How Tendons Work
Hello everyone - I’m continuing today in a series of summaries from Jake Turra’s new book, Tendon Book. Today I’m summarizing his chapter on how tendons work. More specifically . . .
Strain is how tendons function. Tendons do not actively do this, but rather they strain or recoil when their attached muscles and/or bones stimulate them to do so. For instance, muscles can only produce maximal force within a narrow window of muscle length. The tendon can strain so that the muscle fibers do not have to lengthen or contract so much that they are outside of that optimal window. Similarly, muscles and tendons work together to produce force at high speeds since muscle fibers cannot produce the highest levels of force at a high velocity.
Tendons have a low energy state and a high energy state. In the former, the tendon is at rest, and the collagen within the tendon is crimped. In the latter, the tendon is stretched/strained, which stores energy. This increase in energy that is relative to strain/stretch is called stress. The stretch of the tendon is increased if the attached muscle is exerting a pulling force on the tendon (i.e., the tendon is stressed). These two states–relaxed and strained–are the only two ways in which a tendon can be. Moreover, the tendon prefers to be in the relaxed state because it requires less energy.
Tendons can manage stresses and strains because of their viscoelasticity. To be viscoelastic means to demonstrate properties of both a solid (to be elastic) and a liquid (to be viscous). A tendon acts more elastically, like a solid, when it is strained quickly, as in a vertical jump. The collagen and gel inside the tendon become stiff to resist stretch. The tendon “bounces back” from the high energy, strained, state to the relaxed state, like a rubber band. Energy is returned, but some energy is lost as heat (i.e., hysteresis).
On the other hand, a tendon acts more compliantly, like a liquid, when it is strained slowly, as in a heavy squat or isometric hold. During a slow strain, the liquid-like parts of the tendon gel move around, causing the collagen fascicles and fibrils to move, as well. This rearrangement of the tendon parts releases energy to prevent entering the high energy state. Instead of “bouncing back” after the strain, the strain is maintained; however, unlike a rubber band, a tendon cannot hold all of its energy indefinitely, so total energy decreases as the fascicles and fibrils move around. This release of energy is referred to as “stress relaxation.”
Tendons may also demonstrate a process called “creep,” or “gradual elongation over time,” though this phenomenon has mostly been explored in cadavers and not living individuals (who are subject to muscle fatigue). Creep occurs when a tendon, having viscoelastic properties, experiences more and more strain over time when held under constant energy, thus becoming more stretched. For instance, while running for a long distance, one’s tendons may be longer after 20 miles than after 5 or 10.
Tendons experience both tensile and compressive strain. Collagen is organized specially to manage tensile strain, but compressive strain is also present in certain places where the tendon twists around other parts of the body in tight areas. In these areas (e.g., the enthesis, the achilles on the calcaneus, the rotator cuff tendons on the acromion, glenoid rim, and coracoid process, or the patellar and quadriceps tendon on the patella), the tendon is “compressed.” The anatomy of these tendons supports the management of compression, such as having more type 2 collagen and bigger PGs and other structures that address friction.
