Final answer:
Muscle force decreases at very short muscle lengths due to a reduction in the overlap between actin and myosin filaments, impairing cross-bridge formation.
Step-by-step explanation:
The amount of force that can be produced by a muscle drops off when the muscle is activated at very short lengths because there is very little overlap between actin and myosin filaments. During a muscle contraction, actin and myosin filaments slide past one another to produce muscle shortening. The force a muscle can generate is greatly influenced by the length-tension relationship, which is optimal when there is a maximal overlap allowing for efficient cross-bridge formation. If the muscle is too short, this overlap is reduced, resulting in a decrease in the potential force.