In aerobics the floor surface can also be a source of injury. If the floor is hard, bring a piece of padded carpet or a mat to work on. When bicycling, wear a helmet with a label showing ANSI and Snell approval. A proper helmet is a cheap insurance policy. Any good bike shop will help you adjust the height of the saddle and position of the handlebars to give the best workout.
Proper adjustments are important to avoid strains and injuries.
If you are between 30 and 69 the following guide will help you evaluate your aerobic fitness level. Measure an exact mile course on level ground and see how long it takes you to cover it, walking as fast as you can. If you find yourself in the Poor or Fair category, your cardiovascular activity related heart disease risk factor is at a critical level. At a minimum, try to attain the High Average level. In a prolonged workout many vitamins and minerals are lost in perspiration.