Falls represent a significant health problem in the United States, accounting for more than 14,000 deaths a year, and are often associated with a perturbation that occurs during normal activities such as ambulation, stair ascent, or stair descent.1 In addition, slip or trip incidents resulting in falls are frequently linked to less severe injuries, such as those to the head, lumbar spine, shoulder and knee joints. Traditional investigations have focused on liability issues, such as code violations, thereby assuming a causal relationship between the subject event and claimed injuries. However, determination of a causal relationship between an injury and a specific fall incident requires thorough biomechanical analyses of the subject incident for evaluation of injury mechanisms (direction) and force magnitudes associated with the injuries in question.

A biomechanical engineer who is trained in the application of the concepts and methods of mechanical engineering and the physical sciences to medicine and the human body is needed to correlate mechanisms of injuries with the mode and intensity of force transfer associated with the subject incident. While such biomechanical analyses are routinely conducted in automotive accident investigations, there have been limited references to their use in slip, trip, and fall analyses.2,3,4 In light of the limited discussions in the literature regarding biomechanical analyses in slip, trip, and fall incident investigations, the purpose of this paper is to discuss the biomechanics of falls, as well as injury mechanisms associated with common injuries from slip, trip, and fall incidents. In addition, two case studies are included to demonstrate the application of biomechanical analyses in the investigation of slip, trip and fall incidents.

Biomechanics of Falls

The human gait cycle is divided into two separate regions representing the period of time when the foot is in contact with the ground (stance phase) and the period of time when the foot is not in contact with the ground (swing phase), as it moves forward for the next cycle of events.5 Falls occurring during ambulation typically occur from a perturbation during a person's gait cycle. The directionality of the fall forward or rearward is in part dependent on where in the gait cycle the perturbation occurs. The stance phase begins when the foot initially comes into contact with the floor. It is at this point, that the potential for a slip and fall due to inadequate friction between the floor/shoe interface is the greatest. During a slip and fall event, the forward motion of the slipping foot results in a rearward shift of the body's center of mass relative to its base of support, leading to a loss of dynamic equilibrium and rearward fall kinematics.6 Such kinematics lead to the likelihood of contacting the ground with the posterior (back) and side aspects of the body.

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