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.