[author: Karla Cassidy]

Introduction

It’s common that rear-end collisions result in symptoms of whiplash. To determine how these symptoms develop, we must first consider the laws of physics. Newton’s first law of motion states that a body at rest will remain at rest, and a body in motion will remain in motion, unless it is acted upon by an external force. This means that when a vehicle is stationary, the occupant within the vehicle will be at rest or, when the same vehicle is moving, their body will travel at the same speed as the vehicle.

What Are the Occupant Kinematics of a Rear-End Collision?

Let’s apply Newton’s first law to an in-line collision scenario where a stationary vehicle is rear-ended by another vehicle. To assess the occupant kinematics within the target vehicle (i.e., the vehicle that gets struck from the rear), collision reconstructionists must break the collision into three distinct phases: pre-impact, engagement, and separation:

1. Pre-Impact
   Moments before the impact occurs, the occupant within the stationary vehicle is at rest. They will, according to Newton’s first law, maintain that position until a force acts upon them to change their course.
2. Engagement
   At the moment of impact, the bullet vehicle (i.e., the vehicle that does the hitting), will begin to exert a force upon the target vehicle and begin to accelerate that vehicle forward.
3. Separation
   Once the momentum of the bullet vehicle has accelerated the target vehicle forward, the vehicles will separate.

The occupant kinematics (i.e. the motion of the occupants inside the target vehicle) changes during each phase of the collision:

1. Pre-Impact
   The occupants remain seated in their seat either stationary or travelling at a constant speed with the vehicle.
2. Engagement
   As the vehicle is accelerated forward, the seatback and headrest of the rear-ended vehicle move forward, while the occupant will initially remain at rest. The occupant’s back is compressed into the seatback and their head contacts the headrest. The occupant’s head travels slightly rearward relative to their torso.
3. Separation
   Once the occupant fully compresses into the seatback, the occupant rebounds forward until they are stopped by their seat belt. The occupant’s head travels the farthest during this motion; this is when their neck muscles activate to protect their cervical spine from the sudden accelerations.

Understanding the occupant kinematics, it becomes possible to identify the most common symptoms of whiplash from a rear-end collision.

The Most Common Symptoms from a Rear-End Collision

Head, shoulder, or neck injury symptoms are typical symptoms reported after rear-end collisions. In minor and moderate rear-end impacts, a forensic biomechanical engineer will typically see soft tissue symptoms, primarily in the cervical spine. As the severity of the rear-end collision increases, the risk of injury also increases. In moderate and severe cases, we see the soft tissue neck pain turn into a whiplash diagnosis.

Signs of a whiplash injury from a rear-end collision include headaches, shoulder pain, and/or neck pain. When serious whiplash symptoms from a motor vehicle collision are left untreated, the initial symptoms could develop into chronic symptoms. How long the whiplash symptoms will persist depends on physical therapy interventions, the severity of the collision, and other unique factors that are case and occupant dependent.

What Can a Forensic Biomechanical Engineer Do?

Biomechanical engineering analysis applies the concepts of mechanical engineering to biological structures of the human body (such as joints, bones, muscles, tendons, or ligaments). This is a forensic biomechanical engineer’s area of expertise and it helps determine how different parts of a person’s body would respond to the forces resulting from rear-end collisions.

A forensic biomechanical engineer can also assess the severity of the collision, along with other contributing factors, and compare all the evidence to the risks of short-term and long-term whiplash, as well as other reported injuries from a rear-end collision. A forensic biomechanical engineer can also take into account an occupant’s past medical history when assessing these risks.

Acknowledgments

We would like to thank Karla Cassidy, B.A.Sc., M.A.Sc., P.Eng. for providing insight and expertise that greatly assisted this research.