Which muscle action involves both shortening and lengthening while generating force against an external load?

The action that involves both shortening and lengthening while generating force against an external load is isotonic contraction. Isotonic contractions are characterized by the muscle changing length while maintaining a constant tension against a resistance. This includes two subcategories: concentric contractions, where the muscle shortens as it contracts (like lifting a weight), and eccentric contractions, where the muscle lengthens while still producing tension (like lowering a weight).

In practical terms, during an isotonic muscle action, the muscle is actively working against a load, allowing for movements that involve both lifting and lowering phases. This is essential for various types of physical activities and exercises, such as squats or bicep curls, where the muscle must perform both actions for effective and efficient motion through a complete range of motion. This dynamic aspect of muscle action is vital for enhancing strength, endurance, and overall functional fitness.