Exercise is key to good health. But we tend to limit ourselves to one or two types of activity. People often stick to what they enjoy, or what feels the most effective, so some aspects of exercise and fitness may be ignored. In reality, we should all be doing aerobics, stretching, strengthening, and balance exercises. Here, we list what you need to know about each exercise type and offer examples to try. (Talk to your doctor first if you have not been active recently or if you have any injuries