Why Do We Lengthen?

A patient is unlikely to submit himself to the surgeon’s knife unless he knows that the operation’s benefits outweigh the discomfort involved. It is similarly unlikely that a person will tolerate the unpleasant sensations involved in changing long held habits of body and mind if they are not aware of the benefits to be gained. The purpose of this article is to describe these benefits.

Habits have consequences. A person with the habit of brushing her teeth every day is less likely to develop gingivitis, cavities, and other dental problems. A person with the habit of smoking tobacco is more likely to develop lung cancer. Similarly, the quality of a person’s habitual posturality has an effect on the length that viscoelastic body tissues such as tendons, ligaments, and fascias must be stretched to before forces applied to them will transfer to a bone*. This has significant consequences for that person’s stamina, agility, and accuracy of movement.

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*In particular, "posturality" here refers to the manner in which a person creates, maintains, and alters the geometrical relationships between all of the various bones and soft tissues of their body.

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The relationship between muscle, tendon, and bone, is analogous to the relationship between a fisherman, a fishing line, and a fish. In this analogy, the fisherman is the muscle, the fishing line is the tendon, and the fish is the bone.

If the fishing line were an elastic similar to bubble gum, it would have to be stretched to a great length before pulling the fish, allowing the fish ample time to escape. If, however, the fishing line were an elastic requiring minimal stretch before pulling the fish, the fish would likely be caught. Fishermen are well aware of this, and purchase fishing line that has been engineered to produce a lot of force after only a little bit of stretch.

Tendons and ligaments, like different kinds of fishing line, vary in the amount of tension they produce when stretched to a certain length. Unlike fishing line, however, this amount of tension changes drastically depending on the tendon’s or ligament’s recent history of being stretched or slackened.

In a 1992 study at the Biomedical Engineering Institute in Montreal, Quebec, Canada, researchers studied how the amount of tension produced by stretching pieces of human

Lumbodorsal Fascia (LDF, seen in lightest blue in Fig.1) is affected by the tissue’s recent history of being stretched or slackened. They applied whatever force was necessary to stretch the LDF sample to 106% of its initial length. Afterwards, the sample was left to rest at its initial length for 30 minutes. Again, the sample was stretched and returned to its initial length. However, this time, the amount of force required to stretch it to 106% of its initial length was more than doubled (see

fig. 2) . The process of stretching the LDF sample and then allowing it to rest made it exhibit elastic properties less like bubble gum and more like a strong elastic. As a result, more force is required to stretch the tissue to a length that would damage it. If, however, the LDF tissue remains slackened for too long, it returns to its prior state.

This may seem irrelevant or uninteresting until one takes a look at the geometrical relationships between body segments in living people. The LDF tissue examined in the 1992 study, along with several other large ligaments, tendons and fascias are often visibly and unnecessarily shortened by most modern humans. Shortening occurs when a person protrudes the abdomen while inhaling, (this shortens the central tendon of the diaphragm by reducing the upward push exerted upon it by the abdominal contents) as well as when a person curves the lumbar spine while standing up from a chair (the increased curve slackens the LDF tissue from the Montreal study).

A person who shortens these tissues causes them to behave more like the fishing line made of bubble gum, and less like the fishing line made of strong elastic. As a result, when the person contracts a muscle attached to one of these tissues, more of the force generated by the contraction is spent taking the slack out of the tissue, so that less is left to produce an actual movement or stabilization of a bone.

In contrast, a person who consistently stretches these tissues causes them to behave more like the fishing line made of strong elastic and less like fishing line made of bubble gum. This has three notable consequences for people who create and maintain length in these tissues. Firstly, they will fatigue at a slower rate. Secondly, the time it takes for them to translate an intention of movement (efferent nerve signal) into a production of that movement (a muscular contraction and subsequent transmission of force to bone) will be shorter. Thirdly, the accuracy of the movements they make will be improved.

To validate the claims about fatigue and time lag, we must return to the analogy of the fishermen. Imagine that the fishing line is made of the lumbodorsal fascia (LDF) examined in the 1992 study. However, one of the fishing lines was repeatedly stretched, (we call this the recently stretched LDF line). The other fishing line hasn’t been stretched (we will call this the chronically shortened LDF line).

