Cycling is a hugely popular sport across the world, but if you are cycling on the wrong size of bicycle or with the bikes saddle and handlebars set at the wrong height, you may tire sooner and get all sorts of aches and pains.

Your handlebar on a your bike should be about 3cm lower than your saddle, maybe a cm lower still on a mountain bike or if you are racing you could go lower still for comfort and streamlining.
This said, feel free to raise the handlebar as much as you'd like, just don't raise the bar so much that the warning marks on the stem show, you could break the bike or yourself! Check that the saddle on your bike is level. Your crotch should be cupped by the ends of the saddle. If you are sliding forward, you'll be putting too much weight on your arms, which will have to be supported by your back. If the seat is tilted backwards, your posture will be bad and you might strain your lower back.

Sit on your bike with your hands on the brake hoods, with your arms in a natural and unlocked position, your line of sight through the handlebar should bisect the front wheel's hub. If your handlebar sticks out too far ahead, you'll be straining your back. You'll have to buy a shorter stem. This is a very common problem with women, who often have shorter torsos. If your stem is too short, you'll be sitting more upright than you probably need. While this shouldn't hurt your back, you might not like the added wind resistance.

Cycling requires you to ride bent over the top tube while keeping your back straight. If your back is not stretched out, you'll be riding hunched over the top tube. That will cause back pain. This is a very common problem for beginner cyclists who get a bike set up for racing. Their bodies just aren't ready to assume that position and they could injure themselves, the racing position should be moved into gradually.

Try these exercises once a day to stretch out your muscles and make assuming the best cycling position easier and more comfortable.

Lay down on your back. Pull your knees to your chest while keeping the small of your back on the floor. Straighten your legs at the knees. You should feel it pull on your lower back. Stop straightening your legs before it becomes uncomfortable or painful. Hold the stretch for 15-30 seconds. Repeat 2-3 times.

Abdominal Crunches

Lie on your back, with your knees up and the small of your back on the floor. Lift your shoulders off the floor. Breathe out as you come up. You should lift by squeezing your lower abdominal muscles. Don't lift with your arms or your hips. Lower yourself back down and repeat. Try 2-3 sets of 30-40 repetitions or more. Do them at least 3 times a week.

This works your lower back. Lie face down on the floor. Put you open hands, palm down, under your face. Keep your feet, knees, and hips on the floor. Lift your arms and shoulders in one motion off the floor while breathing out. Lift with your back, not your head. Do 2-3 sets of 15-30 repetitions 3 times a week.

More specifically, what to do in case of a cycling knee injury