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First, consult with your doctor to be sure you will not do further injury.  Continue with your existing upper body exercise provided there is no pain, no risk of further injury, and your exercise plan has been cleared by your doctor. For safety reasons, be sure you have someone with you, both at home and at the gym, to guard against falling or further injury. Do regular chin-ups using a reverse grip with your palms facing you and your hands a little closer than shoulder-width apart.  Pull yourself up until you can touch either your chin or your upper chest to the bar.  Do five sets with six repetitions in each set.  Be sure someone is with you to help you position yourself gently on the ground when finished with each set. A type of chin-up exercise called the gironda sternum exercise works different muscles and is also more difficult to perform.  The goal is to position your body in a slant and touch the lower portion of your chest to the bar.  Depending on the type of injury you experienced and the form of brace on your leg, this may not be possible. The ability to engage your upper leg muscles is needed to maintain your body in the slant position.  If you are able to perform this exercise, do 5 sets with 6 repetitions in each set. Be sure someone is with you for safety. Keeping your injured leg straight instead of bent, position yourself so your legs are either flat on the ground or resting on a slightly raised bench.  The injured leg needs to remain straight, but the uninjured leg can be bent at the standard 90 degree angle.  Place your hands on either side of your head but do not lock your fingers behind your head.  Push your lower back into the floor, and slowly lift or roll your shoulders off the floor. Do not raise your shoulders more that about 4 inches from the floor.  Focus on slow and controlled movements while keeping your lower back pressed to the floor to get the greatest benefit. Do five sets with 30 reps in each set. Position your body so that your toes are on the ground and your hands are directly beneath your shoulders.  Keeping your back straight and your head level, push yourself up from the ground.  Lower your body back towards the ground until you feel a stretch in your chest and shoulder area. Hold that position for one second, then push your body upwards keeping your back and head straight.  Repeat five sets with 20 repetitions in each set.  Depending on the type of injury and the type of cast, this may not be a good choice for you. Any exercise that causes pain at your injury site should not be done. Be sure you can do this safely. Your healthy leg will need to be very strong for you to do this exercise, and you'll need to have excellent balance and mobility. It may help to have a friend standing beside you so you can grab her arm if you lose your balance. Make sure your ankle in particular is warmed up and flexible.  Keeping your back straight, lower yourself down to a sitting position, with the thigh of your good leg at a 90 degree angle to your hips. Keep your injured leg in a straight position out in front of you, parallel to the floor.  Return to a standing position using the strength in your uninjured leg. The exercises described e are only examples.  Whatever you choose to try, be sure someone is with you to help, you can do the exercises safely, and the exercises do not cause pain. Work with your doctor, your physical therapist, or trainer, to develop a solid workout routine that includes both cardio and muscle fitness exercises that are safe for you.

Summary:
Continue your upper body workout. Use a chin bar. Try an advanced version. Consider crunches. Consider push-ups. Try squats using one leg. Be creative.