The distance the fishing line must be stretched beyond its initial length before it can move a fish of any size is twice as much for the chronically shortened LDF line as it is for the recently stretched LDF line. This means a fisherman using the shortened LDF line will have to wind his reel more than the fisherman using the recently stretched LDF line, and exert more effort in the process. The difference in necessary effort becomes more significant as the weight of the fish increases.

Human bones weigh as much or more than many fish, and the force required for a person to move an end of one of their bones (for instance, the shin) over a given distance is much greater than the force required to reel in a fish of the same weight over the same distance. This is because tendons are attached very far from a bone’s center of mass, and very close to the axis around which the bone rotates. If a person attempted to lift a shovel loaded with dirt the way a tendon lifts a bone, they would hold the shovel by the handle with their hands as far away from the blade as possible.

If fish were reeled in the same way tendons pull on bones, the fishing hook would be attached to the end of a long stick close to the fisherman, with the fish attached to the opposite end of the stick, as depicted in Fig. 3.

It will also take more time to reel in the fish with the chronically shortened fishing line than with the recently stretched fishing line, because the acts of winding a reel and contracting a muscle are not instantaneous. This difference in time, like the difference in effort, becomes more pronounced as the force required to move the fish increases.

To demonstrate the impact of viscoelastic tissues on accuracy of movement, we will use an illustration of two maces and chains. One has a chain made of “recently stretched LDF”, and the other has a chain made of “chronically slackened LDF”. Fig. 4 is an image of the “Witch King of Angmar” from J.R.R. Tolkien’s “The Lord of The Rings”. Let us imagine, for a moment, what it is like to be him in the midst of battle.

Fig. 4

As the witch king swings his mace to crush the skull of a fallen enemy, a soldier he had previously thought to be dead rises and stabs at his left side. Seeing this new threat, he diverts the trajectory of his mace in a new direction to parry the blow.

To survive in a battle, it is necessary for the witch king to adapt to unforeseen circumstances. This adaption requires a closed loop feedback system, where sensory input influences motor output. Whether sensory input or motor output came first is a mystery equal to that of the chicken and the egg, so in analyzing the combat scene, we will start with motor output.

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Motor output #1: The witch king swings his mace and chain at the fallen enemy, and turns his head to the left to watch its trajectory.

Sensory input: The new field of peripheral vision available to the witch king after turning his head shows a soldier thrusting a blade at him.

Motor output #2: In order to parry the blow, the witch king swings the handle of the mace in a different direction than he initially intended.

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In between the sensory input and motor output #2, there is an inevitable time lag. If the witch king is to accurately parry the blow, the chain must pull the mace into a precise position at a precise time. If the chain behaves like chronically slackened LDF, it will take longer for the mace to change direction when the witch king pulls on the handle (it has to stretch further), and he may be too late to parry the blow. If, however, the mace exhibits the properties of the recently stretched LDF, its direction will change relatively quickly, and the witch king will save himself from injury. Only in a hypothetical world where every force that would act on a person’s body has been accurately predicted by that person would accuracy of movement be independent of the speed of adaptation to sudden changes in the environment.

Just as the chain that connects the handle to the mace must be stretched to a certain length to move the mace, the tendons on either side of a muscle must be stretched to a certain length before they can move a bone. As a result, a person cultivating a posturality that consistently stretches the lumbodorsal fascia, central tendon of the diaphragm, and iliotibial tract will increase their potential for speed and accuracy of movement. Nearly all human movements require force to be transmitted through these tissues.

One could argue that the work done by muscles in order to maintain tension in elastic tissues would actually cause more fatigue. Fortunately, however, it is possible to arrange the parts of one’s body such that appropriate tensions in certain elastic tissues are partially maintained by gravitational pulls. Lessons in applied biomechanics help students to model such an arrangement in their minds, and to adjust their own posturality to replicate this model. The details of this model (called a “posturality of mechanical advantage”) are beyond the scope of this article.

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References:

Yahia, L., Pigeon, P., & DesRosiers, E. (1993). Viscoelastic properties of the human lumbodorsal fascia. Journal of Biomedical Engineering, 15(September), 425-429.

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Fig 3. Image by Greta Joynsen- https://www.instagram.com/greta_joynson/