Biomechanics is the study of mechanical physics applied to the structure and function of living organisms. The guidelines of mechanical physics help us understand forces involved in various injuries. They also help us maximize treatment effectiveness.
In this article, we explore the mechanical principle of force in relation to injury conditions and massage techniques. We encounter mechanical forces every day but routinely take them for granted. Understanding key components of mechanical force will improve your assessment and treatment skills.
You can think of a force as either pushing or pulling. A force has four key characteristics that help us evaluate the impact it will have on the living body.

1. Magnitude

Force magnitude is a measure of how great the push or pull is. A common example of measuring force magnitude is a weight scale. Simply resting your hand on a scale will register as less than if you leaned heavily on the scale. Large magnitude forces are a key factor in soft-tissue damage, though small forces can also render injury as well, especially if repetitive. Consider the amount of force applied to the median nerve in carpal tunnel syndrome. The magnitude is small, but the length of time the force is applied is what leads to the primary tissue damage.

2. Direction

The direction of a force indicates the angle at which it interacts with the tissues of the body. Force direction is important in determining how the force is dissipated by various tissues when they are impacted. For example, suppose a baseball player is hit directly on the thigh with a ball thrown from the pitcher. The direction of force is straight into the body and a greater amount of that force will penetrate into the tissues (Image 1). Suppose instead the thrown ball just glanced off the side of the body. The direction of force is not straight into the tissue. Much less of the force’s magnitude penetrates into the tissues and less damage is sustained.

3. Point of Application

When a force encounters another object (the body in this case), the point where contact is made is called the point of application. In massage treatment, sometimes that point of application may be a broad-based contact, such as the entire palm of the hand. In other cases, the point of contact is smaller, such as the thumb during a trigger-point treatment. The point of application’s size is important for determining how the force penetrates the tissue. When the contact area is larger, the force is much more dissipated. A 30-pound force applied with the entire forearm will not 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 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 lumbodorsal fascia. Conversely, the attachment site of the hamstrings on the ischial tuberosity is small, so the forces are more concentrated there. When high-tension forces are concentrated in a small area, there is an increased risk of tension injury.
Injuries. Tension forces play significant roles in soft-tissue injuries, with the most common examples being muscle strains and ligament sprains. In these injuries, the two ends of the soft tissue are pulled apart. The tissue will stretch to some degree first, but will eventually tear if tension forces are maintained or increased.
Another example of tension forces producing soft-tissue injuries is neural tension disorders. In these conditions, nerve tissue is overstretched and begins to produce common neurological symptoms such as paresthesia, pain, and numbness. Neural tension injuries are frequently mistaken for some type of nerve compression problem because the symptoms are identical. That is why taking a thorough history and analyzing the biomechanics of any potential injury or movement is essential. Nerve compression and tension injuries will be treated with different strategies, so it is important to make this distinction.
Treatment. Tension forces are used extensively in soft-tissue treatment as well. Numerous theories of movement restriction suggest that range-of-motion losses can be reversed by attempting to stretch or relax tissues. Tensile forces are applied to these tissues in an effort to encourage range-of-motion increases. New research emphasizes the role of tensile force applications in decreasing neurological resistance to a muscle’s fully elongating.

3. Torsion

Torsion is a twisting force. Most commonly, torsion force injuries involve a joint or joints (as in the spine). They happen when one body structure is held in place as the connecting structures are twisted. It would be a rare occurrence for an individual soft tissue that has attachments at both ends to experience pathological torsion, unless it is an extreme event.
Injuries. It is easier to perceive torsion forces when applied to a larger region, such as a joint. For example, when a person is running and immediately attempts to change direction by planting a foot and turning the body, there is a torsion (rotary) force on the knee joint as a whole. There is torsion to the entire joint, but the individual ligaments of the knee are not actually being twisted. Another example of a torsion injury would occur when twisting and bending to pick something up. The impact of the torsion stress is exaggerated when combined with a compressive load such as lifting something heavy.
Treatment. Rotational stretching is an example of how torsion is used in treatment. Another application in treatment involves twisting the skin, such as wringing your hands around a limb (Image 3). These maneuvers are often used in superficial myofascial techniques or those aimed at affecting the subcutaneous nerve tissue.

4. Bending

Bending force is most relevant when you are discussing a rigid structure like bone. It is a combination of compression on one side of a structure and tension on the other. If you think about bending a metal rod, on the concave side there is increased compression, and on the convex side there is increased tension.
Injuries. We don’t think of bending force with soft tissues. However, bones are vulnerable. Think of a long bone that is bending in the middle. Keep in mind that it takes a significant force load to cause a hard tissue like bone to bend. When impacted from the side, a long bone will bend somewhat before it breaks.
Treatment. Bending isn’t a prominent factor of any soft-tissue treatments in terms of individual tissues. Bending a body segment, as in stretching, is frequently used.

5. Shear

Shear is a type of force that involves two structures sliding in relation to each other. Shear forces are at play in many different situations in the body. Friction is the result of shear forces, and there are numerous locations where the body has adapted to reduce friction from shear force stress. A great example of reducing friction from shear forces is the synovial sheaths that surround tendons in the distal extremities. These sheaths reduce shear between the tendon and binding retinacula.
Injuries. Tenosynovitis is a condition that develops from excessive shear force. This is a condition involving inflammation and adhesion development between a tendon and its surrounding synovial sheath. Collagen degeneration (tendinosis) in a tendon can also develop from a tendon rubbing against a bone with shearing forces during movement.
Another pathological condition that involves excessive shear force is spondylolisthesis. In this condition, one vertebra (usually L5) slips forward in relation to the sacrum (Image 4). The suffix listhesis refers to sliding down a slope, and this is essentially what is happening to the L5 vertebra as it moves forward on the sacrum.
Treatment. Shear force is used a fair amount in soft-tissue treatment. Most techniques of massage involve some type of gliding or sliding on the skin where there is a shear force between the therapist’s hands and the client’s body. We know that the gliding hand produces therapeutic effects on the nervous system, even though we are still discovering much more about how this actually works.

Conclusion

Biomechanical forces play a major role in injury conditions as well as treatment choices. A skillful clinician can analyze these biomechanical forces and make predictions about tissues that have likely been injured. These analytical skills are a foundation of effective assessment and play a major role in constructing appropriate treatment plans. Next time you have a client describing an injury condition, see if you can tease apart the various biomechanical forces that were likely at play. In addition, when performing various massage techniques, consider the type of force applications you are using and what their primary physiological effects are likely to be. The more you practice and consider these concepts, the better you will get at using them.