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. 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). This month, a new study shows that the 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). 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.

The Study
Researchers analyzed running biomechanics for 110 male and female runners who were:
• 18 to 60 years old,
• injury free for at least six months,
• ran at least five miles per week, and
• had no diseases or infirmities that would interfere with running.
Most of the runners had been running for more than 10 years.

The study found that with aging, there was a decrease in:
• running speed,
• foot strike force, and
• ankle torque and power. Hip torque and power did not change.
The older runners were slower because of decreased stride length; their cadence did not change.

This study suggests that you can keep running as you age and avoid injuries if you reduce the force of your feet striking the ground by taking shorter strides, and restrict the movement of your ankles as you raise your feet to take their next step.

Why Running Causes So Many Injuries
When you run, one foot is always off the ground, so each foot strikes the ground with a force equal to three to five times body weight (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 (Medicine and Science in Sports and Exercise, Dec 1999;31(12):1828-33). The shorter stride lessens the force of their heel striking the ground and places it forward 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.

Pronation Helps to Prevent Running Injuries
The runners who are most likely to become injured have feet that hit the ground with the greatest force and roll the least after hitting the ground (Med Sci Sports Exerc, September, 2000). If your feet hit the ground flat-footed, the shock could break your bones. Instead, most people land on the outside bottom of their feet and then their feet roll inward from the outside bottom on the fifth toe side, toward the big toe side. This is called pronation.

If you still suffer frequent injuries, you can try the firm inserts that fit underneath your feet and fill the space underneath your arch. To hold your feet on the inserts, use running shoes that have a firm counter surrounding your heel that extends forward to your arch and a saddle that runs from the sole to your laces. If injuries persist, see a podiatrist who may prescribe custom inserts.

Barefoot Running is Not the Answer
In the 1960s, doctors thought that most running injuries were caused by excessive pronation. Because the foot rolled inward toward the arch to dissipate the tremendous heel strike forces, the lower leg twisted inward. They blamed the high frequency of running injuries on the inward twisting motion of the leg, so they invented running shoes with special arch supports to limit inward rolling, and padded heels to cushion some of the shock of the heel hitting the ground.

However, Harvard evolutionary biologist Dan Lieberman thinks that today’s running shoes cause even more injuries (Nature, January 2010). The modern shoe features encourage runners to 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 on the side of one end, 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.

Dr. Lieberman has shown that barefoot runners are more likely to land on their forefoot or mid-foot. His elegant experiments show that landing on the front part of the foot reduces the force of the foot strike very significantly. This led many runners to try barefoot running or to use special thin-soled shoes that mimic running with bare feet. However, more recent research shows that runners get this same benefit just by shortening their stride, without having to give up their comfortable shoes. The shortened stride reduces the injury-causing force of the foot hitting the ground and also causes the runner to land further forward on his foot.

My son, Dr. Gene Mirkin, DPM, 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.

Won’t Longer Strides Help Me 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 trying to do so. 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.

Why Aging Shortens Strides
Your muscles weaken with aging, no matter how much exercising you do. Since weaker muscles generate less force, all older people will naturally shorten their strides as they age. Last month, 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 (American Journal of Physiology – Cell Physiology, December 2015). Surprisingly, they found 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.

Age-group competitions in every sport compensate for this inevitable weakening of muscles. Meanwhile, runners who want to keep on running into later life should learn to reduce foot strike force by running with shorter strides.

Checked 11/11/16