Final answer:
Twitch duration decreases with heavy loads. Hence the correct answer is option B
Step-by-step explanation:
The correct answer to the question 'What happens to twitch duration with heavy loads?' is b. decreases. Twitch duration refers to the amount of time it takes for a muscle fiber to contract and relax in response to a stimulus. When heavy loads are applied to the muscle, the twitch duration decreases because the muscle fibers are recruited to generate more force, resulting in faster contractions and relaxations.