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According to the passage, exercise to make the heart muscle stronger ----.
Fitness is determined more by the intensity of exercise
than the duration. Workouts should be energetic enough
that the muscles are somewhat sore the next day but
fully recovered the day after that. To strengthen the
heart, exercise must be performed at an intensity that
increases heart rate at least 20 beats above the resting
heart rate. The harder a person exercises, the faster the
heart beats and the stronger the heart muscle becomes.
Heart rate is determined by how hard the skeletal
muscles contract. When a person starts to exercise, the
skeletal muscles contract and squeeze the veins near
them, forcing blood towards the heart. When the skeletal
muscles relax, these veins fill with blood. The alternating
contraction and relaxation of the skeletal muscles serve
as a second heart, pumping extra blood to the heart. The
increased blood flow causes the heart to beat faster and
more forcefully. So the harder the skeletal muscles
contract, the faster the heart beats.