Isometric contraction requires a muscle to increase its force from rest to a maximum or sub maximum value over a given time, to sustain this force for another set interval and then to decrease this force to rest.
You may distinguish between explosive isometrics, which requires rapid acceleration upto the maximum force value and slow isometrics with a much longer onset to maximum force.
Each type of isometrics produces its own individual training effects. Usually, explosive isometrics will develop explosive strength which can translate to power in a sport, should the athlete need it.
The isometric contraction can be voluntary or involuntary by the reflex response of the muscle between the eccentric and concentric phases of plyometric exercises such as the depth jump.
There are many advantages to isometric training such as very little increase in body mass compared to dynamic strength work, which is great if you are an athlete that needs to compete in weight classes.
However, isometrics fatigue the nervous system and only enhance performance close to the specific joint angle that the exercise is targeting, so this needs to be taken into consideration.
In regards to non explosive isometrics, it is important to:
Hold maximal force for no longer than 8 seconds.
Perform maximal contractions for 6 repetitions.
Rest for 5-10 seconds between repetitions.
Limit isometric training to 10 minutes per session.
Finish the session with relaxation exercises.
Isometric exercises can be varied so specific exercises will depend on the reasons for their use.
Generally, most athletes will benefit from isometric core exercises such as the plank and side bridge to stabilize the spine and allow for greater force output in the arms and legs.
