Lecture 2

Compression: A force that pushes together fibers of a material.
- Example: The weight of a building applies compression to its foundation.
- In the context of the spine, the intervertebral disc (IVD) is the principal component for carrying compression.
- Experiences compression even when standing still.
Tension/Distraction: A force that elongates fibers of a material.
- Example: Stretching a rubber band applies tension, elongating its fibers.
- In spinal flexion, tension applies a pulling or stretching force axially through the body.
Shear: The intensity of force parallel to the surface on which it acts.
- Shear force causes one portion of an object to slide or displace relative to another portion.
- Notably important at L5/S1 due to lumbar lordosis (curvature of the lower spine).
- Occurs when the trunk bends forward and is affected by gravity acting on the upper body.
Torsion: Rotational forces acting around the long axis of a structure.
- Example: Torsional fractures of the tibia demonstrate effects of torsion.
- Objects under torsion develop internal shear stress, maximally at the periphery and none at the neutral axis.
Bending: Occurs when an eccentric force is applied, creating compressive stress on one side and tensile stress on the opposite side.

Typical FSUs range from C2/C3 to L4/L5, moving in relation to adjacent vertebrae.
Atypical joints include:
- C0/C1 (Atlas/Occiput)
- C1/C2
- L5/S1

Configuration of a path geometric center describes when moving through its range of motion.
Determined by the orientation of facet joints and intervertebral discs.

Definition: Rotation or translation about one axis is consistently associated with another motion about a different axis; one primary motion is always accompanied by a secondary motion.
- Normal and expected pattern of motion.

Occurs when the usual movement pattern does not occur, often linked to instability and tissue deformation.
Example: FSU intended to flex (+θX) displays extension (-θX).

Causes deformation over time of associated joints, impairing their ability to manage forces and loads adequately.

Kinematics: Study of motion in rigid bodies, disregarding the forces causing motion; focuses on range and patterns of motion in spinal joints.
Kinetics: Examines relationships between forces acting on a body and resultant changes in motion; considers internal and external forces, such as:
- Gravitational forces acting on an athlete during a sprint.
- Internal forces generated by muscles, tendons, ligaments for stabilization and movement control.

Act upon the spine from outside; encompass:
- Gravitational Forces
- Applied Loads: Actions like lifting.
- External Resistances: Encountered during activities (e.g., pushing/pulling).
Example: Lifting a heavy object includes both the object's weight and gravitational force acting on the spine.

Generated within the body to counteract or respond to external forces, provided by muscles, ligaments, and tendons to maintain stability and manage loads.
Example: Contracting spinal muscles create internal forces that stabilize and control movements when lifting objects.

Definition: General term for applying a force or torque to a structure.
- Example: Lifting weights leads to loads on the spine, particularly on the L5 vertebra due to upper body weight and external loads.
Categories of Load:
- Static Load: Constant load over time.
- Dynamic Load: Load that changes over time.
- Ultimate Load: Maximum load a structure can endure before failure.

Body weight acts as a force through gravity.
Structural anatomy influences how loads are experienced, including:
- Tension in spinal ligaments and surrounding muscles.
- Intra-abdominal pressure and any applied external loads.

Upright Standing: Upper body center of gravity is anterior, creating a constant forward bending moment needing counteraction by back extensor muscles.
Flexed Trunk: Increases moment arm, increasing flexor torque and compensatory extensor tension.

Due to spinal muscle moment arms being small, large forces must be generated to oppose flexion torques commonly resulting from muscular activity.
Compression on lumbar spine dynamics:
- Increases when sitting and more so during spinal flexion or slouched positions.
Comparative Forces: 50 Nm of extension torque yields 800 N of compression at L4/L5; 50 Nm of lateral flexion and rotation may yield 1400 N and 2500 N of compression at the same level respectively.

Interactive learning by using spine models with colored wool to explore:
- Spinal structures experiencing various forces (compression, tension, shear, torsion, bending) during movements such as flexion, extension, lateral flexion, and rotation.
Complete the in-class activity worksheet available on Moodle!