Question

the ______ shorten(s) during muscle contraction
z lines
actin
thick filament
sarcomere

Explanation:

• Z - lines are structural components that define the boundaries of a sarcomere and do not shorten during muscle contraction.
• Actin (thin filament) and myosin (thick filament) are the contractile proteins. However, during muscle contraction, the length of actin and myosin filaments themselves does not change. Instead, they slide over each other.
• A sarcomere is the basic unit of muscle contraction. During muscle contraction, the sarcomere shortens as the actin filaments slide over the myosin filaments towards the center of the sarcomere.

Answer:

sarcomere