What structure in skeletal muscle cells functions in calcium storage?

Choice A reason: This choice confuses metabolic processes with mechanical muscle activity. While creatine phosphate and ATP are essential for muscle contraction, their resynthesis is not specific to isotonic contraction. These energy molecules are replenished during recovery phases and are not the defining feature of isotonic contractions.

Choice B reason: This describes isometric contraction, not isotonic. In isometric contractions, the muscle generates force without changing its length—such as holding a plank position. Isotonic contractions, by contrast, involve a change in muscle length.

Choice C reason: This is the correct answer. Isotonic contractions involve the muscle changing length while maintaining constant tension. There are two types: concentric (muscle shortens) and eccentric (muscle lengthens). These contractions are typical in dynamic movements like lifting or lowering weights.

Choice D reason: This is incorrect. Muscles do not remain relaxed during isotonic contractions. Instead, they actively contract and generate force to produce movement. Passive relaxation does not contribute to the mechanics of isotonic contraction.

Choice A reason: A meatus is a canal-like passageway through bone, such as the external auditory meatus in the skull. It does not describe a rounded projection.

Choice B reason: A foramen is a hole or opening in a bone that allows the passage of nerves and blood vessels. It is not a projection.

Choice C reason: A condyle is a rounded articular projection that typically forms part of a joint. It allows for smooth articulation with another bone, making it the correct answer.

Choice D reason: A fossa is a shallow depression in a bone, often serving as an articular surface or for muscle attachment. It is not a projection.