2.6 Muscle Characteristics.pdf

[00:00:00] >> Muscle characteristics and excursion, these are the objectives. Forces applied to tissues produce stresses to those tissues. A stress is a force or load that is applied to a body segment or tissue. Stresses can occur as compression destruction, shear torsion bending, twisting or any co...

[00:00:00] >> Muscle characteristics and excursion, these are the objectives. Forces applied to tissues produce stresses to those tissues. A stress is a force or load that is applied to a body segment or tissue. Stresses can occur as compression destruction, shear torsion bending, twisting or any combination. The muscles and the connective tissues resists the stresses in a similar fashion. [00:00:28] If they are able to withstand the stress injury occurs. Strain is the amount of deformation that something can tolerate before it could succumb to this stresses. Extensibility is the ability to stretch, elongate, or expand, and creep is elongation of a tissue from having a low level stress or load over a period of time. [00:00:57] We will use creep in rehab to help us stretch tissues. Viscosity is a resistance to an external force that causes a permanent deformation. It's a term that we usually think of with fluids, think of syrup versus water. Syrup has more viscosity than water. If syrup is heated, it can become less viscous and more easily movable. [00:01:22] Human tissue also has viscosity, we use this when we might add heat to a tissue before stretching it. We also know that lowering the tissue temperature and making it more cold might make something more stiff. Elasticity is the ability to succumb to an elongating force and then return to your normal length. [00:01:43] Think of a rubber band, viscoelasticity is the ability to resist changing its shape, but if the force is sufficient to cause the change the tissue cannot return to its original shape. All structures have their own specific relationship between stress and strain. This is called the stress-strain curve, or stress-strain principle, which is what we see here. [00:02:10] It varies from structure to structure, the curve here is shown for connective tissue, which serves to represent a generic stress-strain curve for human tissue. The initial part of the stress-strain curve is called the toe region, so it's this part here. In a resting state, tissue has a crimped or wavy appearance. [00:02:33] When a stress is applied to it, the slack is taken up within that region of the stress-strain curve. Once the tissue is elongated to the point at which the slack is taken out of the structure so it becomes the top, the stress moves the tissue into the elastic range. [00:02:53] Here, the elastic range is a point which the tissue's elastic property are stressed. The tissue strain and the amount of stretch move through a linear relationship when there's a direct relationship between the amount of stress applied to the tissue, and the tissue's ability to stretch. If the force or load is released at any time, either of those two ranges, the tissue returns to its normal length. [00:03:14] On the other hand, if a force continues to increase, the tissue moves from the elastic range into this plastic region. In this range, there is a microscopic damage to the structure and some of the tissues rupture because it's unable to withstand that amount of stress. At this point, permanent change in the tissues length occurs. [00:03:33] At the forces released at this point, the tissue is elongated compared to what it was prior to the stress application. If the amount of stress continues past the plastic range, the tissue moves into the necking range. At this point, more and more microscopic ruptures incur until the tissue becomes macroscopically damaged. [00:03:53] Is at this time that the force or load required to create tissue damage is less than previous sleep because the tissue is weakening. If the stress continues immediately before the tissue ruptures entirely, a give in the structure is felt and then the tissue rips apart moving into the failure range. [00:04:13] The functional excursion of a muscle is the distance to which the muscle is capable of shortening after it has been elongated as far as the joint or joints over which it passes allow. Muscles across more than one joint have the greatest excursion measures. There is a large variation from one muscle to another and the ability to shorten. [00:04:38] From a clinicians perspective, they usually don't estimate that any muscles ability to shorten is a maximum of 70% of its resting length. When a person attempts to make a tight fist with the wrist fully flexed, do so right now, the active shortening of the finger and wrist flexors results in passive lengthening of the finger extensors on the back side of the hand. [00:05:09] So, tissues on this side are shortened and these are all lengthened. If the length of the finger extensors is not enough to allow that full range, both the wrist and the fingers, then you might see that the finger will start to extend or you will have a limitation and the amount of the finger flexors to make a fully tight fist. [00:05:35] Active muscle insufficiency occurs when a muscle is not capable of shortening to the extent required to produce a full range of motion at all the joints that it crosses simultaneously. For example, like we saw in the last slide, the finger flexors cannot produce a closed fist when the wrist is fully flexed, as when they can in a neutral position. [00:05:59] If they cannot do so, that would be considered active muscle insufficiency. When the muscle is shortened, actin and myosin overlap reduces the number of sites available for cross-bridge formation. Passive muscle insufficiency occurs when the two-joint muscle cannot stretch to the extent required for full range of motion in the opposite direction at all the joints crossed. [00:06:24] For example, when making a closed fist with a wrist fully flexed, the actin shortening of the finger and wrist flexors results in a passive lengthening of the finger extensors. If, in this example, the length of the finger extensors is not sufficient enough to allow full range of motion at both the wrist and the fingers, we would call this passive muscle insufficiency. [00:06:52] If the muscle is in a lengthen position compared to its optimum length, the actin filaments are pulled away from the myosin heads to a point that they cannot create as many cross-bridges.