Eccentric exercises are considered to be the gold standard for treating tendinopathy. An eccentric exercise, such as a heel lower in the Alfredson’s Protocol for the Achilles tendon, is an exercise where the muscle lengthens as it contracts. There are many theories on how an eccentric exercise helps with Achilles tendinopathy but the exact mechanism is not well understood. In this post, we will discuss how from a biomechanical standpoint can be used to treat Achilles tendinopathy.  [embed]https://youtu.be/KK9l6XgW2Lc[/embed]An article by Geoffrey Varral published in the Scandinavian Journal of Medicine and Science in Sports investigated how physical science principles can be applied to the Achilles tendinopathy and eccentric exercises. The article focused on using Hooke’s Law to determine the load placed on the tendons. 

Force =  E * CSA * Δk / L 

Where E stands for elasticity, CSA is cross-sectional area, k is displacement and L is original tendon length. Increasing the elasticity, cross-sectional area, or displacement of the tendon will increase the amount of force that the tendon can tolerate. While mathematically decreasing the original length of the tendon would increase the force the tendon can absorb, it’s not actually possible to change this length because the tendon is fixed. When there is a sudden increase in load on the tendon, the tendon needs to respond quickly. In the pathological tendon, increasing the cross-sectional area of the tendon through swelling is a mechanism of how the tendon increases its tolerance to load in a short period of time. The tendon increases the production of proteoglycans which will draw fluid into the tendon, increasing the cross-sectional area of the tendon and therefore increasing its tolerance to load. In the continuum model of tendinopathy, this is referred to as reactive tendinopathy.With sufficient rest, the swelling in the tendon decreases and the structure of the tendon returns to normal. However, with persistent overloading on the tendon, the swelling inside of the tendon causes increased collagen separation and matrix disorganization. Other structural changes in the pathologic tendon include increased collagen cross-linking and a transition from type I collagen to type III collagen. Type III collagen is more commonly found in ligaments. The transition in collagen composition over a long period of time makes the tendon more flexible which will decrease its stiffness and increase its displacement. This is referred to either as tendon dysrepair or degenerative tendinopathy. The article proposes three different mechanisms on how eccentric exercises may help with tendinopathy. One of the following mechanisms or a combination can be involved.

1. The tendon adapts by decreasing its stiffness to allow for more displacement, improving its ability to tolerate force
2. Altering the mechanical properties of non-injured areas to decrease the amount of force applied to the tendon
3. Altering the gait pattern to decrease the force applied to the tendon

The study concludes that physical science principles support the use of eccentric exercises for tendinopathy and the mechanism is to decrease stiffness in the tendon to increase the displacement. There are a couple of considerations to take into account with this study. Even though eccentric exercises are considered to be the gold standard for tendinopathy rehabilitation, there has been a debate about whether they are more effective than other loading strategies. A systematic review by Peter Malliaras found that there was little evidence to support the use of eccentric exercises in isolation and that a concentric-eccentric loading program could be used. This was also supported by a randomized controlled trial by Rikke Beyer which compared a heavy, slow resistance protocol to the Alfredson eccentric exercise protocol and found that the outcomes were similar between the groups. This all to say that these mechanisms may not be specific to eccentric exercises but to the response to loading by tendons in general. [embed]https://youtu.be/fsMt8-4Fj8s[/embed]The capacity theory is a common explanation for the development of tendinopathy, with overloading considered to be the primary mechanism of tendinopathy. While Hooke’s law helps to describe tissue capacity of the tendon, there are other factors that contribute to the development of tendinopathy and the structural changes associated with overload. In an article by Ebonie Rio, et al looked into the mechanism of tendon pathology and found evidence for physiological and pathophysiological mechanisms. Exercise may help with the physiological mechanisms of tendinopathy suggested with Hooke’s law, but exercise may also help with the pathophysiological mechanisms as well. The relationship between structural changes associated with overloading and pain is also not clear. There are multiple studies which have found tendon abnormalities in the asymptomatic Achilles tendons, patellar tendons, and rotator cuff tendons. While Hooke’s law helps to explain how exercise can improve the tissue tolerance of the tendon to loading, it does not provide insight into why some people develop pain and others do not. Load management and exercise remain the primary treatment options for tendinopathy. Physical science principles such as Hooke’s law help to improve our understanding of how to improve tissue tolerance in tendons, but there are likely other factors leading to the development of tendinopathy and how exercise helps in the recovery.