Biomechanics

Biomechanics

Prepared by Dr. Alper DUNKI

Spot Knowledge

• Biomechanics applies physics and engineering to study forces and motion in biological systems.
• Kinematics = motion description (position, velocity, acceleration).
• Kinetics = forces and moments that cause motion.
• Human joints combine translational + rotational movements.
• Free body diagrams and degrees of freedom (DOF) are essential tools.

Definition & Importance

• Biomechanics investigates how energy and forces act on biological tissues.
• .In orthopaedics, it explains injury mechanisms, guides surgical technique, and informs implant/prosthesis design4. Biomechanics

Basic Concepts

• Scalar: magnitude only (e.g., speed).
• Vector: magnitude + direction (e.g., velocity).
• Mass vs Weight: mass = constant; weight = gravity-dependent.
• Center of Mass (COM): coincides with center of gravity in clinical use.
• Displacement: shortest path between two points.
• Velocity & Acceleration: rate of displacement/velocity change.
• Force: push/pull changing motion; unit Newton (N).
• Moment & Torque: rotational effects of force.
• Equilibrium: sum of forces/moments = 0.

Newton’s Laws of Motion

1. Inertia: Body stays at rest/motion unless external force acts.
2. F = ma: Force equals mass × acceleration.
3. Action–Reaction: Equal and opposite reactions.

Fundamental Tools

• Free Body Diagram: shows all acting forces.
• Degrees of Freedom (DOF):
  2D = 3 DOF (2 translational + 1 rotational).
  3D = 6 DOF (3 translational + 3 rotational).
• Coordinate System: X (sagittal), Y (coronal), Z (transverse).

Kinematics vs Kinetics

• Kinematics: motion analysis without forces; crucial for joint kinematics and prosthesis design.
• Kinetics: analysis of forces/moments causing motion.
• Motion: translational, rotational, or combined.

Examples of Joint Mechanics

Elbow (90° flexion, holding weight):

• Forces: forearm weight (Wf), external load (Wo), biceps (Fmuscle), joint reaction (Fjoint).
• Moment equilibrium: pFmuscle = qWf + rWo.
• .Example: Fmuscle = 860 N, Fjoint = 735 N4. Biomechanics

Hip (single-leg stance):

• Forces: abductor (M), joint reaction (J), partial body weight (W).
• Moment equilibrium: M = (W·a)/b.
• .Example: M = 1000 N, J = 1500 N4. Biomechanics

References

• Zhang L, Li B. Knee joint biomechanics in physiological conditions and common degenerative changes. Front Bioeng Biotechnol. 2020.
• Savage M, et al. Altered knee biomechanics & post-traumatic OA outcomes: systematic review & meta-analysis. Sports Med. 2025.
• Chatterjee A, et al. Multiscale biomechanics correlations in ovine stifles. Sci Rep. 20254. Biomechanics