Concepts in Human Factors Engineering (Part 5 of 11)
Concepts in Human Factors Engineering (5): The Body as a Machine
by Dennis R Andrews PhD, PSP, CECD
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Biomechanics has been around for many years. Relatively recently universities have decided to create a curriculum devoted to the subject. Biomechanics is simply a term used to describe the combining of the human body with the mechanics. Biomechanics involves mathematics, physics, anatomy and physiology among other subjects. The body is considered to act as a machine using mechanical parts. A long bone such as in the arm or leg could be considered a moment arm or lever, joints and articulated surfaces are considered to be frictionless but we know that is not true especially as one gets older. Tendons and tendon sheets can be considered sliding surfaces or cables. Muscles are the motors of the human machine. One must remember a metal lever is not as strong as a long bone therefore a direct correlation is not possible unless the specific strength is known with various degrees of freedom. There is much research pertaining to biomechanics and the human body, most of which has a very narrow application. In motor vehicle safety anthropomorphic dummies are used in an attempt to resemble human beings. It is accepted in the biomechanical scientific community that these types of dummies are inadequate if used to predict the probability of injuries to humans. I have been personally involved in analyzing the great volume of research pertaining to the probability of injuries to humans during motor vehicle impacts and other venues. Unfortunately some scientists who use human subjects misinterpret or misrepresent the results of their research. In order to use human subjects a license or certification must be obtained from the Federal H. H. S. Department. This certification explicitly established that human subject testing couldnít expose humans to the probability of injuring beyond "minimal risk". Minimal risk is defined as the probability of injury, which may occur during daily activities. If the researcher knows an injury cannot occur but continues with their research and makes the claim that the research determined the threshold of injuring then the researcher is proffering a fraudulent conclusion and negates the research in total.
While considering the human body as a machine you must also remember just as with machines there can be occasional breakdowns and maintenance is a constant consideration. The machine must be closely monitored, replenished and renewed. The subtle symptoms alert you to potential problems while food and exercise nourish and strengthening the machine apparatus, aka the human body for this discussion. Like most machines, which are in constant use stress and strain, play important role in wearing the machine parts out prematurely. Stress as everyone realizes can be both physical and mental. Either one can produce devastating consequences if not monitored and kept within reasonable limits. Stress and strain is a large part of biomechanical studies and consequently can be a large part relating to workers health and well being. Mechanics considers forces and their effects on objects. Statics is a state in which all forces are at equilibrium or completely in balance. Equilibrium forces are not a large concern to the human body except to maintain the balance of forces. What is most important when dealing with the probability of injury is the dynamic forces or kinematics. Kinematics considers motion such as velocity, acceleration and jerk, and itís affect on the body, it does not including forces. Kinetics considers the forces and its affect on objects. Newtons first, second and third laws of physics pertain to biomechanics, kenesiology and dynamics. Newtonís second law states force equals mass times acceleration is predominant in biomechanical studies. There are several units to quantify forces such as newtons, kg, foot-pound, and pound, etc. Gravity is crucial in calculating forces and probability of injury. Torque is a product resulting from force over the length of the lever arm, such as tightening a bolt with a wrench. If one knows the force and the length of the lever arm the torque can be easily calculated. Acceleration such as that of gravity can be stated either as 32.2 fps squared or 9.81 m per second squared. This is simply the velocity an object gains from a freefall near the earth's surface.
Simple equilibrium calculations of the arm are a good example of torque. The lever arm could be considered the distance from the elbow to the palm and the force can be considered an object resting in the palm such as a 10-pound weight. To determine the force needed by the bicep to keep the lower arm in equilibrium or at 90 degrees to the body we would simply multiply the distance of the lower arm with the 10 pound object, i.e. 11 inches times the object 10 pounds equals a force of 110 pounds. This is a simple mechanical model, if the bicep connects to the lower arm at an angle the trigonometry function is also considered. Of course this example is not considering any friction between the joints or muscle tissue and tendons. This simple model is a clear indication that large forces can be present at the joints. Using the above example one would ordinarily attempt to determine where the distal part of the bicep connects to the long bone in the lower arm, this can be very difficult from an anatomic point of view. Usually a relatively accurate result can be obtained within a range of one or two inches. This method of analysis is most useful in determining forces the human body must deal with during daily activities. If you consider a more complicated area such as the spine or knee a free body diagram showing the muscles, tendons and bones with their accompanying angle must be prepared. The free body diagram allows one to plot trigonometry and force calculations in an orderly fashion. There are also tools, which can be used in determining these forces such as force plates, strength springs or weights. The example just given is of equilibrium a dynamic example would use the formula of force equals mass times acceleration, for angular moments the formula would be moments equal moments of inertia times angular acceleration. In either case an appropriate quantitative value can be accurately determine.
