For comfortable and safe cycling, you must have a bicycle of the proper size. A good test for determining the correct frame size is to straddle the top tube with both feet flat on the ground. If you can lift the bicycle more than two inches, the frame is too small.
Don't be afraid to adjust your bicycle for comfort and maximum efficiency, and keep on adjusting it until it feels right. As a starting point, the saddle should be raised until there is only a slight bend in your knee when your foot is at the bottom of the pedal stroke. Too high a saddle will cause you to roll back and forth with each pedal revolution, which can lead to early fatigue and saddle sores.
After adjusting the seat for proper leg extension, check the handlebars. The distance from the front tip of the saddle to the center of the handlebars should be the same as the distance from your elbow to the tip of your outstretched middle finger. You can move the seat back and forth, or purchase handlebar stem extensions in different lengths. The handlebars can be level with the saddle; however many cyclists prefer to have the bars 1/2 in. to 1 inch lower than the line of the saddle.
Surprising as it may seem, many cyclists don't listen to their own bodies, and so push themselves well past the safe level of their own endurance. When something starts to ache or hurt, stop cycling and find out why.
So many cyclists suffer from saddle sores that this condition is accepted by many as a part of the cycling experience. It doesn't have to be. Your body is telling you that your saddle isn't broken in correctly, that it isn't adjusted to the right height, or that you are wearing clothing that is rubbing and irritating your skin. Do something about it!
Another very common problem that cyclists try to ignore is knee pain, often brought on by cycling in too high a gear. The knee is a complicated mélange of ligaments, bones, cartilages, and tendons; the latter are subjected to huge amounts of force with each turn of the crank. Knee strain is pretty simple to detect even in the early stages. Continued strain can result in damage to the knee tendons and/or tissues. When you feel it coming on, take a break, and then shift to a lower gear.
If the knee pain continues, stop and check your saddle position. You may have your saddle situated back too far, which causes your body to sit back too far for an efficient pedaling posture. When your pedals are horizontal to the road, with your feet on the pedals or in toe clips and straps, the center of your knee (slightly bent at this point) should be just about directly above the center of the pedal.
Cyclists who should pay more attention to body signals often ignore a stinging and numb feeling in the hands. The problem is caused by continued compression of a nerve in the palm of the hand between the handlebars and the bones of the hand. In extreme cases this can lead to damage to nerves or a partial paralysis of the hand. The simple solution is to use well-padded cycling gloves, a padded handlebar, and the good sense to change your hand positions frequently.
This last technique will also help alleviate the wrist, shoulder, and back strain. If you continually suffer from sore shoulders or upper back pain when cycling, check your position. It is likely that your top tube length is incorrect, either forcing you to sit up too high, or (more likely) to stretch too far to the handlebars. This can in many cases be solved by replacing your handlebar stem with a longer or shorter model.