is the ability of isometric training to activate stabilising musculature. Planks are a good example of this - though if you are squatting heavy with good form your core stabilisers are being passively activated in a more functional pattern.
Holding any posture, with or without resistance, for a prolonged period of time leads to an exhaustion of the muscles that are typically prime movers - the large muscle groups (thighs, pecs, lats, etc) as the stabiliser muscles come into play (erector spinae, transverse abdominis, rotator cuffs, etc.) to maintain the posture. These deeper muscles (or closer to the spine as in the case of the erector spinae) are typically less under the conscious control of the central nervous system and their functions are more autonomic and under the control of the subconscious (than the prime movers) - which explains why certain emotional states can affect our posture for better or worse.
|A farmer's hold with light to moderate weights helps to reinforce|
spinal alignment and prepares the breathing musculature
to work under pressure.
In all the standing lifts (eg. - squats, deadlifts, overhead press, powercleans, etc) the spine has three forces to oppose against - tension, compression, and leverage. Tension is the force of the weight pulling down on the spine, as in the top of a deadlift. Compression is the force of weight pushing down on the spine from above, as in the squat and overhead press. Leverage is force pulling against the spine when it is horizontal to any degree as in the start of a deadlift (this force is not a factor in the bench press as the weight is not loaded on the spine). Isometric holds can be used as part of a general warm up to prepare the body to protect the spine. A very submaximal weight would be appropriate in the general warm up so as to not pre-fatigue before your workout. The goal here is to change the breathing patterns to prepare for abdominal bracing, develop kinesthetic awareness and to over time strengthen the connective tissues.
Supramaximal holds, which involve holding a weight much heavier than your max, are an excellent way to fire up the nervous system before maxing out in a working set. The overcompensation resulting from shock to the nervous system will typically make the weight of the working set seem light. Supramaximal holds can be used in the top position of the squat, lockout position of the deadlift, top of the bench press, etc. One proponent of this method, Ryan Munsey, recommends using a weight that is 120-150% of your 1RM ( here you will find a 1 rep max calculator), however this only involves unracking the bar and holding in the top position (as in squats and bench presses). For more information on Ryan Munsey's method click here. Also remember when attempting this, be sensible and have a spotting partner for safety or at least have the safety pins set up just below lockout point in the power rack.
Blasting through sticking points on our lifts requires active recruitment of more muscle fibre, potentiating contraction. Especially for those who would prefer to gain maximum strength without too much weight gain, understanding how to tap into the full power of the nervous system is crucial. Utilising isometrics we can minimise risk of injury by enhancing stabilisation and overcome plateaus in our progression - ensuring that we keep on making gains well past our middle ages.