Alexander RM. Tendon elasticity and muscle function. Comparative Biochemistry and Physiology, 2002, Part A, 135: 1001 -1011
At Gray Institute®, there is a constant search for the TRUTHS (or Principles) of human movement. The Truths / Principles serve as the basis of the Strategies we use to train or rehabilitate patients / clients. Truths are lawful ways that the body behaves in our environment. These Truths or Principles should transcend the type of movement. They should be the same for the different limbs, joints, or muscles that make up the human “machine.” This paper provides a perspective beyond human function that forces us to consider Principles that transcend species. Its specific focus is the properties of muscle-tendon units that allow us to walk, run, and jump efficiently.
Since it is a review paper, a vast number of research articles are discussed to provide a coherent view of the storage and return of energy that occurs when animals move. Although much information has been added to the “story” since its publication, the Principles remain true. One of the key Principles that guide our training / rehab programs is that to move the body must load before it explodes. The loading motions at the joints, often driven by gravity, lengthen the muscle-tendon units. During deceleration of the loading motion, energy is stored in the muscle-tendon unit to be used when the motion reverses into the desired explode. The storage and return of that energy is the primary focus of Alexander’s paper.
One of the primary points of the article is that when muscles lengthen and shorten, metabolic energy is used to generate force at the actin-myosin cross-bridges in the muscle. If a muscle generates force in an isometric condition, less energy is used compared to lengthening and shortening. So less change in the length of the muscle itself is preferable. Motion can occur with limited change in muscle fiber length if the tendon can lengthen. Tendons have been shown to lengthen up to 9 percent in certain conditions. The storage, and more importantly the return, of energy by tendons can be as high as 93 percent. This reduces the need for energy utilization by the muscle. The article also points out that tendons can shorten much faster than muscles. So storage of more energy in tendons rather that muscles is not only more efficient, but also more effective.
So why does this matter for function? The ability to store energy in tendons via lengthening can decrease. Tendon elasticity decreases with aging, and other factors likely to include the amount of physical activity a person engages in. At Gray Institute®, this truth influences our strategy regarding stretching / flexibility exercises. Global movements that elongate the tendons while the muscles are contracting are preferred over passive stretching. Short excursion of the body into and then out of the Transformational Zone (where movement and motions change direction) creating the deceleration and acceleration are chosen over the static holding of a position in most instances. With the muscle contracting, the movement will create more tension in the tendons. This causes the tendons to lengthen more. The lengthening helps to maintain the “stretchability” (elasticity) of the tendon.
Other Principles of human function would tell us to use global movements that include the three planes of motion. If the tendon will be stretched in three planes during function, then our programs need to incorporate three-dimensional movements. Since gravity and ground reaction force provide much of the energy that loads the tissue (and lengthens the tendon), the upright position of the body relative to gravity is an important feature of most movements. The proprioceptors in the muscle and tendon need to be conditioned with movements that replicate the activities the client needs to perform.
Tendon lengthening to maintain or improve elasticity is part of what would be called Functional 3D Flexibility at Gray Institute®. All aspects of these programs are designed around a Strategy that emanates from the Principles of function. We may discover that many Principles that dictate human movement, also apply to other species. Certainly tendon elasticity is an important tissue characteristic that is leveraged to reduce the metabolic cost of accomplishing a task.