Biomechanics of Sports Injuries

Biomechanics Basics - Force & Failure Fun

• Stress ($\.sigma\.): Force/Area ($F/A$). Types: tension, compression, shear.
• Strain ($\.epsilon\.): Deformation/Original Length ($\.Delta L/L_0\.$).
• Stress-Strain Curve:
  • Elastic Region: Reversible. Slope = Young's Modulus (E) (stiffness).
  • Yield Point: Elastic limit.
  • Plastic Region: Permanent deformation.
  • Ultimate Strength: Max stress.
  • Failure Point: Fracture.
• Material Properties: Ductile (deforms), Brittle (fractures easily), Toughness (energy absorption).
• Viscoelasticity: Time-dependent.
  • Creep: ↑ strain @ constant stress.
  • Stress Relaxation: ↓ stress @ constant strain.
  • Rate-dependent: Stiffer at ↑ loading rates.
• Wolff's Law: Bone adapts to loads.

  ⭐ Bone remodels along lines of maximal stress.

• Failure Modes:
  • Load Types: Tension, compression, shear, bending, torsion.
  • Fatigue: Repetitive sub-maximal loads.
  • Acute: Single traumatic overload.

Injury Mechanisms - Snap, Crackle, Pop!

• Primary Forces Causing Injury:
  • Tensile: Stretching/pulling (e.g., muscle/tendon strain, ligament sprain).
  • Compressive: Direct impact/crushing (e.g., contusion, cartilage damage).
  • Shear: Parallel forces in opposite directions (e.g., ACL tear, blister).
  • Torsional: Twisting/rotational (e.g., spiral fracture, meniscal tear).
• Injury Classification by Onset:
  • Acute (Traumatic): Single, identifiable high-energy event. Results in macrotrauma.
  • Chronic (Overuse): Repetitive submaximal stress. Leads to microtrauma accumulation.
• Tissue Biomechanical Response: Load → Stress → Strain. Exceeding elastic limit → Plastic deformation → Tissue failure.
• Characteristic Sounds (Audible Clues):
  • Snap: Often indicates acute ligament or tendon rupture.
  • Crackle/Crepitus: Suggests fracture fragments rubbing or chondromalacia.
  • Pop: Common with ACL tears, meniscal injuries, or joint subluxation.

⭐ The stress-strain curve illustrates tissue behavior: elastic region (reversible deformation), yield point, plastic region (irreversible), and ultimate failure point.

Joint Hotspots - High-Stress Zones

• Knee Joint: High valgus & rotational loads (cutting/pivoting).
  • ACL Injury: Non-contact deceleration, hyperextension, valgus + tibial rotation. Audible "pop".
    • 📌 Unhappy Triad (O'Donoghue's): ACL, MCL, Medial Meniscus tear (ATM).
  • Meniscal Tears: Twisting on flexed, weight-bearing knee. Locking/catching. Bucket-handle type.
• Shoulder Joint (Glenohumeral): ↑ROM compromises stability; overuse risk.
  • Rotator Cuff Tears: Overuse (overhead sports), impingement. Supraspinatus most common; pain on abduction.
  • Dislocations (Anterior > Posterior): Abduction, external rotation, extension (FOOSH). Bankart & Hill-Sachs lesions.
• Ankle Joint: Most frequently injured in sports.
  • Lateral Ankle Sprains: Inversion & plantarflexion (ATFL most common, then CFL).
  • High Ankle Sprain (Syndesmotic): Forced dorsiflexion/external rotation (AITFL/PITFL). Squeeze test positive.

⭐ The anterior talofibular ligament (ATFL) is the weakest and most commonly injured ligament in lateral ankle sprains.

Prevention & Performance - Biomechanical Shield

• Injury Prevention Strategies:
  • Technique analysis & correction: Optimizes load distribution, minimizes joint stress.
  • Equipment modification: Custom footwear, appropriate protective gear (helmets, pads), taping/bracing.
  • Targeted conditioning: Enhances strength, flexibility, endurance, and proprioception.
  • Load management & recovery: Prevents overuse injuries, allows tissue adaptation.
• Performance Enhancement:
  • Movement efficiency: Reduces energy cost, delays fatigue.
  • Force optimization: Maximizes power output and skill execution.

⭐ Eccentric strengthening programs are highly effective in preventing muscle strains, particularly hamstring injuries in athletes (e.g., Nordic hamstring exercise).

High‑Yield Points - ⚡ Biggest Takeaways

• Tensile loading causes ligament sprains (e.g., ACL) and muscle-tendon strains.
• Compressive forces lead to cartilage damage, meniscal tears, and impaction fractures.
• Shear forces are critical in rotational injuries, causing chondral defects or avulsions.
• Repetitive microtrauma underlies overuse syndromes like tendinopathies and stress fractures.
• Eccentric muscle actions frequently cause muscle fiber damage and DOMS.
• Valgus stress is key for MCL tears, often with ACL/meniscus (e.g., O'Donoghue's triad).
• Altered kinematics and excessive loads are primary sports injury risk factors.