Final answer:
Muscles during resistance exercise are contracting against an opposing force, which involves either isotonic contractions (concentric or eccentric) or isometric contractions.
Step-by-step explanation:
During resistance exercise, muscles are contracting against an opposing force. This is a key characteristic of anaerobic exercise, where muscles work against resistance such as in weight lifting. Isotonic contractions occur when muscle length changes, but muscle tension remains the same. Within isotonic contractions, there are concentric contractions where the muscle shortens, and eccentric contractions where the muscle tension diminishes and the muscle lengthens. However, during an isometric contraction, the muscle tension changes but the muscle length remains the same, and the load is not moved. Examples of isometric contraction include trying to lift an immovable object or maintaining posture.