HPW Biomechanics provides biomechanics & human factors experts that specialize in Personal Injury Accidents that emphasize the interactions between people and their environment including perception response/reaction, expectations, and actions (human performance, human factors)  and the consequences of their actions (biomechanics).  HPW Biomechanics, Inc.'s core expertise is in Personal Injury Accidents / Injury Biomechanics in areas such as motor vehicles / pedestrians, workplace / industry, sports / recreation, slips / trips / falls, and sexual assault / rape.

A Human Performance Model

Barry T. Bates, B.S.E., Ph.D., FACFE

From: Proceedings of the 64^th Convention of the Oregon State Bar, 1999  

I. INTRODUCTION TO ‘HUMAN PERFORMANCE’

The evaluation and analysis of most personal injury accidents requires a multidisciplinary approach that focuses on the interactions between people and their environment including perception response/reaction, expectations, actions and the consequences of those actions. More specfically, analysis techniques and procedures can distinguish a human performance approach from other approaches.

In simple terms, engineers apply the laws of science and mathematics to the designing and building of structures, machines, products, systems and processes. Their concern with the nature and characteristics of the human component varies depending upon their background and training. The biomechanist and biomechanical engineer apply the same laws of science and mathematics to human systems. Human factors engineers specialize in the relationships of human-machine environments from a behavioral perspective. A human performance approach focuses on the overall functioning of the human body as it interacts with the environment and typically approaches the problem from a broader perspective. A human performance approach/specialist is concerned with: 1) what people do, 2) why they do it (Human Performance, Human Factors), 3) how they do it and 4) the consequences of doing it, i.e. tissue damage/injury (Biomechanics). Given these definitions, it is possible that any one of these individuals/approaches might be appropriate for a particular case.

All accidents are the result of error on the part of one or more individuals as they interact with their environment and or a human machine system. The individual(s) involved can be in an active or passive state. In the passive condition, an unsuspecting individual is acted upon by external forces and his/her actions are determined mostly by the laws of physics with typically limited or no ability to react. This is predominantly a biomechanics problem. Examples include an unsuspecting occupant involved in a rear impact collision or the movements of an unbelted occupant in a vehicle rollover. Independent of the incident, the individual could be between actions (momentarily passive) at the time of the incident such as having stopped at a light and waiting for it to change before proceeding. A second type of biomechanics problem can occur when the individual is exposed to forces that are vastly greater than his/her capability to counter or control the effect of the forces such as can occur in high speed frontal collisions. These accidents can be analyzed scientifically using biomechanical principles in conjunction with the appropriate injury criteria. These types of accidents are not the focus of this discussion.

Active state accidents result when an individual encounters a condition/hazard (known or unknown) that propagates into some type of error within the environment and/or human-machine system. All complex systems/environments at times exhibit perturbing, error producing events, however they are mostly infrequent, recognizable and usually compensated for prior to propagating into and injurious outcome. These errors can be the result of choice or chance. Chance accidents typically occur when an individual encounters an unexpected condition and must function using exploratory/none practiced behaviors that are outside his/her normal experience. "Choice" accidents on the other hand occur when an individual purposely creates a dangerous situation to which he/she must similarly respond. In both situations, the error can become magnified and result in an accident. An accident becomes even more likely if additional potential error producing events are encountered. In addition, the likelihood of making or propagating an error increases as the necessary time of response shortens since the individual no longer has the opportunity to choose the appropriate response and must rely on reflexive (previously learned) behavior patterns. The causal factor of all accidents is human error due to such things as perception, expectancy, cognition, memory, attention and attitude.

As previously discussed, accidents sometimes occur as a result of error propagation, i.e., one error creates a situation that causes or increases the chance of a second error and so on. Under normal conditions the individual is capable of addressing each situation on an independent basis but when unexpectedly perturbed is unable to compensate and perform the appropriate behavior to avoid an accident. These situations/types of accidents are often the result of a failure to identify/recognize normal/predictable behavior by the operator/user in a man machine system. Independent of the cause of the initial error the error propagates a second error which must be dealt with usually in the absence of prior experiences requiring the use of exploratory/non-practiced behavior typically with an adequate time. For example, an individual operating a lift truck/forklift hits a bump causing him to lose his balance. In response, he reaches for something to grab to regain his balance but reflexively grabs an accelerator lever which causes the machine to speed up and results in an even greater loss of balance and a possible fall accident.

Errors can be the result of a volitional choice by an individual (crossing a street at night or outside of a crosswalk) or the result of an interaction with a machine (driver of an automobile losing control) or the result of an interaction with a machine system. Accidents occur as a result of error caused by an inappropriate behavioral response. The error/response can result from human judgment/error or a chance combination of events/circumstances. Consequently, the appropriate behavioral question to ask is whether most or any other individual could have avoided the accident under identical conditions. To answer this question often requires a multidisciplinary approach.

A Model for Human Performance Analysis

The remainder of this discussion is focused primarily on the behavioral aspects of active state accidents. In order to evaluate these accidents from a human performance perspective, it is essential to have an appropriate model. The analysis must include both a behavioral and biomechanical component. This discussion provides the scientific basis for evaluating accidents from a human performance perspective. Although the discussion that follows is rather detailed (and possibly complex), the model is only used in the analysis stage. The results obtained from the evaluation must then be put into a simple and understandable form for consumption by involved individuals including members of the jury in the case of litigation.

As previously indicated, most accidents involve one ore more individuals in an active state. They are doing something that causes the incident which may or may not be their fault. For example, the premise for many product liability cases is that the actions of the person were normal and predictable and that the manufacturer of the product should have foreseen the behavior and provided the appropriate safety features (design and/or warning). In a more complex situation, such as when a pedestrian is hit by an automobile, both people (driver and pedestrian) are active and it must be determined whose actions were inappropriate and to what extent.

The actions of all individuals, regardless of the situation, can and must be evaluated within the contect of the accident. This is a human performance problem and is far more complex than the passive biomechanics problem. It involves a knowledge of how the various body systems function along with their contribution to action or activity. Fortunately, all human performance/action patterns are performed using a similar information processing procedure that is dependent upon past experiences of the individual and their resulting expectations and perceptions. Past experiences in the same or similar environments influence the individual’s expectations and the assumptions he/she makes regarding the environment and the task to be performed. These expectations/assumptions in turn influence his/her attentiveness in detecting the sensory input data necessary to select the appropriate behavior to successfully complete the task (Figure 1). Successful detection is also dependent upon unambiguous cues and the absence of distractions. The decision-making factors are identified in .

Summary of Model Application

In summary, given our knowledge of behavioral patterns and this general behavioral model, we can analyze any incident/accident involving an individual(s) interacting with any component in his/her environment (such as a product liability issue). It is also essential to evaluate the biomechanics of the incident and determine the relationships and consistency between the actions as determined by the human system in conjunction with the laws of physics and the observed tissue damage/injuries (Figure 4). A brief description of representative cases using this approach to determine cause and effect relationships follows. Some of the cases are relatively straightforward biomechanical problems while others are far more complex human performance problems.

II. REPRESENTATIVE CASE EXAMPLES