For a person in top shape, a broken ankle is a big setback. Broken ankles require wearing a cast for about six weeks to heal properly. Since six weeks of non-activity reduces the fitness of an athlete, it is imperative to find a way to stay in shape with a broken foot. Since doing regular exercises such as running and cycling isn't possible, it takes a little creativity and change of routine to keep in shape during those six weeks.
Focus on upper body. There are plenty of exercises that don't require lower body use. Push ups, sit ups, crunches and working on the arms with dumbbells are great exercises to increase muscle tone to keep it from deteriorating due to lack of use.
Get in the pool. With a waterproof cast, a good choice to stay in shape is aqua jogging. Aqua jogging is jogging in the deep end of the pool, kept afloat by a floating vest. The feet never touch the bottom of the pool, so there is no impact on the ankle during this exercise.
Use crutches. Just getting around on crutches is a tough workout. Besides working on the arm muscles, traveling around on crutches also increases the heart rate.
Lift weights. Go to the gym and use the equipment for the upper body. Get a good workout for the arms, back, chest and abdominal muscles and stay off the broken ankle.