It’s easy to forget that other people don’t know what I know. Developmental psychologists describe this characteristic as normal in children of a certain age, but of course their brains change. I find the roots remain in most adults, myself included.
In daily life, I find this tendency remains deeply ingrained in my subconscious. We all do this on the highway, right? I think of road rage situations where we expect other drivers around us to be mind readers. We do this with our spouses or significant others too when we anticipate they will understand what we are thinking or feeling.
As a running specialist PT, I spend time each day reading running research and working with runners. It’s easy to forget that other physical therapists don’t know what I know. Most PTs will not get to work with patients on running related issues. As you might imagine, I am shocked when I hear colleagues casually mention how ‘bad running is for the knees.’
Wait! What?! Do people still think that?
I hear this from all kinds of medical practitioners. Long before PT school, I saw a physician for my first running related knee pain. Diagnosis? Irrelevant. His advice in three parts: stop running (because it’s ‘horrible’ for the knees), take up cycling, and if I insisted on continuing to run, consider wearing an extra pair of socks to add some cushion. Fortunately the PT he referred me to had been a runner for more than 30 years: my first exposure to PT!
Fourteen years later, I’ve run thousands of miles, including two marathons, three 50-kilometer races (31 miles), and a 40 mile mountain race. My knees feel great, my x-rays are normal, and my body weight and blood pressure are more ‘normal’ or ‘healthy’ than when I was 22 years old. Is this really the ‘medical’ advice we should be giving people in a country that is struggling with obesity, heart disease, diabetes, and inactivity more than ever before?
In this post, I wanted to bring some evidence to the conversation. It’s an important topic, yet surprisingly still controversial. Nearly all children run. Nearly all athletes run. Yet at some point, many of us stop running altogether and often at a fairly young age. Name another species that does this (dolphins and butterflies don’t count.) If you got a puppy and it stopped running after it was 3 years old, that would be concerning, right?
So many have had personal experiences with having to give up running because of knee issues: one cannot falsify another’s personal experience. Most people take their own experience as ‘evidence’ or truth, but that’s not how science works. This topic deserves more than opinion or personal testimony.
Let’s break the science down in just a few of perhaps a hundred different ways of answering this question.
Running and Knee Joint Cartilage
What current evidence is available regarding how running affects cartilage? If you’ve ever eaten a chicken drumstick before, that shiny and slick white material at the ends of the bone is joint cartilage. Cartilage in the knee joints is used both as a shock absorber as well as a friction reducer. Cartilage allows joints to glide smoothly.
A loss of joint cartilage or generalized damage to cartilage is a hallmark of osteoarthritis as well as other joint diseases and acute sports injury. Often the first thing an orthopedist will mention in regards to x-ray imaging is the amount or lack of cartilage in the joint.
A 2018 study analyzed how variation in knee loading mechanics during running affected breakdown of cartilage. Researchers measured the levels of cartilage metabolites in blood. By simulating two different running scenarios that altered impact forces and knee joint angles, cartilage metabolism appeared to be more affected by how much the joint moved at impact, moreso than the amount of impact force.
Visualize 500 miles on a pair of shoes for someone who scuffs their feet and another pair for someone who does not.
Excessive knee joint movement at the time of impact is often exaggerated when quads strength is inadequate. In situations with high volumes of down-hill running, the quads are being asked to work harder. This, without surprise, is a big catalyst for injury in the untrained. A similar situation occurs in knee pain with descending steps in the non-athletic population.
A 2011 study compared the effects of a 30 minute run to 30 minutes of hopping down from a platform. The study sought to compare high-volumes of both moderate and high impact activities. Cartilage, although heavy in collagen, is like a sponge with very high and variable water content. Repeated high impact is known to squeeze out water over time, only to rehydrate overnight while you sleep.
When 30 minutes of drop down training was compared to 30 minutes of running, the physiological stimulation of cartilage production was near equal. Regarding impact related deformation of the cartilage (squeezing out water), the drop down training, with the highest amounts of force, produced less deformation than the running.
Another study from 2005 looked at both physical deformation and metabolic changes in cartilage after running. The findings are interesting, not showing any clear relation between the amount of cartilage deformation and intensity of loading. Similar to the other study above, when adequate strength allowed the joint angle to change very little at impact, the cartilage appeared to better maintain its volume.
In this study, impact forces alone couldn’t explain changes in cartilage and COMP concentration, however the timing of co-activation between flexors/extensors could. These findings again demonstrate how specificity of tissue loading determines the regenerative properties of the tissue. In this case, cartilage appeared more resilient when acting as a shock absorber rather than taking on greater shearing forces.
A 2010 study looked at the persistence or longevity of biomarkers associated with cartilage loading. Even though they appeared elevated after 30 minutes of running, they reduced dramatically soon after, presumably as the body began to repair itself.
It is well known that resistance training or weight lifting largely works by eliciting microscopic ‘damage’ or ‘trauma’ to tissue in order to stimulate muscle growth and strength gains. It is also commonly understood that it’s good practice to take time between heavy resistance workouts to allow for full recovery of the tissue. Is it so hard to imagine application of this concept in relation to cartilage and running?
