If your plan is to get visible abdominals in a week, it's time to set a fitness goal that's a little more realistic. Given that it's typically safe to only lose a couple pounds of fat per week, it's impossible to transform your body to the point of having abs in such a short time frame. If you're serious about this goal, however, a strict diet and training regimen that includes cardio and strength training can lead you to your goal. Although no concrete time frame exists, it might be possible to see some signs of abs in a couple months.
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Overhaul your diet for the short term. Because no amount of abdominal exercise will eliminate the fat covering your abs, you'll have to burn as much of it as you can through traditional means such as diet. For the duration of the week, eliminate all carbohydrates in your diet, with the exception of breakfast, and replace them with protein. This will serve to decrease your total caloric intake, causing your body to burn fat, and provide your abdominal muscles with the necessary protein to grow and recover. Don't worry about the fat in protein sources; low carbohydrate diets consistently outperform their low-fat counterparts.
Perform cardiovascular exercise for at least 30 minutes every day of the week. If you're able, increase this duration to 60 minutes. Choose an activity that will reliably raise your heart rate for the entire duration of the workout, such as jogging or swimming. If possible, choose something you also enjoy; you'll be doing it all week. This program not only bursn excess calories and fat, but also increases your metabolism over the course of the week, allowing your body to burn even more fat while idle.
Schedule three sessions of direct abdominal work during the week. For each session, perform the following exercises, each of which has been demonstrated to be significantly more effective than the standard crunch in eliciting activity within the abdominal muscles: bicycle crunch, captain's chair, exercise ball crunch and vertical leg crunch. For each of these exercises, perform two sets of as many repetitions as you can complete without sacrificing proper form. Rest for between 90 seconds and two minutes between sets.