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Concepts in Human Factors Engineering (Part 3 of 11)
Concepts in Human Factors Engineering (3): The Energy Force of Our Frame
By Dennis R Andrews PhD, PSP, CECD (ExpertPages member profile page)
The skeleton receives its force for movement through the power of the muscles. Muscles work against each other (isometrics) or in concert with each other toward a predetermined task. The task may be a simple matter of picking up a coin or as complex as running complicated machinery. Muscles are the only power, which the human body has to complete desired movement and tasks. Muscles will either abduct or adduct body parts; this movement is necessary not only for safely reaching for the operation of machinery but for all complicated tasks. The forces exerted by the muscles on the bones can be considerable and can even at times break bones by overexertion. If one imagines a mechanical crane one could clearly visualize muscles using the bones as lever arms. The forces exerted by muscles are directly linear in the size of the specific muscle. Muscles of the human body are smooth, skeletal or cardiac. Cardiac muscles obviously pertain to the major pump of the human body call the heart. Smooth muscles work in conjunction with their cardiac brother to assist in the flow of blood. While the smooth and cardiac muscles are important for living in a static condition skeletal muscles are paramount for dynamic movement and work performed. As an example the bicep muscle would be considered a skeletal muscle since it is used to move the arm up and down and the smooth muscles contribute to the flow of blood into vessels and internal organs.
|Concepts in Human Factors Engineering is a series containing eleven articles:|
The muscle has three dynamics, the stretch or elongation stage, contracted or shortening stage and the relaxed or passive stage. The change to the muscle fibers creates friction this friction is relieved through the sheathing of the striated bands of the muscle. Contraction of the muscle takes place during lifting with the bicep. The stretching of the muscle occurs as described previously when pulling the arm back in anticipation of the foreword movement of throwing a ball. Nerve fibers within the muscles are used to innervate several muscle fibers, this normally occurs when using human subjects in perception reaction and crash testing where the subject anticipates an impact or reaction. This innervation is what receives most of the criticism of human subject testing when a real world response is needed. If the muscle fiber for a fast twitch is 40 milliseconds and the crash impulse is 100 milliseconds one can readily understand how a human subject can overcome the response to act naturally prior to a crash test event. Muscle fatigue is a safety concern in the industrial workplace. The endurance and strength of a particular muscle is dependent upon the amount of exertion. Muscle strength can be maintained at the maximum level for only a short period of time and decreases rapidly from this maximum level to 10 to 15 percent over a long period of time. This muscle fatigue causes concern for anyone working overhead with arms raised such as working on a ceiling.
The amount of tension or strength directly correlates to the number of muscle fibers of the specific muscle. It is believed maximum tension of skeletal muscles is between 16 and 61 Newton/cm squared. Since muscle tension is within the muscle fibers itself direct muscle force of the body cannot currently be measured. In biomechanics forces are measured at the output of the force such as at the hand, foot etc. The measurement of muscle force is produced by muscle tension transferred inside the body and along links and joints to the measurement point. Torque in a human body segment is similar to the mathematical torque in mechanical engineering. Torque is equal to the length of the arm long bone times the muscle force pulling at the angle. In other words the force is reduced by the appropriate trigonometry function multiplied by the amount of force and the lever arm distance. The knowledge of anthropomorphic data relating to ranges of strength is important in designing hand tools where strength to safely operate the tool and the on off mechanism is important. Strength of course depends on many factors including fitness of the individual, the size of the muscle, muscle cross-section area, etc. If an individual is involved in strength training the thickness of the muscle fibers will increase, not the number of fibers. This increase in thickness is what gives the human body the chiseled and well-defined muscular appearance.
The tension of muscles follow these steps: feedforward is the segment relating to contracting the muscle and the internal transmission through the nerves, feedback is the segment whereby sensory activity is compared with the brain, the third segment is the output which is the exertion at the output point such as a hand. Observing and documenting muscle movement is still very complicated. Observations can be done through surface contact or intrusively in side the body. Most testing is done through the surface of the skin and consequently is adequate for some research but is inadequate for others. The small difference in time could be and has been misleading in some research, mainly perception reaction times of motor vehicle operators. Measuring the feedback to the nervous system poses greater problems since electrical signals currently cannot be distinguished from each other. Output measuring which means the measurement of the muscle against a measuring device is still the easiest and most accurate. Static strength is measured in both static and dynamic modes. Static strength is when the muscles are in equilibrium such as isometric exercises, while dynamic strength is where the muscle changes in length. As the muscle lengthens the force and store energy increase. Kinematics addresses the issue of motion while kinetics investigates the force of the motion. Kinetics and kinematics are most important when addressing the issue of injuries from motor vehicle accidents or falling objects. Measuring devices must be calibrated periodically to insure that all the devices measure accurately and there is no discrepancy in the measurement from one device to another.
