Why do our muscles get tired?

What is the first step that causes a muscle to contract?

• The muscle runs out of energy.

• The brain sends a signal through motor neurons.

• Lactic acid builds up in the muscle.

• Calcium is released inside the muscle cell.

What happens to the charged particles (ions) on the muscle cell membrane when it receives a signal to contract?

• Sodium ions flow out, and potassium ions flow in.

• Both sodium and potassium ions flow out.

• Both sodium and potassium ions flow in.

• Sodium ions flow in, and potassium ions flow out.

What molecule is responsible for providing the energy needed for muscle contraction and for resetting the balance of ions?

• Acetylcholine

• Lactic acid

• ATP

• Calcium

What is the primary reason a muscle becomes fatigued and can no longer contract effectively?

• The muscle runs out of ATP.

• Too much lactic acid accumulates in the muscle.

• There are not enough necessary ions near the muscle cell membrane to generate an electrical signal.

• The brain stops sending signals to the muscle.