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Human system
About the chapter
This chapter introduces the reader to human anatomy, physiology, and psychology from a system’s perspective, and is intended to provide necessary background for the following chapters. The discussion includes a review of muscular–skeletal, sensory, and support systems. Some of the subtopics covered are movement and force exertion, work-related disorders, human variability in shape and size, strength, endurance, and work capacity.
Two final objectives of this chapter are to familiarize readers with various measurement units employed in ergonomics, and to show how well the human body is engineered.
Introduction
Ergonomists must deal with the fact that people come in different sizes and shapes, and vary greatly in their strength, endurance, and work capacity. A basic understanding of human anatomy, physiology, and psychology can help ergonomists find solutions that deal with these issues and help prevent problems that can cause injury to workers. To provide some perspective regarding the scope of this problem, let us begin by considering some of the component systems of the human body. Figure 2.1 describes the human body in terms of four component systems:
1. The sensory systems (vision, hearing, position, touch, taste, and smell) are stimulated by energy sources (e.g., light, sound, or heat) or materials (e.g., airborne chemicals, acid on skin, and salt on tongue) in the outside environment.
2. The central information processor (brain and nervous system) processes information acquired from the sensory systems.
3. The effector systems (arms, hands, eyes, legs, etc.) are consciously controlled to mod- ify the environment and acquire information.
4. The support systems (circulatory, digestive, metabolic, heat-regulatory, etc.) act in var- ious ways to keep the other systems functioning.
One general observation is that the four major systems perform fundamentally different functions (i.e., sensing, processing, acting, and support). Each of these systems contains a sometimes overlapping set of subsystems. For example, people can use their fingers to read Braille (as sensors) and type (as effectors). The components of systems and subsystems are intricately interfaced throughout the body. For example, the skeletal and muscular subsys- tems, contain nerves and sensors, as well as muscles and bone, and are controlled by the central processor. The circulatory system, an important component of the body’s support system, similarly connects with the effector system through veins and arteries that supply the muscles with nutriments. One of the more important points that should be clear from the previous discussion is that although each system performs a different function, overall performance involves a complex interaction between the different subsystems.
The following discussion will first address the human skeleton, joints, and muscle systems. This sets the stage for introducing the biomechanical concept of levers, and their implications regarding human strength and speed of movement. Other systems of the human body will then be discussed. Focus will be placed on sensory operations, blood circulation, metabolism, and body heat regulation.
Skeletal subsystem
The human skeleton consists of two principal systems of levers, the arms and legs, which are flexibly connected to the central body structure consisting of the skull, spine, rib cage, and pelvis. Joints between these bones allow for body movements and the shape of these joints constrains those movements. The human skeleton contains over 200 bones and a corresponding set of joints. Most of these articulations are of little consequence to the ergo- nomic specialist who is designing work methods and products used by people. Analyses normally focus on the extremities (people’s arms, wrists, hands, legs, and feet) and the lower back.
Extremities
Bones in the extremities are structured like pipes, with closed ends near the joints. Muscles are attached to the bones by tendons. A bone depression or protrusion is normally present at the spot where the tendon attaches. The surface layers of a bone are hard and dense, and tend to be smooth except for roughened areas where ligaments and tendons are attached.
Several small holes allow arteries, veins, and nerves to pass into the soft and spongy inte- rior of the bone.
Joints occur at the locations where bones come together, or articulate. Joints tend to be complex structures, made from many different materials beside bone. Within a joint, liga- ments and muscles hold the bones together. Most ligaments and tendons are made from inelastic collagen fibers. Some joints (especially in the spine) are held together by stretchable ligaments made from elastic fibers. The contact surfaces of bones in a joint are normally covered with a thin, smooth, and very slippery layer of collagen fibers, referred to as car- tilage. This cartilage layer acts as a shock absorber and also helps minimize friction forces.
Synovial and cartilaginous joints are two important types of joints found in the human body. The former make up most of the articulating joints of the upper and lower extremities,
Vision, audition, position, and
temp.
sensing
Brain and nervous
system
Arms, legs, trunk, head, fingers, and
toes
Output Physical
energy Waste Nervous
Nervous Central
process
Effector subsyst.
bones muscles Input
Air, food,
and other Support subsystems of the human body lungs, heart, circulatory Sensory
subsyst.
eyesears energy
Figure 2.1 Subsystems of the human body.
while the latter are found principally in the spine. In synovial joints, the mating ends of the two bones are covered with articulator cartilage as shown in Figure 2.2. A joint capsule made from collagen fibers surrounds the joint and helps sustain stresses. This capsule also has a synovial membrane lining that produces a lubricant and nourishing fluid.
