Question

Why are muscles able to contract and expand?

Answer 1

Muscle contraction and relaxation is a link between nerves and muscle.
Each muscle fiber is composed of hundreds of smaller filaments or threads called myofibrils. Each myofibril contains bundles of threadlike proteins or filaments called myofilaments, which can be either thick(made up of myosin) or thin(made up of actin).

When a muscle fiber gets a signal from its nerve, proteins and chemicals release energy to either contract the muscle or relax it. When the muscle contracts, this pulls the bones it's connected to closer together.

Mechanism:
The electrical signal send by CNS stimulates the release of calcium ions stored within the muscle fiber.The ions attach to the thin myofilaments and remove their protective coverings. The arms of the thick myofilaments then reach out, pull the thin myofilaments toward the center of the sarcomere. This shortens the sarcomere. Muscle contraction occurs when these filaments slide over one another in a series of repetitive events.