before exercising. With your doctor's permission, you can often start stretching in the hospital. Try to stretch at least once a day to get your body ready for exercise. Remember to relax and breathe through the stretches. Keep your joints slightly bent and never locked during stretching to prevent injuries. You should also avoid bouncing your muscles. Instead, stretch smoothly and hold the stretch for 10 to 30 seconds. Repeat stretches three to four times. Stretching does not improve muscle strength or heart efficiency, but it does improve your flexibility, allow you to do different types of exercises more easily, improve your balance and will relieve muscle tension. Whether you were a marathon runner before the heart attack or a couch potato, your exercise program after a heart attack starts with a walking program. Do a walking warm up for three minutes. Then work up to a pace at which you are breathing heavier than when you’re sitting, but you can still talk and carry on a conversation. Walk for approximately 5 minutes at this pace. Add about one or two minutes to your daily walk each day until you are walking 30 minutes every day.  Walk with a partner for the first several weeks and stay close to home in case you become uncomfortable or very out of breath. Carry a cell phone with you in case you need help from home or to call 911 in case of an emergency. Remember to cool down after your workout. Avoid strenuous activity during the four to six weeks after your heart attack. It takes your heart approximately six weeks to heal enough for moderate to heavy exercise even if you were in fairly good condition before your heart attack. Avoid things like: heavy lifting or pulling, vacuuming, scrubbing, sweeping, painting, running, mowing or sudden bursts of movement. You can start doing things like walking on a flat surface for a few minutes at a time, cooking, washing dishes, shopping, light gardening and light housework.  Increase your exercise time and intensity gradually without ever crossing over into...

===
Write a summary of the previous text in Vietnamese:
Trước khi luyện tập, hãy thực hiện các bài tập giãn cơ. Bắt đầu chương trình luyện tập với đi bộ. Cẩn trọng khi tăng thêm hoạt động. Tăng dần bài tập. Kiểm tra với bác sĩ trước khi thêm bài tập sức mạnh. Duy trì hoạt động trong suốt cả ngày.