lecture 54

• Kinematic chain- any linked system of rigid segments or bodies.

  • the degrees of freedom can be determined from the nature of the articulation, the number of links, and If the chain is open or closed
  • in planar/2D, where articulations are hinge joints = 1 degree of freedom for each articulation
  • if kinematic chain is closed, there is a reduction in the degrees of freedom for the entire chain
• Kinematic Pair- two adjacent links connected with one joint.
• the degrees of freedom are often called mobility
• Mobility is increased by lifting one foot off the ground and having an open kinematic chain but stability is reduced.
• Closed kinematic chains are associated with decreased mobility but increased stability.
• Form Closed Pair -connection is maintained by the shape of the pair ends (bone).
• Force Closed Pair - connection is maintained by externally applied forces (muscles, ligaments).
• hip joint = ball and socket = closed pair
• knee joint = force closed pair (held together by ligaments)
• (ACL) strain was 5-17 times greater between 0 an d 22 degrees of flexion when compared to weight bearing exercises like the half squat
• the force vector representing the pull of the quadriceps muscle on the tibia has a vertical component as well as an anterior (horizontal component). The anterior component is a shear force across the tibial plateau and requires the ACL to pull posteriorly to force close the articulation.  As the knee becomes more flexed (dotted lines), that shear component of force is reduced and can even become posterior. 
• the force of the quadriceps on the patella causes a large compression against the femur at the more flexed positions.  This explains the finding that patellofemoral pain is increased when exercising at the more flexed knee angles.
• pushup would be considered a closed kinetic chain because the hands (distal) are fixed on the floor
• bench press would be open kinetic chain because hands are NOT fixed, rather are free
• the squat exercise produced less ACL strain in the zero to 22 degree range because of hamstring activity and soles activity.

  • they both have posterior shear component that reduces the quadriceps anterior shear

  SQUAT IS BETTER THAN SIMPLE KNEE EXTENSION
• simple knee extension exercise is called an open kinetic chain exercise with 1 degree of freedom
• moving weight closer to the patella reduces the moment arm of the weight and increases the magnitude of the weight = REDUCING SHEAR FORCE

  
• Exercise machines can be used as an exoskeleton to provide resistance to muscles and prevent shear stress on ligaments in the same way a knee brace stabilizes an ACL deficient knee
• Additionally, if the user of simple knee hinge exercise could use straps to rigidly secure the leg to the moving arm and the thigh to the seat, the hinge could provide the form closed pair that would eliminate shear forces to the tibia and ACL
• The role of exercise in the prevention of an ACL injury includes plyometrics and neuromuscular activities emphasizing proper technique, balance and stability.

  • For these activities, free weights and multi-segment movements requiring the recruitment and coordination of multiple muscles and joints and proprioception may be needed.
• Machines that isolate a single muscle group and use an exoskeleton to unload ligaments may have advantages in rehabilitation but are counter indicated for prevention with the healthy knee.