Final answer:
During isotonic concentric contraction, the muscle shortens while generating force and the muscle length decreases as the muscle contracts. This is observed when lifting a weight in a bicep curl.
Step-by-step explanation:
During an isotonic concentric contraction, the muscle actively shortens as it generates force. This type of contraction occurs when the muscle develops enough force to move a load. An example is when you perform a bicep curl; as the biceps muscle contracts, it shortens, bringing the forearm up towards the shoulder, while the tension within the muscle remains constant. This shortening of the muscle while under constant tension aligns with the definition of an isotonic concentric contraction, which is characterized by a decrease in muscle length as the muscle contracts.
Isotonic concentric contraction is critical for understanding how muscles work during various physical activities, such as lifting weights. Unlike isotonic eccentric contractions, where the muscle lengthens, or isometric contractions, where the muscle length stays the same, isotonic concentric contractions involve active shortening of the muscle fibers. Therefore, the correct answer to the student's question is:
B. The length of the muscle decreases with contraction. This is consistent with the illustrated principle that during isotonic contractions, such as a bicep curl, the muscle shortens as it moves a load.