If a muscle is spindle-shaped with parallel fibers, the muscle fiber pattern is classified:

Choice A reason: Elasticity refers to the ability of muscle tissue to return to its original shape after being stretched or contracted. It is a fundamental property of muscle.

Choice B reason: Extensibility is the ability of muscle to be stretched without being damaged. This allows muscles to accommodate movement and joint flexibility.

Choice C reason: Excitability is the ability of muscle cells to respond to stimuli, typically from the nervous system. This property is essential for initiating contraction.

Choice D reason: Secretion is not a property of muscle tissue. It refers to the release of substances from glands or cells, such as hormones or enzymes, and is not involved in muscle function.

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.