## 3 Examples
### Spinal Injury During Lifting  
Attorneys seek biomechanics experts to opine about the forces acting on the internal biological tissues associated with the injury involved in litigation. The request can be to opine on whether the forces exceed some generally accepted threshold established by a government agency or consensus standards organization. This is relevant to the question of negligence of the employer. The National Institute for Occupational Safety and Health (NIOSH), which is part of the Centers for Disease Control and Prevention in the United States, issued a guideline of 3400 N of spinal compression force at the L5/S1 spinal level [5; 6]. NIOSH stated this was the level at which jobs \"are hazardous to all but the healthiest of workers.\" Therefore, the biomechanical would conduct an analysis of the lifting task the plaintiff claimed caused the injury to determine if the spine experienced more than 3400 N of compression force. NIOSH estimates of compression force used to develop this threshold were computed using a deterministic static biomechanical model developed by [PERSON] [7], which has been nicely presented in a common textbook on occupational biomechanics [3]. The model was two dimensional and had rigid body segments joined by hinges. The body segments were hand/forearm (combined), upper arm, torso/head/neck (combined), pelvis, thigh, lower leg, and foot. Postures for all segments, except the pelvis, were input by the user based on measurements made in the lab or field. L5/S1 intervertebral disk orientation, which defined pelvic orientation, was estimated from the torso and knee angle using a regression model [8]. The mechanical analysis was top-down, meaning that it sequentially computed the moments and forces at the elbow, shoulder, L5/S1 disc, hip, knee, and ankle. Erector spinae muscle force was computed using a simplified anatomic representation of the lumbar region extensor musculature. The reaction force at L5/S1 was decomposed into orthogonal components to get shear and compression. [PERSON] [4] implemented [PERSON]'s model [3] as a hybrid Bayesian network so that model inputs could be treated as random variables. Following publication of the hybrid Bayesian network model, it was observed that the model could easily be extended to directly address the \"more likely than not\" criterion. The remainder of this subsection describes the method by which the model described in [PERSON] [4] was developed and extended to address the civil litigation \"more likely than not\" standard of proof. It begins with the first five steps of the methodology proposed in Section 2:  
1. **Identify the deterministic model based on principles of engineering mechanics**. The lifting model described by [PERSON] was selected for implementation (it is described above and presented in more detail in [3,7]), but the effect of intra-abdominal pressure was not included because its effectiveness depends on breath control [9]. 2. **Represent model as a directed acyclic graph**. Nodes (written in _italics_) were made for input variables (mass in hands and body angles), joint reaction forces and moments, included knee angle, L5/S1 intervertebral disc angle, and erector spinae force. Directed edges were added to indicate relationships between forces and moments at ends of body segments, static equilibrium at the L5/S1 intervertebral disc, the regression equation relating included _knee angle_ and _torso angle_ to _L5/S1 disc angle_, and the trigonometry required to decompose the L5/S1 reaction force into _L5/S1 shear force_ and _L5/S1 compression force_ components. The disc injury portion of the model was completed by adding directed edges from the _L5/S1 compression force_ and _disc compression strength_ to _disc injury_ nodes. 3. **Identify nodes that can be modeled as random variables**. Variables associated with input nodes (_mass in hands_ and joint angles) were considered to be appropriately modeled as random variables because of measurement error.