Biomechanics relies to a great extent on anthropomorphic data fit into a mechanical model. As discussed earlier anthropomorphic data must be meaningful and of value to the subject at hand. Sometimes this data can be very difficult to obtain therefore only ranges can be calculated. The body consists of many joints or articulated points where the body moves with several degrees of freedom. These degrees of freedom must be identified and considered for the portion of the body under investigation. When preparing an analysis accurate identification of the body part must be described such as medial (midsagittal) plane, frontal or coronal, horizontal, distal, anterior, posterior, etc. Biomechanical researchers use an axis system identified as XY and Z. The positive X axis is horizontally towards the front the positive Y axis is horizontally to the right of the body and the positive Z axis is vertically down through the center of the body. Of course the negative values are in the opposite direction of their respective positive positions. Inertia is an important value giving the different geometric shapes and mass sizes of the different body parts. Inertia is the resistance to change or movement. For example it is much harder to start to push a stalled vehicle and overcome inertia then it is to continue pushing the same vehicle once inertia has been overcome. Inertia is a product of geometry and mass. Body volumes must be considered as moments of inertia. Links and joint centers are particular importance when considering the portion of the body, which is articulated. Joints are considered a point at which specific body parts are connected and articulate while links are the point at which the joint or the subject body parts are located. As stated previously anthropomorphic data can be used to determine ratios of body part length and proportion, which is of value in various calculations.
Regression equations are use to determine the best fit for statistical analysis and help in determining the degree of accuracy of the analysis. Regression analysis is most accurate when there are a large number of data to plot. The smaller the population for analysis the greater the potential for error or deviation. Designing safe equipment and workstations depend greatly on the relative weights and ratios of body segments. The center of mass is an important location since walking is crucial pertaining to slip and fall injuries. Center of mass is determined basically the same way with most objects. With a living human it may be more difficult due to the breathing of the living person. The basic method is to determine the point at which equilibrium takes effect. The human body when thought of as a stick person can easily demonstrate the links and rotation in the three degrees of freedom, XY and Z. Movement of the hand and foot begin with the rotation or torque of the link attached to it. For example hand movement requires elbow, forearm and wrist movement, if the movement is exaggerated the movement of the links would not only be larger but the upper arm and shoulder links may become involved. The same is true of the foot and ankle movement.
When describing motion of the human body the terms could be confusing. The terms were described as anatomical positions. Flexion is a movement of the head normally forward or downward while extension is a movement of the head rearward. Abduction is easy return to remember since to abduct something means to take away, so abduction would be moving the leg away from the centerline of the body. The word pivot would indicate rotation or a movement up or down. Some of the motion can occur in more than one plane. To some it is easier to think of the movements in terms of a standard coordinate system.
In summary biomechanics is the simplification of the human body by attaching mechanical properties to specific body parts. The muscles are the driving energy source while the tendons act as stabilizers for the bones or levers. Thinking of the human body in mechanical terms creates some limitations and may have inherent unreliability. Using a mechanical analysis makes it easier to explain and visualizing. Of course everything, which is simplified, which may be complicated creates oversimplification and subtle conflicts may be overlooked or not addressed. Body segments or ratios can be easily calculated from available data. Kinematics can be more easily understood from a mechanical standpoint. The human gait or walking can easily be analyzed to address safety problems. When using the laws of physics and mechanics it is relatively simple to calculate forces upon certain parts of the body.
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