Which hormone increases osteoclast activity to release more calcium ions into the bloodstream?

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: Irregular bones have complex shapes and serve various functions, such as protection and muscle attachment, but they are not specialized for weight-bearing or resistance to twisting.

Choice B reason: Trabecular bone, also known as spongy bone, is lightweight and porous. It helps absorb shock and supports bone marrow but is not optimized for resisting twisting stresses.

Choice C reason: Interstitial bone refers to remnants of old osteons found between newer ones in compact bone. It is not a distinct tissue type adapted for mechanical stress.

Choice D reason: Compact bone is the correct answer. It is dense and organized into osteons, which provide strength and resistance to compressive and torsional forces. It forms the outer layer of bones and is especially prominent in the diaphysis of long bones.

Choice E reason: Spongy bone is adapted for shock absorption and lightness but lacks the dense structure needed to resist twisting and heavy loads.