External and internal forces are always acting on the body, and it is important to understand their effects and how these forces can facilitate movement or, conversely, cause damage to the skeletal system.
The skeletal framework is a series of long and short bones connected at joints. Each joint has a specific design that allows for movement in certain directions. Movement at joints is produced by either internal or external forces. Internal forces are generated by muscle contraction, whereas the most important external force is probably gravity, inasmuch as it is always present and must always be taken into account when the factors responsible for movement are considered. Other external forces exist, such as the mechanical force produced by body contact in sports or physical impact during accidents. During movement, both internal and external forces need to be controlled precisely.
All people are aware of using gravity to facilitate movement. When people move in the direction of gravity, the prime factor in producing movement is gravity itself, and muscles are used to control or enhance the effect of gravity so that the required movement takes place. All downward movements, such as sitting down, stepping down, and bending down, are produced primarily by gravity; the muscles are used only to initiate the activity and control the speed of movement.
Some external forces are capable of causing injury during movement. Each time a person takes a step and the heel strikes the floor, a "ground reaction force" is transmitted through the whole body. As the speed of walking increases, so do the ground reaction forces, which leads to greater forces through the lower limb and an increased likelihood of injury. The human body has evolved mechanisms to attenuate the effects of ground reaction forces so that it is not damaged by their repeated occurrence. When the heel strikes the ground, the relatively soft heel pad on the sole of the foot absorbs some of the initial force. The knee flexes as the limb takes the load of the body, and this knee flexion movement, up to approximately 40 degrees, also has a shock absorption effect. Through small movements at the hip and spine, residual forces are further absorbed, which ensures that the effect of these forces is minimal by the time they reach the skull and brain. The healthy muscles are major cushions to absorb the physical forces from all directions to protect the tendons, ligaments, and bones. If the muscles are fatigued or injured, they cannot function as cushions and bone fracture may occur, such as stress fracture in the tibia in running and other sport activities.
Physical forces are transmitted in straight lines. Every time they meet an interface between tissues, some are absorbed by the soft tissues and bones, and others are deflected by anatomic design. No single, straight structure runs the length of the body, and thus force has no direct route to travel. All the long bones in the body are curved, as is the vertebral column. As externally applied forces are transmitted through the bones, the curvature of the bones absorbs or deflects the forces. By the time the forces have reached the cervical spine and the skull, much of the effect has been removed, and the impact on the skull and the vertebral column is kept to a minimum.
The curved bones transfer energy. When a force is applied longitudinally to a curved structure such as the femur, the bone deforms and the curvature increases, absorbing some of the forces rather than transmitting them to more vulnerable structures such as the cervical spine and skull. The absorbed forces form elastic (potential) energy, which is released as kinetic energy when the bone returns to its resting shape. This energy transfer is useful in many human movements. For example, a higher jump can be achieved if compressive forces are applied to the long bones of the lower limbs immediately before takeoff.
Nevertheless, there are limits to how far the skeletal framework can sustain internal and external stress. It is not uncommon that a sudden powerful muscle contraction or a repeated external force can cause bone fracture. One of the purposes of dry needling acupuncture therapy is to lower the stress in muscles so that they can absorb more force during sporting activity and thereby prevent or reduce stress-related injuries to the musculoskeletal system.
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