Final answer:
Muscle fibers produce maximal force at an optimal length, known as Lo. A fiber producing 50g of force is likely not at its maximal force since it is away from Lo. The actual force produced by a fast fiber compared to another type cannot be precisely doubled without additional information.
Step-by-step explanation:
If a muscle fiber that is NOT at Lo (optimal length) produces 50g of force, it implies that the fiber is not generating its maximal force. This is because muscle fibers generate the most force when they are at an optimal length where the overlap between the thick and thin filaments in sarcomeres is ideal for cross-bridge formation. Stretching the muscle fiber would indeed increase its passive tension due to the elastic elements being stretched, and also it could increase active tension if the fiber was previously less than its optimal length, allowing for more cross-bridges to form upon activation. However, it does not necessarily mean that the fiber is experiencing fatigue; other factors could be influencing its length and tension, such as the position of the muscle or neural input. The force value of 50g cannot be taken as its maximal force without knowing the conditions, such as Lo or the recruitment level of motor units. Fast fibers, as opposed to slow fibers, should produce a higher force because they hydrolyze ATP faster and cycle cross-bridges more quickly, but without baseline values, we cannot confide that a fast fiber would produce exactly 100g of force.