Always try to keep at least a slight bend in your knee when you run or ride a bicycle. When you run, you are supposed to land on each foot with a partially-bent knee. Otherwise you transmit the shock of your foot hitting the ground directly onto your knees, hips and back. Straightening your knees when you pedal a bicycle markedly increases risk for knee pain by increasing the force on your knee joints.

Shorter Strides Keep You from Straightening Your Knees 
I do not run anymore, and virtually all of the runners who ran with me in the 1940s through 1960s don’t run either, mostly because of the very high rate of running injuries. Up to eighty percent of long-distance runners suffer injuries that force them to take time off from running each year (Br J Sports Med, Aug 2007;41(8):469-80). Most wear-and-tear running injuries are caused by the high impact of your foot hitting the ground, which is determined most by the length of person’s natural stride (Scan J Med & Sci in Sports, May 30, 2018). Unnecessarily high impact is often caused by over-striding.  Frequently-injured runners take longer strides than those who are not injured (J Phys Ther Sci, 2022 Apr; 34(4): 327–334). Runners who are most likely to be able to continue to run as they age are the ones who take shorter strides (Med Sci Sports Exerc, Jan 2016;48(1):98-106), and you take shorter strides by never fully straightening your knees. Shorter strides help to reduce the tremendous ground foot-strike force that tears muscles and tendons, cracks bones, and injures joints. To convince yourself, place your hands on the huge quad muscles in the front of your upper leg while you run. Each time your foot strikes the ground, you will feel the muscles shake like jelly. This force is transmitted up your legs to your hips and back, and done repetitively, it can cause injuries (Br J Sports Med, Apr 2016;50(8):450-7). Runners who are injured frequently are likely to benefit most by shortening their strides, which then coincidentally increases likelihood of their landing on the front part of their feet, rather than on their heels. Contrary to common belief, it is not important whether you land on the front of your foot or the heel. However, the more you overstride, the more likely you are to land on your heel. Landing on the front of your foot does not prevent injuries, it is only a marker that you are not overstriding. A study of the 2017 IAAF World Championships showed that 54 percent of the men and 67 percent of women landed on their heels (Journal of Biomechanics, May 22, 2019).

Lower Seat to Keep Knees Bent While Pedaling
The most common cause of knee pain in bicycle riders is having the seat set so high that it forces you to fully straighten the knee as the pedal reaches its lowest level. You are never supposed to fully straighten your knee when you do any kind of exercise, particularly cycling or running. If you set your seat too low, you will bend your knee excessively and be at high risk for developing pain behind your knee cap. Other common causes of knee pain are over-training, setting your seat too far forward or backward, not having the cleats on your bike shoes set correctly, or not having the correct crank length. For other causes of knee pain on a bicycle, see Knee Pain in Bicycle Riders

Why Running Causes So Many Injuries
When you run, both feet are momentarily off the ground at the same time, and each foot strikes the ground with a force equal to three times body weight at six-minute-mile pace. The faster you run, the greater the force of each foot strike. Walking is much safer because when you walk, you always have one foot on the ground, so the force of a walking-foot strike almost never exceeds your body weight. As runners start to feel tired, they naturally shorten their strides and this decreases the force of their foot striking the ground (Med Sci Sports Exerc, Dec 1999;31(12):1828-33). The bent-knee shorter stride lessens the force of their heel striking the ground and places it forward on the foot to the area behind the big toe. To compensate for the shorter stride, they move their legs at a faster cadence. Shortening your stride will help to protect you from injuries by shifting your foot strike force forward. You can keep your speed by moving your legs at a faster cadence.

Today’s specially-cushioned running shoes were mistakenly thought at one time to increase risk for injuries (Nature, January 2010), but they increase injury rate only if you also overstride. Specially-padded heels encourage runners to straighten their knees, extend their strides, and land on their heels first. Hitting the ground with the heel first generates tremendous force because it stops the foot suddenly, while landing on the front of the foot allows the foot to keep on moving as the heel is lowered toward the ground to distribute the forces throughout the entire lower leg. You can demonstrate this by dropping a pen on its tip. The pen hits with great force because it stops suddenly when it hits the ground and then falls forward. However, if the pen is dropped at an angle, it hits the ground with much less force because after hitting on that end, the force is distributed as the pen falls backward to the other end. Whether you land on your heel or the front part of your foot is determined by your stride length, which is determined by whether you keep your knee bent or not. Runners get the same benefit just by never straightening their knees, which shortens their stride, without having to give up their comfortable shoes. The bent-knee, shortened stride reduces the injury-causing force of the foot hitting the ground and also causes the runner to land farther forward on his foot.

