Arnold Schwarzenegger is basically employing isometric contractions as he flexes his arm. Charles Atlas became famous for promoting isometrics and selling isometric courses. Or did he… according to Wikipedia, “Contrary to popular opinion, The Charles Atlas “Dynamic tension” Course did not use any true isometric exercises, but rather dynamic self-resistance, that is, pitting one muscle group against another and calisthenics.” However, in the same Wiki article it says, “Resistance in isometric exercises typically involves contractions of the muscle using:
The body’s own structure and ground
Structural items (e.g., pushing against a fence)
Free weights, weight machines, or elastic equipment (e.g., holding a weight in a fixed position)
Pressure-plate-type equipment that has a digital display of maximal force.
Wikipedia is by no means the bottom line and sometimes writers wish to stand out by being contrarian or nuanced. Is this just a matter of semantics? If you are holding a weight in a fixed position, are not resisting gravity? Can it not be said that if the angle of the joint does not change and there is resistance then it is isometric and not isotonic?
Although isometrics have been around since and before Yoga and Martial Arts, physical culture, which was the precursor to modern bodybuilding, brought isometric contraction awareness to the modern era.