Isometric exercise
An isometric exercise involves muscles producing tension while the joint position and muscle length remain approximately the same.
In everyday words
Holding a position or pushing against an object that does not move can involve isometric muscle work. The effort happens even though there is little visible movement.
What to keep in mind
Still does not necessarily mean easy or suitable for everyone. The exercise, effort, and hold duration need to fit the person. Ask for advice if you are unsure, especially if you have cardiovascular concerns.