Tendon Elasticity

Achilles tendinitis and patellofemoral syndrome are common overuse injuries occurring in runners and other athletes that affect the Achilles and patellar tendons. Both can be painful and debilitating, and when left untreated, they can lead to degeneration of tendon tissue that causes irreversible damage.

Tendon elasticity has been getting significant attention recently as a critical factor for efficient and injury-free running. Elastic tendons are able to store energy and release it, contributing to force production and sparing muscles from overuse and energy depletion. In general, the stiffer the tendon, the greater its capacity for energy storage and release.

During training and rehab, it is important to keep in mind that tendons are designed for unique functionality, and undergo different loading patterns during different sports activities.

Tendons are tough connective tissues that are continuous with muscles and attach them to bones. Tendon tissue is made up of 70-80% collagen. The remainder is made up of proteoglycans and glycoproteins that include elastin, fibronectin, and tenascin-C. Glycoproteins give tendons elastic properties that allow them to return to their resting length after being stretched during loading. Tendons are encased in epitenon, a sheath of parallel fascicles that prevent them from adhering to neighboring tissues when shortening and lengthening.

To understand tendon elasticity, think of two rubber bands, one thin and one thick. The thin band stretches easily and returns to its original length without much force. When overstretched, a thin band with quickly lose its elastic properties. The thick band is difficult to stretch, and snaps back to its original length with tremendous force. Tendons are like the thick band. When stretched, they contribute significantly to force production by releasing elastic energy as they recoil to their original length.

Now, imagine that you stretch the thick rubber band repeatedly, elongating it to the point where its elastic fibers begin to fray. Eventually, it becomes less elastic and does not completely resume its pre-stretched form. The same thing can happen to tendons. When they are repeatedly overloaded with strain beyond their physiological limits, tendon stiffness is reduced, lowering elastic energy, causing failure and leading to irreversible plastic deformation.

During running, the soleus and gastrocnemius (calf) muscles work to plantar flex the foot, pointing the toes downward as the runner moves into the toe-off stage of the gait cycle. Simultaneously, the Achilles tendon is shortening, releasing elastic energy that contributes to force production to propel the runner’s body upward. The elastic stretch and recoil of the Achilles tendon contributes substantially to total energy storage and return by as much as 35%.

The patellar tendon is stiffer and thicker than the Achilles tendon, and it enhances knee mechanics during running by releasing elastic energy that helps the quadriceps muscles to extend the knee. As running speed increases, both knee flexion and contact time with the ground decrease, demanding a quick and strong release of elastic energy from the patellar tendon. Efficient running mechanics relies heavily on healthy elastic tendons, and injured or painful tendons should not be ignored.

Muscle and tendon loading patterns vary among athletes, depending on the sport. A 2017 study by Wiesinger et al. compared the patellar and Achilles tendon stiffness of water polo players, runners, ski jumpers and a control group.

The research team found that:

• Compared to water polo players, the knee extensors of runners and ski jumpers had a lower muscle-to-tendon cross-sectional ratio.
• Tendon energy storage and return was similar for all groups.
• The patellar and Achilles tendons of runners and ski jumpers displayed lower hysteresis (energy loss during loading) compared to controls.
• Recovery of patellar tendons strain energy was 50 percent higher in ski jumpers compared to water polo players and controls.
• Recovery of Achilles tendon strain energy was 40 percent higher in runners and ski jumpers compared to controls.

Another study by Cristi-Sanchez et al. (2019) found that elite soccer players have significantly stiffer patellar tendons compared to a control group, but found no significant difference in Achilles tendon stiffness. It is important for trainers and therapists to keep sport-specific differences in mind when training athletes for performance, and when rehabilitating tendon injuries.

Plyometric (jumping) exercises are the best way to enhance tendon elasticity, but plyometric training needs to be tendon- and sport-specific. With that in mind, we would train a soccer player differently than we would train a runner.

Examples of plyometric exercisers include:

• front and lateral box jumps
• trampoline jumping
• explosive squat jumps
• lateral skater jumps
• lunging scissors jumps
• jumping agility drills
• burpees

Plyometric training is essential, but it requires an intelligent approach that takes into account tissue stiffness, the tendons at stake, and sport-specific loading patterns. Done incorrectly, plyometric work can easily provoke symptoms.

At NYDNRehab, we utilize a comprehensive protocol for restoring tendon elasticity. Our methodology includes:

• Force plate jump testing complemented with sonoelastography to evaluate the capacity of the muscle/tendon complex for elastic recoil and force production.
• Kineo Intelligent Load system to measure eccentric muscle and tendon strength.
• C.A.R.E.N dual force plate and platform perturbation combined with Kineo varied loading to restore tendon elasticity. Platform movement during jumping allows us to study tendon response to landing on an angled surface.
• Tendon tolerance testing, along with SMI (superior microvascular imaging) to monitor micro inflammatory responses, and to establish load tolerance for training and rehab.

Resources:

Cristi-Sánchez, Iver, et al. “Patellar and Achilles Tendon Stiffness in Elite Soccer Players Assessed Using Myotonometric Measurements.” Sports health 11.2 (2019): 157-162.

Robi, Kelc, et al. “The physiology of sports injuries and repair processes.” Current issues in sports and exercise medicine (2013): 43-86.

Wiesinger, Hans-Peter, et al. “Sport-specific capacity to use elastic energy in the patellar and achilles tendons of elite athletes.” Frontiers in physiology 8 (2017): 132.