Connective tissue has two components: (1) the cells and (2) the extracellular matrix. The two cells of primary importance in connective tissue are the fibroblast and the myofibroblast. The fibroblast synthesizes all the inert components of connective tissue, including collagen, elastin, reticulin, and ground substance.1–5 The myofibroblast is a specialized cell that contains smooth muscle elements and has a capacity to contract.^6–9

The extracellular matrix of connective tissue includes connective tissue fibers and ground substance. The connective tissue fibers include collagen (the most tensile), elastin, and reticulin (the most extensible). Collagen, elastin, and reticulin provide the tensile support that connective tissue offers. Extensibility or the lack of it is driven by the relative density and percentage of the connective tissue fibers. Tissues with less collagen density and a greater proportion of elastin fibers are more pliable than tissue with a greater density and proportion of collagen fibers.1–3

The ground substance of connective tissue plays a very different role in the connective tissue response to immobility, trauma, and remobilization. The ground substance is the viscous, gel-like substance in which the cells and connective tissue fibers lie. It acts as a lubricant for collagen fibers in conditions of normal mobility and maintains a crucial distance between collagen fibers. The ground substance also is a medium for the diffusion of nutrients and waste products and acts as a mechanical barrier for invading microorganisms. It has a much shorter half-life than collagen and, as will be discussed, is much more quickly affected by immobilization than collagen.^4,10

Connective tissue is classified according to fiber density and orientation. The three types of connective tissue found in the human body are (1) dense regular, (2) dense irregular, and (3) loose irregular (Table 2-1).^11,12

Dense regular connective tissue includes ligaments and tendons (Fig. 2-1).⁵ The fiber orientation is unidirectional for the purpose of attenuating unidirectional forces. The high density of collagen fibers accounts for the high degree of tensile strength and lack of extensibility in these tissue. Relatively low vascularity and water content account for the slow diffusion of nutrients and the resulting slower healing times. Dense regular connective tissue is the most tensile and least extensible of the connective tissue types.

Dense irregular connective tissue includes joint capsules, periosteum, and aponeuroses. The primary difference between dense regular and dense irregular connective tissue is that dense irregular connective tissue has a multidimensional fiber orientation (Fig. 2-2). This multidimensional orientation allows the tissue to attenuate forces in numerous directions. The density of collagen fibers is high, producing a high degree of tensile strength and a low degree of extensibility. Dense irregular connective tissue also has low vascularity and water content, resulting in slow diffusion of nutrients and slower healing times.⁵

Loose irregular connective tissue includes, but is not limited to, the superficial fascial sheath of the body directly under the skin, the muscle and nerve sheaths, and the bed and framework of the internal organs. Similarly to dense irregular connective tissue, loose irregular connective tissue has a multidimensional tissue orientation. However, the density of collagen fibers is much less than that of dense irregular connective tissue. The relative vascularity and water content of loose irregular connective tissue is much greater than dense regular and dense irregular connective tissue. Therefore, it is much more pliable and extensible, and exhibits faster healing times after trauma. Loose irregular connective tissue also is the easiest to mobilize.⁵

Connective tissues have unique deformation characteristics that enable them to be effective shock attenuators. This is termed the viscoelastic nature of connective tissue.1–3,13 This viscoelasticity is the very characteristic that makes connective tissue able to change based on the stresses applied to it. The ability of connective tissue to thicken or become more extensible based on outside stresses is the basic premise to be understood by the manual therapist seeking to increase mobility.

