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Kinematics of Trauma

Category: Core Concepts Last updated: 2026-04-05 San Juan College EMT-B Program — New Mexico

Overview

Kinematics is the physics of motion. In trauma, kinematics describes how energy is transferred to the body during injury and predicts where that energy is likely to cause damage. Understanding kinematics lets you anticipate injuries that are not visible — internal hemorrhage, spinal injury, tension pneumothorax — before the patient is even fully assessed.

The goal is not to diagnose. The goal is to raise or lower your index of suspicion for specific injury patterns and decide how aggressively to assess and how fast to transport.

The Physics Foundation

Kinetic energy = ½mv²

  • m = mass (weight of the moving object or body)
  • v = velocity (speed)

The critical variable is velocity, squared. Doubling the mass doubles the energy. Doubling the speed quadruples the energy. A 50 mph MVC does not have twice the energy of a 25 mph MVC — it has four times the energy.

Energy is not created or destroyed — it is transferred. When a body in motion strikes a surface, or a projectile strikes a body, the kinetic energy is transferred to tissue, causing disruption. The more energy transferred, the greater the tissue damage.

The Three Collisions of an MVC

Every motor vehicle crash actually involves three separate collisions:

Collision 1: Vehicle strikes object - The vehicle decelerates and absorbs energy through crumpling, airbag deployment, etc. - Modern vehicle safety engineering is designed to manage this collision

Collision 2: Occupant strikes vehicle interior - The occupant continues moving at vehicle speed until they contact something: steering wheel, dashboard, windshield, door - The location and pattern of this contact predicts injury location

Collision 3: Internal organs strike body wall - The solid organs (liver, spleen, brain, heart) continue moving inside the body after the body has stopped - This is how a patient with no external injuries can have a ruptured spleen or aortic tear

Understanding the three collisions explains why someone "walked away from the accident" but has a splenic laceration — their body stopped in collision 2, but their spleen continued in collision 3.

Restraint Systems

Seatbelts

Seatbelts are life-saving but create predictable injury patterns by transferring energy to specific body regions: - Lap belt only (older vehicles): flexion of lumbar spine, abdominal organ compression, small bowel injury, lumbar fracture (Chance fracture) - Shoulder belt (or 3-point harness): clavicle fracture, rib fractures, sternal contusion, pulmonary contusion - Seatbelt sign: linear bruising across the abdomen or chest from belt contact — document, assess for internal injury, transport even if patient feels "fine"

Airbags

Airbags deploy in 30–50 milliseconds at ~200 mph. They prevent the occupant from striking the steering wheel or windshield (collision 2).

Airbag-specific injuries: - Facial abrasions and burns from airbag material - Forearm fractures (from impact against deploying bag) - Eye injuries - Chest wall contusion (in small patients)

Critical caveat: airbags are effective only with a seatbelt. Without a seatbelt, the occupant may already be moving toward the airbag at high speed, and the airbag becomes an additional surface to strike.

Deployed airbags = evidence of significant energy transfer even if the airbag "caught" the patient.

Falls

Energy = mass × gravity × height (potential energy converted to kinetic at impact)

General rules: - > 3× the patient's height = significant MOI for an adult - > 10 feet or 2–3× height = significant MOI for a pediatric patient

Landing surface matters: Hard concrete transfers more energy to the body than soft grass. Body position at impact determines injury pattern.

Landing position: - Feet-first: calcaneal (heel) fractures, pilon fractures, tibial plateau fractures, femur, and then axial compression up the spine (lumbar fracture, thoracic fracture, chance of spinal injury) - Head-first: obvious TBI risk; C-spine compression - Side: hip fractures, shoulder, clavicle

In NM oil fields, falls from elevated equipment, tanks, and scaffolding are common. Ask about height and surface.

Penetrating Trauma

Low-Velocity (Knife, Ice Pick, Most Handguns < 2,000 fps)

  • Energy is deposited along the wound track
  • Injury is limited to the direct path of the penetrating object
  • Assess by wound location: head/neck/torso penetration = high risk for vascular and organ injury
  • Handgun rounds: not all handguns are low-velocity — consider caliber and range

High-Velocity (Rifle, Military Weapons > 2,000 fps)

  • Much higher kinetic energy due to velocity² relationship
  • Creates a temporary cavity as the projectile passes through — tissue is thrown outward and then collapses back
  • The temporary cavity can be significantly larger than the wound track
  • Organ damage may occur centimeters from the wound channel
  • Entry wound underestimates injury extent; exit wound is typically larger than entry
  • Fragment wounds from high-velocity rounds follow multiple tracks

Practical implication: A small entrance wound from a rifle round can have devastating internal injury. Any penetrating trauma to the torso = priority transport.

Significant MOI Criteria

These triggers prompt a rapid trauma assessment (head-to-toe DCAP-BTLS) rather than a focused exam, and generally indicate priority transport with advance ALS notification:

Motor vehicle crash: - Speed > 40 mph - Significant intrusion into passenger compartment (> 12 inches at occupant, > 18 inches at any site) - Rollover (unrestrained occupant or any rollover with ejection risk) - Ejection from vehicle - Death of another occupant in same vehicle - Airbag deployment without seatbelt

Falls: - > 20 feet (adult) - > 10 feet or 2–3× patient height (pediatric)

Penetrating trauma: - Any penetrating wound to head, neck, or torso

Blast injuries: - Any explosion — three mechanisms: blast wave overpressure, fragmentation, and secondary impact from being thrown

Motorcycle/bicycle/pedestrian vs. vehicle: - Unprotected body absorbing vehicle energy; predictable patterns including head injury, lower extremity fractures, torso injury

Other indicators: - Patient ejected from horse or ATV - Crush injuries with significant force - Near-drowning, hanging, strangulation

Predictable Injury Patterns by Mechanism

Mechanism Suspect
Restrained driver, frontal MVC Sternal contusion, rib fractures, pulmonary contusion, aortic tear (high speed), knee/patella, ankle
Unrestrained driver, frontal MVC All of above + TBI (windshield), facial trauma, C-spine, abdominal organ laceration
T-bone / lateral impact Head injury ipsilateral, clavicle, shoulder, rib fractures, pneumothorax, liver/spleen (depending on side)
Rear-end collision Cervical hyperextension (whiplash), lumbar injury
Pedestrian struck Lower extremity fractures (bumper impact), contralateral torso/head (secondary fall impact), pelvis
Feet-first fall Calcaneal fractures, ankle, tibia, knee, hip, lumbar spine, compression fracture sequence up spine
Dashboard knee impact Posterior hip dislocation, femur fracture, knee injury
Handlebar bicycle/motorcycle Abdominal organ laceration (spleen, liver, bowel), sternal injury

Application to Assessment

Kinematics is used at scene size-up (scene-size-up) and drives: 1. MOI determination — significant vs. non-significant 2. C-spine consideration — based on MOI before patient contact 3. Assessment path — significant MOI → rapid trauma assessment (full DCAP-BTLS head-to-toe) 4. Transport decision — significant MOI with any complaint → priority transport

The physical exam (secondary-assessment) confirms or refutes what kinematics predicted. You may find more (unexpected injuries) or less (patient absorbed energy without tissue damage). But you start with the mechanism to know where to look.