Final answer:
Muscle contraction involves the shortening of muscle fibers to produce movement or increase tension. Isotonic contractions change muscle length while maintaining tension, with concentric contractions specifically involving muscle shortening as seen in bicep curls.
Step-by-step explanation:
Muscle contraction occurs when there is either an increase in muscle tension or a shortening of the muscle itself. Isotonic contractions are a type of muscle contraction in which the muscle length changes but the tension remains the same, allowing a load to be moved as the muscle shortens. This is further divided into concentric and eccentric contractions.
Concentric contraction is a subtype of isotonic contraction where the muscle actively shortens to produce movement and decrease the angle of the joint. For example, when lifting a hand weight, the biceps brachii contracts, and this causes the angle at the elbow to decrease as the forearm moves toward the body. Muscle fibers shorten as sarcomeres contract and the actin and myosin filaments slide past each other.