Every joint in the body undergoes shock absorption and rebound. Ligaments provide shock absorption and muscles provide rebound.
Although shock absorption can be an independent driver of joint movement, it occurs in coordination with the action of muscles. As the joints move, the ligaments within them sense direction, speed and acceleration, and regulate the muscles to maintain balanced tension for joint integrity. Thus, in a normal joint, a rebound muscle will remain in a mild state of tension in preparation for shock absorption, increase activation during the process, and fully activate once the bottom point is reached.
On the other hand, some muscles help drive the joint into shock absorption; these muscles reciprocate with the rebound muscles and help provide balance to the structure. While one side of a joint, or of the body, goes into shock absorption, the opposite side goes into rebound, and vice versus; the muscles must be coordinated to accomplish this, which is done through the ligaments.
Most joint injuries occur at the bottom point of shock absorption. Once the joint is injured, the muscles that drive the joint into shock absorption become inhibited, while the rebound muscles remain in a constant state of increased tension in preparation for the next movement. Both tense and inhibited muscles will have reduced circulation and may become painful; this relationship remains until the joint is stabilized and/or healed. These principles are true in any joint in the body, whether it be the foot, SIJ, or spine, etc.
If we consider the entire kinetic chain as one contiguous shock absorber, we can appreciate the relationship between different joints in distant parts of the body. If one joint is compromised, the other joints in the kinetic chain help absorb the force by activating or inhibiting muscles relative to their degree of importance in maintaining both local and global equilibrium. For example, if the knee is injured, the tension units (ligaments, fascia, and muscles) that attach to the femur and tibia are directly affected. The other ends of those same tension units attach at other joints and create an imbalance in tension at the distant joints, as well, in a chain reaction that spreads throughout the structure. Since the sacroiliac joint is the center point of movement and shock absorption in the body with, by far, the greatest mass of ligaments and muscles directly involved in its action, it plays a pivotal role in the musculoskeletal response to shock absorption; muscular adaptations are stronger and their effects are more pervasive.
Please review the opposing actions of the sacrum and ilium during nutation to understand their position as the center point of shock absorption in the Lateral View of Nutation.
When force is transmitted superiorly through the legs, or inferiorly through the spine, a spring like action takes place. Below the sacrum, the feet pronate, the fibulae move inferiorly , and the knees twist as the tibia goes farther into internal rotation than the femur. As the innominate rotates posteriorly, the sacrum rotates anteriorly. At the sacroiliac joint, this movement winds the interosseous ligament, drawing the sacrum and ilium closer  p56 p55, but not together . Above the sacrum, the lumbar curve increases like a compressing spring p54&60 as the sacroiliac joint is forced into nutation. Once the energy is absorbed and the end of range of motion is reached, the body reacts by going into counternutation, and all the above actions are reversed as the spring rebounds. Through this mechanism, the sacroiliac ligaments can be seen as the central spring in shock absorption   p438.
Shock Absorption in Gait
As the right heel strikes the ground, the weight of the upper body is held by the sacrum, forcing the right side of the sacrum to move inferiorly and anteriorly. A corresponding ground reaction force is transferred up through the right leg to the pelvis where it forces the right innominate to rotate superiorly and posteriorly. Both forces induce nutation on the right side. At the same time, the left side of the pelvis goes into counternutation. As one continues, and the left heel strikes the ground, the forces are reversed. This alternating left/right, nutation/counternutation movement travels superiorly though the spine to the head, in a rhythmic oscillation as we walk. Similarly, the reverse motion can occur inferiorly through the spine to the legs . The same pattern will occur with any axially directed force. Whether lifting an object, braking hard, or landing on one’s feet, the force transmitted will create a nutation effect at the center of the body’s shock absorption system, the sacroiliac joint.
According to the principles of Biotensegrity, nutation/counternutation is a subset of whole body shock absorption. Below is a list of other subsets, each of which plays an important role in maintaining balanced tension in the musculoskeletal system.
|Anatomy||Shock Absorption Movement||Rebound Movement|
|Tibia||Internal Rotation||External Rotation|
|Knee||Flexion to 20 degrees||Extension|
|Knee||Internal Rotation (1 degree)||External Rotation|
|Femur||Internal Rotation||External Rotation|
|Hip (Innominate)||Extension (PI)||Flexion (AS)|
|Ilium||Post Inf / Flare In (PI)||Ant Inf / Flare Out (AS)|
|Sacrum||Antero-inferior w/Contralateral Rotation||Post Sup w/ipsilat rot|
|Cervical & Lumbar Curves||Increases||Decreases|
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