Question

the ______ tighten(s) the muscles.
muscle spindles
golgi tendon organ
muscle fibers
myofibrils

Explanation:

To solve this, we analyze each option:

• Muscle spindles: Detect muscle stretch, not tighten muscles.
• Golgi tendon organ: Monitors tendon tension, not for muscle tightening.
• Muscle fibers: Are the cells that contract to tighten (shorten) muscles.
• Myofibrils: Are within muscle fibers, responsible for contraction at a smaller scale, but muscle fibers as the cell level are the direct structures that tighten muscles. Among the options, muscle fibers are the main structures that cause muscle tightening through contraction.

Answer:

C. muscle fibers