Lastly, an RCT (randomized controlled trial) on this topic. In 2013 the question being asked is whether a 12 week exercise program changes how cartilage responds to high impact. Data was collected from participants undergoing three different interventions: running, swimming, and cycling.
After 12 weeks of training, the running group showed no deformation of cartilage immediately after a minute walk. In contrast, 12 weeks of swimming and cycling appeared to show different results. Regular high impact loading resulted in more resilient cartilage.
RUnning and Knee Joint Tendons
Knee tendon injury is almost exclusively related to running related activity. When considering running related tendon injury, achilles tendon should also be considered. What does current evidence say about the effects of running on tendon tissue?
It is important to remember that in running, tendons act as physiological rubber bands, which not only allow the body to move with greater energy efficiency, but aid in greater force production than muscles alone. In simplified terms, the transition from walking fast to running is when tendons begin to be utilized to their full potential.
In a 2010 study, cellular qualities in tendons of running mice were compared to couch potato mice. The study looked at both achilles and patellar tendons.
Running mice compared to caged mice showed increased proliferation of tendon stem cells and tendon cell-related collagen in both tendon types. Collagen, similar to in cartilage, is the predominant component of tendons. According to these data, running appears to fortify tendon tissue.
Another 2014 study collected data in search of running risk factors predictive of injury to the achilles tendon. They identified only two risk factors clearly associated with a lower risk of injury to this tissue: arch height and loading impact.
The first factor, arch height, says much about the interplay between mid-foot mobility and ankle mechanics. More interestingly however, the second risk factor associated with lower risk of injury was high impact running: the higher the impact, the lower the risk of injury.
Tendons, similar to cartilage above, appear to not only get stronger with running, but appear to respond more to higher impact activity. One could argue that a lack of high impact activity could be detrimental to the development of these tissue types. On the opposite end of the continuum, what happens to tendon tissue in disuse?
What happens to tendons in times of sedentary behavior?
A 2004 study put 16 subjects on bed rest for 20 days, with a heavy exercise intervention given only to half. Resultant data demonstrated the daily leg-press exercises to have sustained tendon mechanical properties, whereas the sedentary group was shown to have tendon degradation as evidenced by decreased stiffness and increased hysteresis.
This study also looks at the relationship between muscle strength and tendon stiffness. It takes strong muscles to build strong tendons. Similar in cartilage tissue as above, the presence of relative weakness changes the mechanics and loading of the tissue. Muscle weakness means that the tendon tissue will not be able to function to its greatest potential.
Other studies show cellular changes to the tendon that occur with weakness imposed on overuse. These changes are often thought to be associated with the beginnings of tendinopathy.
There are well described aging related changes in tendons as well. A 2018 systematic review concluded that the age related changes to tendons in older adults were in-part due to biological changes with aging, but significantly more related to inactivity and muscle loss.
Their review suggests that older adults’ tendons are similarly responsive to high level loading in a positive way as their younger counterparts. Authors conclude that “Interventions should implement tendon strains corresponding to high mechanical loads (i.e., 80-90% MVC) with repetitive loading for up to 3-4 months to successfully counteract age-related changes.”
Another 2006 study investigating the age-related effects on mechanical properties of tendons reported similar findings. Authors concluded “Ageing alters tendon mechanical properties; stiffness and modulus were lower in older adults by 10 and 14%, respectively, compared to young adults. Increased levels of exercise loading in old age can however partly reverse this process, as tendon stiffness and modulus were found to increase by 65 and 69%.” This is evidence that physical activity can literally reverse the effects of aging.
Running and Knee Osteoarthritis
In spite of all of the evidence above, injury still occurs in running. Runners still have pain from time to time. Are cartilage and tendon specific studies too narrow a viewpoint? If we step back from tissue specific studies and look at joint disease as a whole, what do the data reveal? Is there a link between running and osteoarthritis of the knee?
Osteoarthritis is a degenerative inflammatory condition of the joint as a whole, involving degeneration of the joint capsule, fluid, and cartilage. We now know that the disease process begins long before degenerative changes to the bone begin to show up on x-ray. In a review from 2017, the author jumps right to it in the title of the paper.
The author found no correlation with running and increased incidence of OA. The author theorizes that running actually makes cartilage more tolerant of loading stresses. This theory is supported by cellular evidence described above.
Another 2017 review of data on a much larger pool of runners had similar conclusions.
In this paper, data is reviewed from 2637 participants in the Osteoarthritis Initiative. Of these participants, 30% reported running for leisure at some point in their life. The authors conclude “There is no increased risk of symptomatic knee OA among [...] runners compared with non-runners[…].” “In those without OA, running does not appear to be detrimental to the knees.”
Another systematic review from 2017 crunched the numbers available from 25 prior studies on this topic.
From this review, there is “moderate to low quality evidence” that running has no causal relationship to knee OA. Some of their analyses indicated that running might even be somewhat protective against the need for a surgical intervention related to knee OA.
