Axial compression is a fundamental concept in biomechanics, describing a force that pushes inward along the long axis of an object or body part. This force causes the object to shorten or become compacted. Understanding axial compression is central to analyzing the forces that impact the human body’s structures and overall health.
Understanding Axial Compression
Axial compression involves forces transmitted directly along the main length of a biological structure. For instance, gravity continuously exerts a downward axial compressive force on our bodies, particularly on the spine and lower extremities. The human body is designed with specific structures that bear these loads, including the vertebral column, long bones like the femurs and tibias, and various joints.
These structures can withstand a specific amount of compressive force before damage occurs. The vertebral bodies and intervertebral discs are major structures that resist compressive loads in the spine, while bones like the tibia and femur are built to tolerate large compressive forces in the lower body.
When forces exceed these inherent limits, the structures can deform or fail. Axial force transmission also occurs between body parts; for example, a force on the hand can transmit through the wrist, forearm, elbow, and shoulder joints.
How Axial Compression Leads to Injury
Axial compression causes damage to biological tissues through specific biomechanical processes. When a tissue, such as bone or cartilage, is subjected to a compressive force, it undergoes deformation. This deformation can be either elastic or plastic. Elastic deformation means the tissue temporarily changes shape but returns to its original form once the force is removed, much like a stretched rubber band.
However, if the compressive force surpasses the tissue’s elastic limit, plastic deformation occurs, leading to permanent changes or even failure. This failure can manifest as crushing, where the tissue is compacted and broken, or buckling, where a slender structure bends and collapses under the load. For intervertebral discs, excessive compression can cause the disc material to bulge outwards or rupture.
Common Injuries from Axial Compression
Axial compression commonly leads to specific injuries in the human body, particularly affecting the spine and major bones. Vertebral compression fractures are frequent spinal injuries where the vertebrae collapse. These fractures often result in a wedge shape if the front part of the vertebra collapses, or a crush fracture if the entire vertebra collapses uniformly.
A more severe outcome is a burst fracture, where the vertebra shatters into multiple pieces that can spread in various directions, potentially damaging the spinal cord. Intervertebral disc herniation is another common injury, occurring when disc material protrudes due to increased pressure from compression.
In children, axial compression can also cause damage to growth plates, which are areas of growing tissue near the ends of long bones. This can lead to impaired bone growth.
Real-World Scenarios Causing Axial Compression
Axial compression forces often arise from sudden, forceful events. Falls from a height are a common cause, especially when a person lands on their feet or buttocks, transmitting the impact force up the legs and spine. The severity of injuries from falls increases with the height of the fall.
Diving accidents, particularly head-first dives into shallow water, can cause axial compression on the head and neck, leading to cervical spine injuries. Car accidents, especially those involving vertical impacts or abrupt deceleration, can also subject the spine to axial loads. Certain sports impacts, such as spear tackling in football or head-first checks in hockey, cause axial compression injuries to the neck and spine.