Sports, Exercise and Health Science | Injury

Welcome to B.3: Injury - Biomechanics in Action!

Hey future sports scientists! This chapter might sound a bit painful, but it's one of the most practical parts of Biomechanics. We spent time learning about forces (B.2); now we learn what happens when those forces get too big, too frequent, or are applied incorrectly.
Understanding injury from a biomechanical perspective is crucial. It means asking: How did the physics of movement cause the tissue to fail? Let's dive in and learn how to keep athletes safe!

1. Classifying Sports Injuries: Acute vs. Chronic

Not all injuries happen the same way. We classify them based on the onset, which helps us understand the underlying biomechanical cause.

Acute Injuries (Sudden Trauma)

Acute injuries occur suddenly due as a result of a specific, high-force event. The forces involved typically exceed the immediate strength or yield point of the tissue.

Key Characteristics:

• Cause: A single, catastrophic moment of high mechanical stress (e.g., sudden impact, rapid twist, or forceful hyperextension).
• Symptoms: Immediate pain, swelling, and loss of function.
• Examples: A fractured bone from a tackle, an ankle sprain when stepping into a hole, or a sudden hamstring tear during a sprint.

Analogy: An acute injury is like snapping a ruler—it happens instantly when you apply too much force.

Chronic Injuries (Overuse and Repetitive Stress)

Chronic injuries, often called overuse injuries, develop gradually over time. They result not from a single massive force, but from repeated low-level forces that don't allow the tissue adequate time to heal and adapt.

Key Characteristics:

• Cause: Repetitive microtrauma or submaximal stress applied repeatedly (e.g., poor running technique performed thousands of times).
• Symptoms: Pain that worsens during activity, gradual stiffness, and tenderness.
• Examples: Stress fractures, tendinopathies (like Achilles tendonitis), and chronic shin splints.

Quick Tip for Struggling Students: Remember, the core biomechanical issue in chronic injury is tissue fatigue. The tissue is repeatedly stressed before it can fully recover its strength.

2. Biomechanical Mechanisms of Injury

In Biomechanics (B.2), we learned about force application. Injuries happen when the mechanical stress applied to a tissue (like bone, ligament, or muscle) exceeds its ultimate failure point. The type of injury depends on how the force is applied.

The Concept of the Yield Point

Every biological tissue has a yield point or elastic limit.

• Below the yield point, the tissue is elastic: it deforms (stretches or compresses) but returns to its original shape.
• Past the yield point, the tissue enters the plastic region: it deforms permanently (suffers injury).
• If stress continues, it reaches the ultimate failure point: the tissue tears or fractures completely.

Did You Know? Training works by safely stressing the tissue just below the yield point, causing it to adapt and increase its strength, effectively raising the yield point for future activities!

Types of Mechanical Stress Causing Tissue Failure

The three primary ways external or internal forces cause tissue damage are Tension, Compression, and Shear.

Tension (Pulling Apart)

This occurs when forces pull the tissue in opposite directions, stretching it.

• Mechanism: Stretching force applied along the long axis.
• Tissues at Risk: Muscles, tendons, and ligaments (which resist pulling).
• Injury Examples: Muscle strains (tears) during explosive movements, or ligament sprains (overstretched or torn ligaments) when a joint is forced beyond its normal range.

Analogy: Imagine stretching a rubber band until it snaps.

Compression (Squashing Together)

This occurs when forces push the tissue together, compacting it.

• Mechanism: Force applied perpendicular to the tissue surface, pushing inwards.
• Tissues at Risk: Cartilage, bone, and intervertebral discs (tissues designed to absorb impact).
• Injury Examples: Bruises (contusions), vertebral compression fractures (often in weightlifting or falls), and damage to joint cartilage from repeated high-impact landings.

Analogy: Imagine standing on a soft object and squashing it flat.

Shear (Sliding/Rubbing)

This occurs when forces act parallel to the surface of the tissue, causing one layer to slide over another.

• Mechanism: Forces acting across the structure, creating a sliding or slicing effect.
• Tissues at Risk: Growth plates (epiphyseal plates in children/adolescents), joint surfaces, and skin.
• Injury Examples: Blisters (skin layers shearing apart), specific types of bone fractures (especially rotational or twisting injuries), and meniscal (cartilage) tears in the knee when the thigh rotates over a planted foot.

