Q: I have started cycling because my doctor recommended it as a safe, low-impact form of exercise. It bothers my knees, though. Is there something that I’m doing wrong?

A: Your seat may be too low—a common mistake made by many recreational cyclists who simply hop on a bike without adjusting the seat height to fit their body. Cycling is a safe activity, but if your alignment is off you can still place excess strain on your joints. For knees, it’s crucial to keep your seat high enough.
If your seat is too low, then you end up cycling in a squat-like position, with your knees bent throughout the entire pedaling motion. This can stress the knees because the leg isn’t able to fully extend at the lowest point in the pedal stroke. If your seat is not far enough away from the pedals, you are most likely to feel the excess pressure in your knees when you are seated and “climbing” (or pedaling against harder amounts of resistance).
So how should you adjust your bike-seat height?
When you are sitting on the bike and your foot is at the lowest point in the revolution—closest to the floor on an upright bike or farthest in front of you on a recumbent cycle—your knee should be mostly extended (bent at no more than 5 to 10 degrees). The leg should not be completely straight—or locked out—because that forces you to stretch to reach the farthest point in the pedaling motion. Your leg should be more elongated than bent, especially if you already feel strain in your knees.
Indoor cycling, or spinning, classes use several techniques to determine correct seat height. Some people stand next to the bike and roughly align the seat height to the height of their hip bone while standing. This is an imprecise measure.
Another approach is to sit on the bike and straighten the leg so that it’s vertical to the floor, then gauge the knee angle. The seat should be placed at the height where there is a 5 to 10 percent knee bend. The problem with this method is that it’s usually done incorrectly—people do this exercise with their legs skewed, so they do not get an accurate view of what their leg extension looks like as it moves through the pedaling motion.
To get the correct knee angle, the pedal and the bar that connects the pedal to the bike both must be properly aligned. The connecting bar should be perpendicular to the floor. The foot also needs to be perfectly horizontal with the pedal and parallel to the floor.
Perhaps the easiest, most functional way to gauge seat height is to get on the bike and start pedaling. Notice how high your knee rises in relation to your hip. If you can see your profile view in a mirror, also notice the line that your thigh makes as your knee reaches the highest point. The knee should not come all the way up to hip level—it should rise to a point that’s below hip level so that the thigh is always diagonal, nor horizontal, or parallel to the floor, as the leg circles around. Generally, your seat and hips should be higher than your knees as they reach the highest point in the pedaling revolution.
If your seat height is aligned properly but you still feel strain when you cycle, make sure that you are not adding too much resistance. You should not get stuck or struggle when trying to pedal. The motion should be fluid and smooth. As your legs get stronger, you can add more resistance, but you should never add so much that you are straining to push the pedals.