Designing products means considering human strength. As stated earlier hand tools must include the range of strength of the hand. Ranges of strength can be obtained from various published data but the data must be analyzed to determine the best and most appropriate ranges for a particular use. There can be a very large range of force exerted by an individual based upon leverage and conditions outside of the body. For example its easier to push a stalled vehicle on a dry surface rather than an icy surface, considering inertia being equal in both cases. Various body part forces must be considered depending upon the tool or task at hand. For example finger, thumb, hand and wrist force must be considered for hand tools while lower extremity force is considered in bicycles and climbing stairs. Articulation of specific body parts must also be considered for product designs such as motor vehicle dashboard equipment, foot pedals and seat adjustment. Recently vehicle manufacturers have been making available as an option brake and accelerator pedals, which are adjustable, based upon the anthropomorphic data available to them. This new accessory makes operating a vehicle safer and more comfortable. A simple change can cause serious safety problems as in the case of the rash of unintended acceleration of certain Jeep Cherokee vehicles. For one model year the designer moved the accelerator pedals slightly to the left from its standard position. This slight adjustment caused some operators to step on the accelerator when they thought they were stepping on the brake. To complicate matters when feet are covered with socks and shoes one cannot tell the difference between the size, texture and shape of the pedals therefore the operator cannot detect they were actually depressing the accelerator when in fact they wanted to apply the brakes. This is a classic example of unnecessary or over designing a product, which was satisfactory in its current condition.
Hydraulic brake systems such as a passenger car requires a specific amount of force on the brake pedal to stop the car in a certain distance while airbrakes do not require specific foot pressure but merely opening the air valves slightly will apply the required brake force. Prolonged foot pressure on the brakes can lead to muscle fatigue and consequently an unsafe condition. This is especially true when traveling down a very long decline, as in driving through mountains. The angle of the knee also contributes to pedal force, the optimum angle is considered to be approximately 160 degrees.
The Message Delivery System
The delivery system can also be called the central nervous system, which controls and regulates the human body to complete chosen tasks. Pathways are known as neurons, which transmit the body's messages. This system is required to maintain the status quo while various efforts requiring energy is used toward work. There are actually two systems one hormonal and the other is nerves. The autonomic nervous system generates the motion and regulates involuntary movement, such as breathing and blood flow. The somatic nervous system deals with mental, and skeletal muscle activities in an awareness state. The brain consists of the cerebral, anterior or frontal section, the cerebellum, the under portion of the posterior section. The brain stem (medulla) is the main highway for the transmission to and from the functioning brain. The ganglia or center brain controls semi voluntary activities such as walking. The spinal cord is the main junction box of the bundle of nerves, which branch out to their respective endings. If the spine is injured the nerves pathway will be interrupted and either a lack of function or pain sensation will occur. Internal sensors monitor information concerning the level of strength of the muscles and respond accordingly by increasing or decreasing movement of the muscle or joint. External receptors relay information system, which monitors the environment outside of the body such as sound, vision, smell, temperature and touch. The spine, which consists of the cervical, thoracic, lumbar and sacral sections of the spine, has nerves, which monitor specific parts of the body. A typical neuron consists of a soma, axon, snyapses and dendrites. The soma is the central body of the neuron the long extension is the axon and the branching from the soma is the dendrite. The neuron has up to several hundred dendrites, which receive signals from axons. Snyapses are similar to bulbs or clubs, which contained a substance, which transmit signals. The velocity of the nerve impulses is a constant for each nerve fiber and ranges from .5 m/s to about 150 m/s. Velocity is faster in a thick fiber than in a thin one.
The spinal cord is where certain actions mainly relating to the limbs are centered and coordinated. The reflex usually begins with a stimulation of a sensor receptor such as touch. A reaction of the muscle can be executed in a few milliseconds after the stimulus was received, since no time consuming higher brain function is necessary. This understanding is crucial when attempting to determine perception and reaction times for various hazardous reactions. Some muscle movements, which require a higher brain involvement, will require much more reaction time. Reactions which require more information during the perception phase are considered discriminatory perception and can greatly increase the time from first identifying a hazard to completing the appropriate reaction. This function is extremely important pertaining to motor vehicle operation and dangerous machinery. If the nerve is damaged or temporarily impaired reaction time could be considerably different. If the nerve transmits a signal to innervate the muscle the reaction may be performed after the reason of the intervation is reached. It is crucial that sensory nerves are not impaired since immediate danger and injury can occur. If the sensory nerves of touch are impaired, the reaction of moving one's hand from a hot surface could be seriously delayed and be a secondary cause of a severe burn. Whiplash syndrome is a serious problem in the
In summary the body controls muscle function through various sensors. The sensors are crucial to guard against injury and to perform selected tasks properly and safely. Feedback through the central nervous system is of paramount importance when considering perception and reaction times and movements. Feedback and feedforward signals are transmitted along the nervous system through neurons, which act as a message center. Signal strength is also of importance when life-threatening hazards are possible.