How to warm up. Warm up right before you plan to start your workout. In general, warm up by focusing first on large muscle groups, such as your hamstrings. Then you can do exercises more specific to your sport or activity, if necessary. A warm-up may cause mild sweating, but it shouldn't leave you fatigued. Here are some examples of warm-up activities: 1  To warm up for a brisk walk, walk slowly for five to 10 minutes. 2  To warm up for a run, walk briskly for five to 10 minutes. 3  To warm up for swimming, swim slowly at first and then pick up the tempo as you're able.