Analogy: Imagine putting your palms together and rubbing them fiercely.

3. Common Biomechanical Injury Types

The structure of the tissue dictates how it fails under mechanical stress.

Fractures and Dislocations (Often Acute)

Fracture: A break in the continuity of the bone.

• Biomechanical Link: Occurs when compression, tension, or shear forces exceed the bone's very high yield point.
• Specific Biomechanics: A stress fracture is a chronic injury, resulting from repeated, low-magnitude compression forces, typically seen in endurance runners (a chronic failure of the bone's repair mechanism).

Dislocation: Complete displacement of a joint surface (bones separating).

• Biomechanical Link: Usually caused by a massive tensile force that stretches or tears the surrounding ligaments and joint capsule, forcing the joint out of alignment.

Sprains and Strains (Soft Tissue Damage)

These terms sound similar, but they affect different structures.

Sprain: Injury to a Ligament (bone-to-bone connector).

• Biomechanical Link: Always caused by tension, forcing a joint past its anatomical limits. Severity is graded (Grade I, II, or III, where III is a complete tear).
• Example: The common ankle sprain, where the foot rolls inwards (inversion), stretching the lateral ligaments too far.

Strain: Injury to a Tendon or Muscle.

• Biomechanical Link: Caused by excessive tension, often during eccentric (lengthening) muscle contractions, or rapid changes in direction.
• Example: A pulled hamstring (muscle strain) during the push-off phase of a sprint.

Memory Aid: Sprain affects Ligaments (L for Lost stability). Strain affects Tendons or Muscles (T for Tear, M for Muscle).

Tendinopathies (Often Chronic)

Injury to a tendon (tissue connecting muscle to bone). This is frequently a chronic, overuse issue.

• Biomechanical Link: Result of repeated high-frequency tensile or compressive loading (especially where the tendon wraps around a bony structure) without adequate recovery. This causes the tendon matrix to break down faster than it can repair.
• Example: "Jumper's Knee" (Patellar Tendinopathy) from repeated high-impact jumping.

4. Risk Factors: Why Injuries Happen

From a biomechanical perspective, risk factors determine how and where excessive stress is applied to the body. We categorize these into intrinsic (internal) and extrinsic (external).

Intrinsic Factors (Internal Risks)

These relate to the athlete's body structure, fitness, and biological characteristics, often leading to inefficient force absorption.

• Alignment and Biomechanical Inefficiency: Poor joint alignment (e.g., flat feet or excessive genu valgum/knock-knees) can change the line of force, causing tissues to be sheared or compressed unevenly.
• Muscle Imbalances: If one muscle group is much stronger than its opposing group (e.g., strong quadriceps but weak hamstrings), the weaker group is highly susceptible to strain during rapid deceleration or movement.
• Flexibility/Mobility Issues: Tight muscles (low flexibility) restrict range of motion, forcing joints to the end of their movement limits abruptly, increasing tensile stress on ligaments and tendons.
• Previous Injury: An injured tissue is often weaker and has a lower yield point, making it highly susceptible to re-injury.

Extrinsic Factors (External Risks)

These involve the environment, equipment, and training methodology.

• Improper Equipment/Footwear: Shoes that provide poor support or cushioning can significantly alter Ground Reaction Forces (GRF), leading to excessive compression forces moving up the kinetic chain (ankle, knee, hip).
• Training Errors: Rapid increase in volume or intensity (too much, too soon). This is the leading cause of chronic injuries because it prevents tissue recovery and adaptation.
• Playing Surface:
  • A hard surface (e.g., asphalt) increases GRF and compression stress on bones and joints.
  • A surface with high friction (e.g., artificial turf) increases the risk of shear injuries when the foot is planted and the body rotates.

Key Takeaway for Biomechanics: Injury is essentially the failure of a biological material (bone, ligament, tendon) when the applied mechanical stress (tension, compression, or shear) exceeds that material's capacity to tolerate the load. By optimizing technique and managing external factors, we minimize unnecessary stress and keep the athlete below that critical yield point.