Hiker’s Knee: Using Physics to Examine Causes and Solutions

Over break I spent a lot of time hiking. By the end of the week, I had a small case of hiker’s knee, or aching knees with each step. Interestingly, knees actually undergo more stress when walking downhill than walking uphill. In both instances, one bent knee bears the entire bodyweight mid-step (front knee for uphill, back knee for downhill). When moving downhill though, there is the additional force of impact of each step on the other extended knee. This front knee, as a mobile weigh-bearing joint, bears both the weight of the hiker (as it does when standing still) and the reciprocal force of impact with the ground per Newton's third law. In fact, in a paper Kuster et al. (1994) published in Knee Surgery, Sports Traumatology, Arthroscopy, it was found that the hiker’s knee can absorb forces up to 8-times greater than the hiker's bodyweight when hiking downhill. Accordingly a 60kg hiker could have 4700N applied to their knee with each step. That much force exerted onto the small surface area of the knee cap results in a lot of pressure felt by the hiker, per the equation F=PA. Over long distances with little rest, this repeated stress can contribute to the aches of hiker’s knee. Plus, since people tend to walk faster downhill due to the assistance of gravity, the steps are often more bounding and result in more forceful impacts.

So, how can you prevent hiker's knees? One option is to use a walking stick or hiking poles. Hiking poles are usually lightweight, so they add little extra mass to the hiker’s load, minimizing additional stress on the knees. They help by redistributing the weight of the hiker, as there are now four points of contact with the ground. When moving uphill, the arms/shoulders now exert some force so that the legs have to do less work per step. When moving downhill, the arms support some of the bodyweight, reducing the force of impact on the knees. Specifically, a study in Journal of Sports Science by Schwameder et al. (1999) cited a 12-25% decrease in amount of force felt by the knee when using hiking poles to walk down a 25° incline compared to using no poles. As an added plus, using poles improves posture so that hikers are more likely to maintain their center of mass above their feet while hiking downhill instead of leaning backwards and causing the muscles to need to expend more energy to compensate.

Another option to ease the stress placed on your knees is to invest in better hiking shoes. Shoes that have more cushion in them increase the time of impact for each step, thereby decreasing the force felt on the knees per the equation ΣFΔt=Δp. Both of these options are common pieces of advice given in the hiking community to help alleviate stress on the knees, and I can now support them with physics.

With that, happy hiking and remember to protect your knees!