It depends on the type of isometric as to whether it can be performed on a daily basis. Overcoming isometrics don’t overtax the nervous system and don’t break down the muscle fibers sufficiently to warrant having to take a recovery day. Yielding isometrics can progress into an eccentric repetition which does break down the muscle, necessitating time for the muscle to repair itself.
What are isometrics?
The term “isometric” comes from the Greek words “Isos” (equal) and “metria” (measuring), describing the fact that, in an isometric contraction, the muscle contracts without changing in length.
By way of example, imagine holding a dumbell in front of you. Your muscles will be contracting just to hold it in place. The length of the muscles will remain fixed and your arms and shoulders won’t move.
Attempting to prevent the movement of a heavy weight in this way is called a ‘yielding isometric’.
If the weight is heavy enough, it won’t be long until your muscles tire and you have to lower the weight. It then becomes an eccentric movement, where the muscle is lengthened under load.
Lengthening a muscle when it’s holding a weight causes micro-tears to form in the muscle. As these tears are repaired by the body, the muscle gets thicker and stronger to overcompensate for the damage that has been done.
Yielding isometrics allows for measurable progress. You can time how long you can keep the weight in a fixed position, increasing it as you become stronger. Care needs to be taken as you begin to manage heavy weights to prevent a risk of injury.
Contracting the muscle when holding a weight also damages the muscle but not to the same extent as an eccentric movement. It’s why you may need more time for your body to recover when performing a yielding isometric if it ends in a lowering of the weight under load.
There is also another type of isometric technique. Instead of holding a heavy weight in place, imagine pushing or pulling against an immovable object with sustained, near maximal effort for around 5-7 seconds at a time. Pushing against an immovable object in this way is called an ‘overcoming isometric’.
In a yielding isometric, the individual is exerting the exact amount of force required to keep the weight fixed in space. In an overcoming isometric, more force can be exerted but the object will remain motionless.
In an overcoming isometric, muscle damage is still being done so the muscle will become stronger but it doesn’t involve an eccentric movement so the muscle damage will be less.
This is the reason why you can train isometrically everyday without overtraining.