Final answer:
A muscle fiber would generate its maximal active force when eccentrically contracting near its optimal length, due to the formation of the maximum number of cross-bridges within the sarcomeres at this length.
Step-by-step explanation:
When considering the scenarios that would lead a muscle fiber to generate its maximal active force, it's important to understand the length-tension relationship of a sarcomere. The optimal length of a sarcomere is where the zone of overlap between the actin and myosin filaments is greatest, allowing for the maximum number of cross-bridges to form. This condition is typically met when a muscle fiber is eccentrically contracting near its optimal length. In this scenario, the muscle is lengthening even though it is contracting, which tends to involve more tension than during shortening (concentric) contractions.
A concentric contraction is when a muscle fiber shortens to move a load, like lifting a weight with the biceps. Though the muscle generates tension, it is usually not at the maximal force possible due to the changing length of the muscle fibers. On the other hand, eccentric contractions tend to create more tension, as they involve controlled lengthening of the muscle fibers while still producing force, such as when lowering a weight in a controlled manner.