Physics: Kinematics and Arthrokinematics

Kinematic Chains

• A kinematic chain refers to a series of articulated segmented links, such as the connected pelvis, thigh, leg, and foot of the lower extremity.
• There are two types of kinematic chains:
  • Open kinematic chain: the distal segment of the chain moves in space while the proximal segment is fixed or stable.
  • Closed kinematic chain: the distal segment is fixed, and proximal parts move.

Examples of Kinematic Chains

• Squat: an example of a closed-chain motion.
• Leg curl from sitting in a chair: an example of an open-chain motion.
• Reaching or bringing the hand to the mouth: an open-chain motion.
• Performing a chin-up: a closed-chain motion.
• Standing to sit and sitting down in a chair: both are closed-chain motions.
• Walking and stair climbing: alternation of closed-chain motion and open-chain motion.

Convex-Concave Principle

• Normal interphalangeal joint movements follow the convex-concave principle.
• Close-packed position: the ovoid surfaces of joint pairs match each other perfectly in only one position of the joint, usually occurring at one extreme in the range of motion.
• Open-packed position: the ovoid joint surfaces do not fit perfectly and are incongruent.

Characteristics of Close-Packed Position

• Maximum area of surface contact occurs.
• The attachments of the ligaments are farthest apart and under tension.
• Capsular structures are taut (under tension).
• The joint is mechanically compressed and difficult to distract.

Characteristics of Open-Packed Position

• The ligamentous and capsular structures are slack.
• The joint surfaces may be distracted several millimeters.
• This position allows the necessary motions of spin, roll, and slide and may decrease joint friction.

Mechanical Advantage of Close-Packed Position

• Provides great mechanical stability to the joint.
• Reduces the need for muscle forces to maintain a position.
• Examples: standing position, gripping action.

Accessory Motions

• Joint surfaces can be moved passively a few millimeters in translatory motion, called accessory movements or joint play.
• Examples: equilibrium, dynamics.

Arthrokinematics

• Three types of motion can occur between the two articulating surfaces: rolling, sliding, and spinning.
• In a pure rolling motion, each subsequent point on one surface contacts a new point on the other surface.
• In sliding and spinning, the same point on one surface contacts new points on the mating surface.
• Most normal joint movement has some combination of rolling, sliding, and spinning.
• The knee joint shows the combination of these movements most clearly.

Arthrokinematics in Sit-to-Stand

• Rising from a seated to a standing position: the femur is extended on the fixed tibia, the femoral condyles roll and slide so that they are always in contact with the tibial condyles.
• In the last part of knee extension, the femur spins (internally rotates on the tibia).

Clinical Importance of Arthrokinematics

• Arthrokinematics (combination of roll, slide, and spin) permits a large range of motion while using a small articular surface.
• If joints possessed only one of these motions, this would limit the range of motion.

End-Feel

• The end-feel is the resistance felt at the end of the range of motion.
• Normal end-feel: hard, firm, or soft.
• Examples: hard or bony end-feel, firm or springy end-feel, soft end-feel.
• Pathologic end-feel: an end-feel that is not characteristic of the normal joint end-feel and occurs at a different place in the range of motion.
• Examples: empty end-feel, hard end-feel, springy end-feel, sudden hard-feel, capsular feel.