In an isotonic contraction, the muscle:
rapidly resynthesizes creatine phosphate and ATP
maintains the same length while generating tension
changes length while maintaining constant tension
remains relaxed and passive during movement
The Correct Answer is C
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.
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Related Questions
Correct Answer is B
Explanation
Choice A reason: Myosin is the thick filament in muscle fibers and is not directly attached to the Z disc. It anchors in the center of the sarcomere at the M line and interacts with actin during contraction.
Choice B reason: Actin is the correct answer. It forms the thin filaments in muscle fibers and is directly anchored to the Z disc. This attachment allows actin to slide past myosin during muscle contraction, shortening the sarcomere.
Choice C reason: Troponin is a regulatory protein complex that binds calcium and controls the position of tropomyosin on actin filaments. It is located along the actin filament but is not attached to the Z disc.
Choice D reason: Myofibrils are bundles of actin and myosin filaments organized into sarcomeres. While they span the length of the muscle fiber and include Z discs, they are not themselves attached to the Z disc.
Choice E reason: Tropomyosin is a regulatory protein that lies along the actin filament and blocks myosin-binding sites in a relaxed muscle. It is not anchored to the Z disc.
Correct Answer is B
Explanation
Choice A reason: Symphyses are cartilaginous joints where bones are joined by fibrocartilage. They allow limited movement, such as in the pubic symphysis, but are not the most movable joints.
Choice B reason: Synovial joints are the most movable type of joints in the body. They feature a joint cavity filled with synovial fluid, which reduces friction and allows for a wide range of motion. Examples include the shoulder, hip, and knee joints.
Choice C reason: Syndesmoses are fibrous joints where bones are connected by ligaments. They allow more movement than sutures but are still relatively limited compared to synovial joints.
Choice D reason: Gomphoses are immovable joints found between teeth and their sockets. These joints are highly stable but do not permit movement.
Choice E reason: Synchondroses are cartilaginous joints where bones are joined by hyaline cartilage. These joints are typically immovable or allow very slight movement, such as in the epiphyseal plates of growing bones.
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