Some joints, such as the elbows and fingers, have a synovial membrane around mus- cle attachments to lubricate that point of friction. Other joints, such as the knees, contain annular disks of fibro-cartilage, which are believed to further reduce friction.
The range of movement allowed by a joint is influenced by many factors including
• The shape of the articulation surfaces
• The distribution of the muscles and ligaments
• Muscle bulk
Joints at the ends of the fingers, knee joints, and elbow joints are known as hinge joints.
Hinge joints have inelastic ligaments stretching down each side that prevent sideways movements. Other joints are less restrictive. Gliding joints allow two-dimensional move- ment at articulations in the wrists and ankles. The saddle joint found at the base of the thumb also allows two-dimensional movement. The hip and shoulder joints are examples of spherical joints (or ball-and-socket joints) similar to trailer hitches. Since the hip joint is a large ball-and-socket joint that is deep within the pelvis, it can carry heavy loads over a small range of movement. The shoulder joint is smaller and not nearly as deep within the shoulder bone, so it cannot take as great a load, although it has a greater range of move- ment than the hip joint.
Pivot joints are joints where a protrusion from one bone fits into a recess of another. For example, at the upper end of the lower arm near the elbow, a protrusion of the radius bone fits into a recess in the ulna bone. Pivot joints restrict movements in some directions. The range of movement allowed by particular joints can be measured using goniometers that are strapped to the outside of the human body at the location of the studied joint. Figure 2.3 illustrates bones and joints in the upper extremities, of particular importance in ergonomic
Medial collateral ligament Muscles Joint capsule
Tendons Synovial membrane
Synovial fluid Bone Cartilage
Posterior cruciate ligament
Lateral collateral ligament
Anterior cruciate ligament
Figure 2.2 Synovial joint.
design. The single humerus bone of the upper arm connects to the scapula in the shoulder.
The forearm has two bones, the radius and ulna, which connect the elbow to the carpal bones of the wrist. Note that the bone on the outside of the wrist is the ulna. The radius is interior to the ulna. The lower part of Figure 2.3 shows bones of the wrist and hand. Gliding joints are found where the carpal bones articulate with each other and with the radius. The joint at the base of the thumb where the carpal bones articulate with the metacarpal bones within the hand is called a saddle joint. The phalange bones form the fingers.
Because human joints are susceptible to a large variety of such disorders, designers of work methods and hand tools must take care that work operations do not cause joints to exceed their natural limitations in terms of force and direction of movement. While such disorders can be caused by single-event accidents, they are more commonly due to a cumulative building of trauma over time. Ergonomic designers must consequently be particularly concerned about device and job designs, which might lead to various types of cumulative trauma disorders.
Joint-related disorders
Synovial joints are very well engineered and work beautifully so long as they are not subjected to stresses beyond their design limits. One potential problem is that joints can be damaged when they are forced to move in directions that they were not designed for.
Sudden forces may cause a sprain or dislocation of the joint, two types of injury that are common in sports medicine and that occur occasionally in industrial operations. Sprains occur when ligaments are torn or stretched beyond their limits. The more the ligaments are stretched, the less likely they are to withstand subsequent stresses. This increases the chance of incurring severe strains or dislocations in the future. Tearing of the protective
Clavicle Scapula
Humerus
Ulna
Carpus Phalanges Metacarpus Radius
Figure 2.3 Bones of the upper extremity.
joint capsule is another common injury. This tearing allows the synovial fluid to leak out of the joint capsule, resulting in disorders such as water on the knee.
Continued misuse of joints over time can cause several types of cumulative trauma dis- orders (CTDs, also called overuse or repetitive-use disorders). Table 2.1 describes a variety of CTD associated with the back (neck to lower back), hand, lower arm, and legs, several of which will be discussed in more detail in the following. These disorders can involve damage to many different parts of the body, besides joints, including ligaments, tendons, tendon sheaths, bursa, blood vessels, and nerves.
