I hate to sound like a broken record or an advertisement, but I’ve recommended the Nintendo Wii Fit system for years. Although not an 'app', the most recent versions have programs for Balance, Strength training, Yoga, Aerobics and now something called training plus.
This is a great system that helps you set goals and track your response. It is also fun and best performed as a social activity with family members. Ask your physician or physical therapist about it.
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