Kinematic Chains and Muscle Force Generation

Open vs Closed Kinematic Chain

• Closed Kinematic Chain (CKC) occurs when the distal segment of the chain is restricted by external resistance, meaning that the end of the limb is fixed in space and cannot move freely. This type of movement typically engages multiple joints and muscle groups simultaneously, enhancing stability and functional strength. CKC exercises are also beneficial for rehabilitative purposes, as they promote joint integrity and balance. For instance, during a squat, the feet remain in contact with the ground, creating resistance through the lower extremity that allows for effective engagement of the hips, knees, and ankles.
• Open Kinematic Chain (OKC) occurs when the distal segment of the chain moves freely without external resistance, allowing for more isolated movements primarily targeting specific muscles. This type of exercise is often performed using machines or free weights, where the movement is accustomed to the direct action of the intended muscle group. The bicep curl, for example, allows the individual to focus solely on the contraction of the biceps brachii while minimizing engagement of other muscle groups, promoting strength and hypertrophy development in that specific area.
• Examples of OKC: bicep curl, leg extension, and bench press, all of which effectively target individual muscle groups while the distal ends of the limbs are not fixed. These exercises can increase muscle mass and joint flexibility.
• Examples of CKC: push-ups, squats, and lunges, which require the stabilization of the body as a whole and engage several muscles in a coordinated fashion. This promotes functional fitness and is essential for activities of daily living.

Muscle Force Generation

• Isometric
  • Isometric contractions occur when a muscle generates tension without changing its length, which can be important for stabilizing joints and maintaining posture without moving the bone structure.
  • In this scenario, the internal torque produced by the muscle is equal to the external torque exerted by the load or resistance. This balance allows the muscle to hold a position, providing the necessary strength to counteract external forces.
• Concentric
  • Concentric contractions involve the shortening of muscle fibers as they generate force, which is crucial during movements such as lifting weights or climbing stairs, where the muscle acts to overcome resistance.
  • During concentric contraction, the internal torque exerted by the muscle exceeds the external torque from the load, allowing for upward movement against gravity. This is indicative of a successful muscular effort to perform work on an object or perform a physical activity.
• Eccentric
  • Eccentric contractions occur when a muscle lengthens while generating force, often during controlled movements like lowering weights or running downhill. These contractions can be particularly effective for muscle growth and strength improvements.
  • In the case of eccentric contractions, the internal torque produced is less than the external torque acting on the muscle, which can lead to the lengthening of the muscle under tension. This process is significant for muscle control and the absorption of impact forces.

Basic Mechanics

• Hysteresis
  • When force is applied and removed from a structure this creates a load–deformation curve
  • Not all energy gained during the lengthening work is recovered during shortening work, some is lost as heat
• Strain-Rate Sensitivity
  • Tissues behave differently when loaded at different rates
    • Stiffness increases and larger peak forces can be applied with rapid loading
    • Creep happens more slowly with rapid loading

Basic Mechanics Concepts

• Stress
  • The force per unit area applied to a material
• Strain
  • A measure of how much the object is stretched
• Stress divided by Strain
  • This defines the modulus of elasticity, an indicator of how likely the object is to deform under force

Stress-Strain Curves

• Three Distinct Regions:
  • Initial linearly elastic region is where slope = elastic modulus (E)
  • Intermediate region exhibits yielding and nonlinear elasto-plastic material behavior
  • Final region exhibits linear plasticity where slope = strain hardening modulus
• Therapeutic Range
  • The elastic region of biological tissue is most important clinically

Viscoelasticity

• All connective tissues are viscoelastic materials:
  • Viscosity (fluid property): material’s resistance to flow
    • High viscosity fluids flow slowly (honey)
    • Low viscosity fluids flow quickly (water)
  • Elasticity: material’s ability to return to its original length or shape after a load is removed
    • Length changes are proportional to applied loads
    • Depends on collagen and elastin content
  • Work and Energy
    • When stretched, work is done (force x distance) and energy in the stretched material increases
• Creep: gradual deformation of a solid material under constant load, over time

Explore the principles of open and closed kinematic chains and how they influence muscle force generation. This quiz covers examples of each type of chain and different modes of muscle contraction including isometric, concentric, and eccentric actions. Test your understanding of basic mechanics related to force application and deformation.