In the viscoelastic model, two components combine to give connective tissue its dynamic deformation attributes. The first is the elastic component, which represents a temporary change in the length of connective tissue subjected to stress (Fig. 2-3). A spring, which elongates when loaded and returns to its original position when unloaded, illustrates this. This elastic component is the “slack” in connective tissue.^1–3

The viscous, or plastic, component of the model represents the permanent change in connective tissue subjected to outside forces. A hydraulic cylinder and piston illustrates this (Fig. 2-4). When a force is placed on the piston, the piston slowly moves out of the cylinder. When the force is removed, the piston does not recoil but remains at the new length, indicating permanent change. These permanent changes result from the breaking of intermolecular and intramolecular bonds between collagen molecules, fibers, and cross-links.^1–3

The viscoelastic model combines the elastic and plastic components just described (Fig. 2-5). When subjected to a mild force in the midrange of the tissue, the tissue elongates in the elastic component and then returns to its original length. If, however, the stress pushes the tissue to the end range, then the elastic component is depleted and plastic deformation occurs. When the stress is released, some permanent deformation has occurred. It should be noted that not all the elongation (only a portion) is permanently retained.^1–3

Clinically, this phenomenon occurs frequently. For example, a client with a frozen shoulder that has only 90° of elevation is mobilized to reach a range of motion (ROM) of 110° by the end of the treatment session. When the client returns in a few days, the ROM of that shoulder is less than 110° but more than 90°. Some degree of elongation is lost and some is retained.

This viscoelastic phenomenon can be further illustrated by the use of stress-strain curves. By definition, stress is the force applied per unit area, and strain is the percent change in the length of the tissue. When connective tissue is initially stressed or loaded, very little force is required to elongate the tissue. However, as more stress is applied and the slack or spring is taken up, more force is required and less change occurs in the tissue (Fig. 2-6). When the tissue is subjected to repeated stresses, the curve shows that after each stress the tissue elongates and then only partially returns to its original length. Some length is gained each time the tissue is taken into the plastic range. This phenomenon is seen clinically in repeated sessions of therapy. ROM is gained during a session, with some of the gain being lost between sessions.^1–3

The older model of connective tissue does not explain completely the quick changes that can occur in connective tissue during manual therapy. Several theories have emerged to explain these quick changes. The most substantial body of literature suggests that connective tissues have a limited contractile ability resulting from the presence of myofibroblasts. Myofibroblasts are differentiated fibroblasts that not only synthesize collagen and ground substance, but also retain the ability to contract. These specialized cells were first recognized to be present in immature scar tissue, and were believed to be responsible for scar tissue shrinkage and contracture.6,7,14–16

More recent literature has documented the presence of both myofibroblasts and smooth muscle fibers in normal connective tissue, including the fascia cruris and the lumbodorsal fascia.^16,17 Myofibroblasts contain smooth muscle of type actin and myosin in the cytoplasm of the cell, and therefore respond to the same stimuli that affect smooth muscle.²⁰ Schleip and associates have described several autonomic mechanisms by which connective tissue “tone” can possibly be affected.^14,15,18,19

First, manual stimulation of interstitial and Ruffini mechanoreceptors present in connective tissue affect the autonomic nervous system in a way that creates changes in local fluid dynamics through vasodilation of local blood vessels and movement of fresh fluids into the interstitial tissue. Second, stimulation of the autonomic system through tissue mechanoreceptors has the effect of “hypothalamic tuning,” which results in a global decrease of muscle tone. Lastly, autonomic effects from manual work create a localized autonomic response that inhibits smooth muscle cells present in connective tissue and relaxes actin/myosin activity in myofibroblasts that are also present in connective tissue. All three of these basic mechanisms can better explain the relatively immediate changes that are palpable after manual therapy.

As an example, consider the patient who is referred for therapy 4 weeks after rotator cuff repair, who has developed significant capsular tightness of the shoulder, and who is reactive and guarded with passive range of motion. Gentle soft tissue work in the shoulder girdle, including the scapula-thoracic area, lateral scapula, upper trapezius, and the pectoralis major and minor usually result in immediate increases in range of motion and less reactivity from the patient. Mechanical changes in the joint capsule are not likely to have occurred in such a short time to explain the increased range of motion. A more plausible explanation would be that gentle manual work around the shoulder complex created an autonomic response, which in turn relaxed the contractile elements present in the connective tissue.