Pushing against an immovable wall with the arms extended is an example of:

Pushing against an immovable wall with the arms extended exemplifies isometric contraction because, during this activity, the muscles are activating but not changing in length. In isometric contractions, the muscle generates tension while maintaining a constant length, which means there is no visible movement occurring at the joint.

When you push against a wall, the muscles in your arms and shoulders are working hard to produce force; however, since the wall doesn't move, the muscles remain at a fixed length. This contrasts with other types of contractions, such as concentric, where the muscle shortens, or eccentric, where the muscle lengthens while resisting a force. Isokinetic contraction, on the other hand, occurs at a constant speed throughout the movement and is not applicable in this scenario because there is no motion. Thus, the correct identification of this action as isometric efficiently highlights the specific type of muscle contraction taking place.