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have the same penetration of pressure as a 30-pound force applied with the thumb. 4. Line of Action The line of action is an imaginary line extending in both directions along the same path as the direction of force. The line of action helps to visualize the effects of force application, especially rotary motion. It is also helpful when visualizing the impact of multiple forces on a structure. For example, both the trapezius and serratus anterior act on the scapula to produce upward rotation during shoulder abduction movements. However, the line of action for these two muscles is significantly different. If one of them is weak compared to the other, the rotational movements will be affected. Recognizing a muscle's line of action can help identify dysfunctional movement patterns. The above factors come together to create a particular experience of the force. Image 2 shows an application of force to the lateral thigh. The magnitude of the force is dependent on how hard the individual presses. The direction of force application is indicated by the white arrow. The point of application is represented by the yellow bar. In this case, the point of application is a broad contact surface with a fist. The line of action for this force is parallel with the direction of application and is represented by the thin black line. Even though this force is being applied diagonally to the thigh, there is an element of it that is gliding along the skin and an element that is pressing directly down into the thigh. FIVE TYPES OF FORCE There are five types of mechanical force application. The first two, compression and tension, are the most important. The majority of injury conditions are the result of some type of compression or 2 tension force. Yet, recognizing the other three—torsion, bending, and shear—is important as well. Each of the five types of force are described here, along with examples of how they occur in injuries as well as treatment applications. 1. Compression Compression occurs when two surfaces are pressed against each other. The contact surface makes a big difference in how a compression force is distributed. If the contact surface during a treatment is broad, such as the entire palm of the hand or a forearm, then the compressive force is dissipated more broadly. If the contact surface is small, like a pressure-point tool or fingertip, the amount of force delivered in that small area is much greater. Injuries. Compressive forces are frequently a cause of soft-tissue injuries. However, large magnitude forces are not always required to cause damage. As noted earlier, low levels of compression can lead to serious pain and debilitation as well, particularly with nerve compression. Two key factors determine the severity of compression force damage to soft tissues: magnitude and time. The body is quite resilient and capable of handling significant compressive loads if they only occur for a short period of time. Banging your "funny bone" can be a high force load, but it is temporary. If that level of pressure were applied for a long period, long-term damage would certainly occur. Conversely, even low levels of compressive force on a nerve can cause damage if they continue for a long period. It can be difficult to recognize ongoing compression issues in a clinical environment because clients may be unaware of various factors that cause low-level compression. Treatment. Compression forces are used extensively in soft-tissue treatments. The intended physiological effects of compression methods also vary. Some techniques focus predominantly on the mechanical aspects of the compression, such as efforts to move tissue fluids during manual lymph drainage pumping techniques. Most commonly, compression techniques produce neurological results that decrease muscle tightness. 2. Tension In biomechanical terms, tension is a pulling force. The terms tensile stress or tensile force are also used to describe these forces. There is some confusion with this term because people often speak of "tension" in their muscles. What they are really referring to is excess muscular contraction. As with compression forces, tension forces can be spread out over a broad area or concentrated. For example, the proximal attachment of the latissimus dorsi is spread out over a broad area as it connects with the Example of force application. Yo u r M & B i s w o r t h 2 C E s ! G o t o w w w. a b m p . c o m / c e t o l e a r n m o r e . 89