Biomechanical Modeling
1. Why, where and when is biomechanical modeling important?
Biomechanics is the study of the effects and control of forces that act on or are produced by living tissue. Its
major classifications are (1) sports biomechanics; (2) clinical biomechanics (including cell, tissue, dental, cardiovascular, respiratory, orthopedic, rehabilitative, spinal, gait biomechanics); and (3) occupational biomechanics.
The most common variables investigated in biomechanical analysis are
- Axial Force - force components acting normal to the tissue surface of interest.
- Shear Force - force components acting tangent to the tissue surface of interest.
- Moment - the tendency of a force to cause rotation about some axis, and is equal to the magnitude of the force multiplied by the perpendicular distance from the center of rotation.
- Stress - force per unit area upon a cross section.
- Strain - the ratio between the change in the dimensions of living tissue under longitudinal loading (tension or compression) and its original size.
- Creep - increasing strain due to constant loading over a long period.
- Fatigue - mechanical failure due to repetitive (cyclic) loading. The lower the loading force, the more cycles that can be applied before failure.
- Elasticity - property of a tissue to return immediately to its original dimensions following loading.
- Plasticity - property of a tissue to retain its change in size and shape when the load causing the change is removed.
- Viscosity- property of a tissue not to deform instantaneously from an applied load.
- Brittleness a property of a tissue that has no or limited plastic deformation.
- Ductility a property of a tissues that exhibits significant plastic deformation.
Biomechanical models are frequently used to (1) summarize a body of information; (2) divide a complex process into unitary functions; (3) divide a complex physiological process into identifiable steps; (4) provide insight into structure-function relationships; (5) summarize one system in order to understand the implications for another system with which it interacts.
2. What are the main issues/problems surrounding biomechanical modeling?
Generally, there are three components in a model: the inputs, the embedded mechanism and the outputs. Given the inputs and outputs, a system identification model is designed to find the embedded mechanism. Given the outputs and the embedded mechanism, a control model is designed to find all the plausible input signals. Given the inputs and the embedded mechanism, a simulation model is designed to find all the possible outputs. No model can represent reality with full accuracy. Models are abstractions; as such, they are necessarily flawed. By nature, models are either overly simplified or overly complicated. The complexity of a model depends on the objectives of the model. A justifiable biomechanical model should have (1) minimal number of variables (e.g., principle of parsimony), (2) sufficient explanatory variability, (3) repeatability, (4) robustness, and (5) validated outcomes.
The general objective of sports biomechanics is maximization of athletic performance. Biomechanical techniques can be applied clinically for the goal of reducing abnormality. The objective of occupational biomechanics is the examination of the physical interaction of workers with their tools, machines, and materials so as to maximize workers performance while minimizing the risk of musculoskeletal disorders. The most reported work-related injuries of musculoskeletal disorders are (1) lower back pain (LBP), (2) injuries related slips, trips and falls, and (3) repetitive motion disorders of the upper extremities. These disorders have a multifactorial pathology and onset can be acute (e.g., due to overexertion) or chronic (e.g., due to overuse).
3. What are main options for addressing the issues raised?
Among the three common areas of musculoskeletal disorders, LBP has the most complex nature, and has the highest cost associated with disability. For years, two basic questions have remained elusive -- (1) is the pain real? (2) where is the pain coming from? Clinical studies suggest that the origin of LBP can range from the discs (herniation/disruption) to the facets, from the ligaments to the muscles. The anatomic levels involved range from the L3/L4, the L5/S1 to the sacroiliac joint. Thus, the pain could be from any location of the lower back area. This area is compact and redundant with various muscles and ligaments, and serves as the bridge between the upright torso and the lower extremities. During physical activities, such as lifting, some muscles act as prime movers or coactivate with other muscles (e.g., psoas and abdominis) to provide stiffness. With very limited mechanical advantage both mobility and stability are achieved. Since the structure performs an important role in transferring the forces and the bending moments from the upper torso to the lower extremities, it is under tremendous stress. Three types of questions are frequently asked by biomechanical practitioners:
- Can the stress due to load, speed or their combination be consistently reduced?
- Can force mechanical advantage be reliably maximized by using mechanical assistance, and/or can the task sequences be alternated to reduce the frequencies of motion?
- Can equilibrium or balance be recovered from errors (e.g., misjudgment on weight, size, or balance) due to unexpected situations?
While some psychologists suggest using the psychophysical approach to determine the maximum acceptable weight for lifting and other manual materials handling, the conventional biomechanical approach is the reduction of spinal loading. This can be achieved by decreasing the load or the moment imposed by the combination of the body and the load. Based on the moment-reduction strategy, several guidelines on lifting techniques have been developed, such as:
- Decrease the load size and weight;
- Decrease the pace of work;
- Avoid coupling the movements of twisting and bending;
- Keep the load within two straddled feet and close to the body;
- Make the load trajectory smooth, and,
- Avoid jerky motions.
Remember to Think before you Lift!
Below is a brief description of a recent biomechanical modeling study performed in the ergonomics lab:
Three Different Lifting Strategies for Controlling the Motion Patterns of an External Load
Coordination of various components of the human body during the course of lifting are very complex and difficult to control. This study hypothesized that strategies used to control the motion patterns of the external load may be applied to control coordination and also to control the level of compressive force on the lumbosacral joint. A simulation of lifting based on the optimization approach was introduced to generate three classes of unique dynamic motion patterns of the external load directed by three different objective functions. The first objective function was to maximize the smoothness of the motion pattern of the external load. The second objective function was to minimize the sudden change of the center of gravity of the body-load system. The third objective was to minimize the integration over time of the sum of the square of the ratio of the predicted joint moments to the corresponding joint strength during the course of lifting. Eight subjects were recruited to perform 40 lifts using each of the three optimal motion pattern of the load. Compressive forces on the lumbosacral joint were computed and compared. The data showed with statistical significance that subjects using the motion patterns of the external load suggested by the first objective function had the lowest compressive force peaks. Thus, this study has satisfied two goals: (1) it indexed and synthesized three motion patterns of the external load by three biomechanically unique objective functions, and (2) it established the association between the spinal loading and the control of the motion patterns of the external load during lifting.
The figures above illustrate the type of lifting task performed. Contact Dr. Simon M. Hsiang for further information regarding this study.