For intramembranous ossification to take place, which of the following is necessary?

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: This is incorrect. Tropomyosin does not block myosin; it blocks the active sites on actin filaments, preventing myosin binding until calcium binds to troponin.

Choice B reason: Tropomyosin is not a chemical activator. Activation of myosin heads occurs through ATP hydrolysis and calcium-mediated exposure of actin binding sites.

Choice C reason: This is correct. Tropomyosin is a regulatory protein that wraps around actin filaments and blocks myosin-binding sites. When calcium binds to troponin, tropomyosin shifts, exposing these sites and allowing contraction to proceed.

Choice D reason: Tropomyosin does not function in the synaptic cleft nor inhibit calcium there. Calcium regulation in the synaptic cleft involves neurotransmitter release mechanisms, not tropomyosin.

Choice E reason: Tropomyosin is not a receptor. The receptor for motor neuron neurotransmitters (e.g., acetylcholine) is the nicotinic acetylcholine receptor located on the sarcolemma of the muscle cell.