Alper DUNKI
Biomechanics
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
Inertia: Body stays at rest/motion unless external force acts.
F = ma: Force equals mass × acceleration.
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