Question

as you are lifting a box, someone places extra weight on top of it. for your muscle to continue contracting and lifting the box, the muscle must
multiple choice
recruit more muscle fibers
lower its threshold
reduce its wave summation
shift from isometric to isotonic contraction
shift from slow - twitch to fast - twitch mode

Explanation:

When extra weight is added to the box, the muscle needs to generate more force. Muscle force generation is related to the number of muscle fibers recruited. Recruiting more muscle fibers (motor unit recruitment) increases the force output. Lowering the threshold (incorrect as it's about activation level, not directly force - increasing in this context). Reducing wave summation (would decrease force). Shifting from isometric to isotonic (not relevant here as the task is still lifting - a form of isotonic if moving). Shifting from slow - twitch to fast - twitch (not the main mechanism for immediate force increase when weight is added; recruitment is more immediate).

Answer:

recruit more muscle fibers