Which of the following is an example of an isometric exercise?

An isometric exercise involves maintaining a muscle contraction without changing the muscle length or moving the joint angle. The plank hold is an excellent example of this type of exercise as it requires the individual to hold a position, engaging the core muscles and other stabilizing muscles while keeping the body steady without any movement.

In contrast, squats involve dynamic movement where the body lowers and raises, thus changing the length of the muscle and the position of the joints. Jumping jacks are also a dynamic exercise that involves multiple movements and does not involve sustained muscle tension in one position. Lastly, bicep curls require movement that shortens and lengthens the muscle during the lifting and lowering phases, making it a dynamic exercise rather than isometric.

Therefore, the plank hold is the only option that exemplifies isometric contraction, where the muscle is engaged while remaining static.