A Need to Praise the Benefits of Running
Let’s put an end to propagating the myth about running being detrimental to the knees. Particularly as healthcare providers, it’s important to give credence to all the benefits that running provides. America is fighting an epidemic of chronic diseases right now that are largely lifestyle mediated. Physical activity is in high demand.
Staying physically active plays a huge preventative roll in the biggest killers out there, heart disease and stroke. Research on exercise, and running specifically, has been able to show decreased risk for diseases like cardiovascular disease, heart disease, diabetes, stroke, and dementia. Many living with these conditions are physically inactive, making matters worse, often progressing the course of disease. Promoting the baseless claim that ‘running is harmful’ is borderline negligent.
Perhaps a better approach is connecting people with physical therapists and other performance specialist when pain arises. A little education can help keep people physically active for life.
Strength Adequacy is a Recurrent Theme
A recurrent theme: when muscle strength is inadequate, soft tissue is unable to function in a productive, restorative way. Weak muscles limit the efficiency potential for cartilage to be used as a shock absorber. Weak muscles do not allow tendons to function at their fullest springy potential. Similar research from the osteoporosis world shows that strong muscles are the key to maximizing bone density.
The brain is very clever. The fact that it allows running in a system with significant strength insufficiency is likely a key factor in the survival of any species. The mechanical systems of the hip, knee, and ankle have endless built in redundancies that allow compensatory movement patterns to take the lead when optimal movement is lost.
This eloquently describes limping, which albeit frustrating to the injured, allows mobility to continue to occur in the setting of potentially immobilizing injury.
Identifying isolated deficiency and movement dysfunction is the ‘nitty gritty’ of my job. As a running specialist, I have to take into context the way a person moves, and correlate this with the strength of individual components: the function of the system as a whole. Proper diagnosis is a challenge: correction of the problem is an art of physiological persuasion.
Staying below the Breaking Point
Every system has a threshold for breakdown or injury. Every runner can run uninjured up until a certain amount of miles. Some studies indicated that on average, a weekly mileage over 35 miles is when most runners begin to experience problems. For perspective, many elite runners log 90-120 miles per week. Is this phenomenon a weakness problem? Is it a problem with progression in miles quicker than soft tissue can adapt?
Weakness is a relative term that must be framed in several contexts. Weakness can be the insufficiency of a body to perform a task with clean, controlled movements. Each person lives a unique life, with unique physical activities and a unique body type. Individualized strength requirements vary from person to person.
Weakness can be the failure of a singular joint during a complex movement. This is common after an injury has healed, but the motor control surrounding the joint was never restored. This is extremely common after ankle sprain, where once the ligaments are thought the have healed, the athlete is thrown back into the game. No attention is brought to the remaining weak ankle muscles, and the athlete is subject to a career of repeated ankle injury.
Strength required for injury-free running is highly specific to the amount and type of running that will be done. A runner who puts down 2-3 miles on the treadmill each day for fitness has different demands than one training for a half marathon. Both runners can have relative weakness, but might have a different mile-mark for when insufficiency becomes injury.
The average runner in the US does not have the body of a professional athlete. It doesn’t make sense to assume that these two types of runners could withstand the same levels of stress and one not get injured. This is why NASCAR uses modified engines rather than factory engines for optimal performance. Be that as it may, they typically start with new car parts, not a vehicle with 300,000 miles on it.
Driving is Bad for Your Car
An automotive analogy helps make sense of this. Think of the condition of your current vehicle: the age in years, the fuel economy, the tread on the tires, and the age of the oil. Maybe you have a slow oil or coolant leak, squeaky brakes, or a ‘check engine’ light that pops up once a month, nothing bad enough to warrant a trip to the mechanic.
Now consider what type of driving your car is suitable for. Would your vehicle easily get you around town for errands? Would it safely get you across the country? Could you take up a side job towing loads of gravel to construction sites? Would you put it on the highway at 120 mph? If you never drove your car, it would essentially outlive you!
All of these situations create unique loading requirements on various systems of your vehicle. In running, one must consider the current state of their body and choose wisely on how find moderation and progress training accordingly. The biggest difference between the car and the body is that body parts get stronger with use, if the progression of activity is appropriate.
Running in and of itself is not bad for your knees. Any activity which pushes the body beyond its functional capacity will eventually result in injury. Activity moderation is critical to avoiding injury.
Not all who come to running will have immediate success. Not all who come to running will do so straight from the factory floor. Some may already have suboptimal joints, cartilage, tendons, or bone density before they lace up for their first mile. The body may need some tune-ups before hitting the highway.
Adversity in sport and physical activity should not be met with giving up or being told to stop. It should be met with qualified healthcare practitioners, master problem solvers, who can help people progress back to activity and lifestyles that give them meaning and happiness.
No one wants to go to the gym because of a medical mandate or a new diagnosis. We come of age using play as a means to pass time, socialize, and develop our motor skills. In other cultures, it is not uncommon for adults to continue to play pick-up games of soccer or other sports, a social thread woven with physical activity. In healthcare we should encourage everyone to find activities that they can participate in and be passionate about.
Until next time, don’t stop moving!