Barefoot Running
The idea that landing on the front part of your foot reduces risk for injuries encouraged some people to try running barefoot. My son, podiatrist Dr. Gene Mirkin, says, “Barefoot running has done more to bring patients into my office for fasciitis, shin splints, and general pain . . . it is good for podiatrists, not for runners. We have evolved into shoe-wearing people.” Stones and broken glass can cause injuries, and most people have such thin skin on the bottom of their feet that they can’t possibly run barefoot.”

Shorter Strides Can Help You to Run Faster
When most experienced runners go as fast as they can, they run at close to the same cadence. For example, a video at the New York City Marathon showed that all of the top 150 runners had the same cadence, taking 92 to 94 steps a minute. The difference between the top runners and the others is that the best runners are able to take longer strides without any special effort. Trying to extend your stride consciously slows you down and increases your chance of injuring yourself. When you try to take longer strides than what is natural for you, you lose energy and run more slowly.

Shorter strides help you to run faster because of stored energy. When your foot hits the ground, the tendons in your legs (particularly the Achilles tendon in the back of your lower leg) absorb some of this energy and then the tendons contract forcibly so you regain about 60 to 75 percent of that stored energy . When you try to take a stride that is longer than your natural stride, you lose a great deal of this stored energy, tire much earlier and move your legs at a slower rate.

The key to running faster in races is to make your leg muscles stronger so you can contract them with greater force so they drive you forward with a longer stride. Competitive runners strengthen their legs by running very fast in practice two or three times a week, and by running up and down hills once or twice a week. If you want to be a faster runner, learn to take stride lengths that feel comfortable, do not try to extend your stride, and try to increase your cadence (Sports Health. 2014 May; 6(3): 210-217).

Why Aging Shortens Strides
Your muscles weaken as you age, no matter how much you exercise. Since weaker muscles generate less force, older people will naturally shorten their strides as they age. Canadian researchers reported on biopsies of the leg muscles of 80-and-90-year-old world champion runners at the world masters track and field championships, and showed that, even though the muscles of the champion athletes were stronger, their muscle fibers contracted with the same speed and force as those of older non-athletes, and with less speed and force than what is generated by muscles of younger non-athletes (American Journal of Physiology – Cell Physiology, December 2015).

How to Make Your Leg Muscles Stronger
The only safe way to increase your stride length is to strengthen your leg muscles to help them contract with greater force. To make a muscle stronger, you have to damage the muscle fibers so they will be stronger when they heal. Competitive runners strengthen their legs so they have longer natural strides by:
• doing interval training (running very fast short bursts), two or three times a week
• running up and down hills once or twice a week

You can also strengthen your legs by using strength training machines, but you should do leg presses or knee and hip extensions only on the same days that you run fast. You cannot do strength training on recovery days because it will delay healing of your muscle fibers that were damaged from the previous day’s intense running. Most runners are better off not using strength machines on their legs because running very fast damages muscles and so does using strength machines. The combined load of running fast and using machines increases your risk for major injuries.

My Recommendations
If you are a regular runner or cyclist, realize that you can become stronger and faster, and gain more health benefits, if you try to pick up the pace during some of your workouts. However, this can increase your chances of injuries. To help to protect yourself from running injuries:
• Run at your most comfortable stride length and do not try to extend your stride length
• When your muscles are fatigued or sore, you may feel better if you shorten your stride length a small amount
• Run slowly on days when your muscles are sore from running fast on the previous days
• Stop your workout immediately if you feel localized pain that does not go away as soon as you slow down

• If your knees hurt while cycling, lower your seat so your knees do not straighten fully when you pedal.  If this does not correct the problem, see Knee Pain in Bicyclists for other adjustments that should be checked.