P90X provides an intense workout via DVDs that you workout along
with. If you don't want to invest in a program such as that, you
can try doing sprints, push-ups, and calisthenics to build
muscle.
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The Navy Seal Workout is actually pretty intense. It includes a
lot of exercises to build core strength as well as cardio. Plan on
breaking a sweat!
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Yes, dance is a great form of fun and is an intense workout!
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The ideal length of time for a cool- down following an intense
workout is 5 to 15 min.