Table 2.1 Some Cumulative Trauma Disorders Back Disorders
• Degenerative disk disease—Spinal discs narrow and harden during degeneration, usually with cracking of the surface
• Herniated disk—Spinal disk rupture causing the fluid center to spill or bulge out
• Tension neck syndrome—Neck soreness occurs, often because of prolonged tension in the neck muscles
• Posture strain—Continual stretching or overuse of neck muscles
• Mechanical back syndrome—Portions of the vertebrae known as the spinal facet joints start to degenerate
Elbows and Shoulder Disorders
• Epicondylitis—Tendonitis of the elbow, also called tennis elbow
• Bursitis—Inflammation of the bursa (small packets of slippery fluid that lubricates spots where tendons pass over bones)
• Thoracic outlet syndrome (TOS)—The nerves and blood vessels under the collarbone are compressed
• Radial tunnel syndrome—The radial nerve of the forearm is compressed
• Ligament sprain—Either a tearing or stretching of a ligament that connects bones
• Muscle strain—Overuse or overstretching of a muscle Hand and Wrist Disorders
• Synovitis—Inflammation of a tendon sheath
• Tendonitis—Inflammation of a tendon
• Trigger finger—Tendonitis of the finger resulting in snapping or jerking rather than smooth movements
• DeQuervain’s disease—Tendonitis of the thumb, usually at the base
• Digital neuritis—Inflammation of the nerves in the fingers, usually caused by continuous pressure or repeated contact
• Carpal tunnel syndrome (CTS)—Pressure on the median nerve Leg Disorders
• Patellar synovitis—Inflammation of the synovial tissues within the knee joint
• Subpatellar bursitis—Inflammation of the patellar bursa
• Shin splints—Very small tears and inflammation of muscles stemming from the shin bone
• Phlebitis—Varicose veins and blood vessel disorder often caused by protracted standing
• Tronchanteric bursitis—Inflammation of the bursa at the hip
• Phlebitis fascitis—Inflammation of tissue in the arch of the foot
Ulnar deviation is a case in point. This disorder occurs when ligaments are stretched to the point that they no longer hold the lower part of the ulna bone in place, resulting in permanent disability. People who work for long time periods with their hands in an unnatural position may be prone to this disorder. Cobblers (i.e., shoe makers of a past era) who manually put in the welts between the upper shoes and the soles often experienced ulnar deviation.
Work-related tendonitis is another CTD that can restrict movements in many different ways. For example, excessive gripping and twisting activity can cause DeQuervain’s disease.
This disorder involves inflammation of the tendon sheaths at the base of the thumb. This syndrome is painful and can severely restrict movement of the thumb.
Trigger finger is a related disorder due to overuse of the finger used in shooting guns, first observed among soldiers who fired rifles on the rifle range for many hours of the day.
Bursitis is an inflammation of the bursa, which often contributes to tendonitis in the shoul- der. Lifting, reaching, and various overhead tasks can cause this disorder. In advanced cases, patients lose their ability to raise the affected arm.
Thoracic outlet syndrome is a disorder resulting from compression of nerves and blood vessels beneath the collarbone, which can cause the arms to become numb and interfere with movement. This syndrome is associated with a wide variety of physical tasks, where people exert forces with extended arms.
Another CTD known as carpal tunnel syndrome involves both the hand and the wrist.
Joints of the finger are operated by muscles in the forearm that attach to tendons encased in sheaths. These sheaths pass over the bones of the wrist in what is known as the car- pal tunnel, which is formed by the U-shaped carpal bones at the base of the back of the hand and the transverse carpal ligaments forming the opposite side below the palm of the hand. A number of tendons in sheaths, nerves, and blood vessels run through that tun- nel. A second tunnel, called the Guyon canal, is adjacent to the carpal tunnel away from the thumb side and provides a conduit for the ulnar nerve and artery. When the wrist and fingers are in their normal position, the sheaths, nerves, and blood vessels and arteries are nearly straight. However, when the wrist and fingers are bent, the sheaths are constricted.
It consequently requires more force to pull the tendons through the sheaths, and damage may occur. The damage may cause irritation and swelling, which then causes second- ary problems. Compression of blood vessels and arteries, due to swelling, is of particular concern because this reduces the blood supply. With oxygen and nutrient supply shut off, and the removal of waste products similarly stopped, muscles are less capable of sustained exercise and require longer periods of time for fatigue recovery. Bending of the wrist can also cause nerve compression, which can result in a variety of problems including carpal tunnel syndrome.
Carpal tunnel syndrome is one of the most common of the CTDs. People who have this disorder often feel numbness or tingling in their hands, usually at night. It is caused by swelling and irritation of the synovial membranes around the tendons in the carpal tunnel and that swelling puts pressure on the median nerve. Associated causes and conditions of carpal tunnel syndrome are repetitive grasping with the hands, particularly with heavy forces, and bending of the wrist. Many other conditions are associated with this disorder, including arthritis, thyroid gland imbalance, diabetes, hormonal changes accompanying menopause, and pregnancy. It is important to identify potential cases before they become serious. Early treatment consists of rest and the wearing of braces at night to keep the wrist from bending and some mild medications. In more severe cases, physicians often use cor- tisone injections or surgery. The best solution is to ergonomically design workstations to help prevent this disorder from occurring in the first place.
Spine
The human spine is a complex, load-bearing structure. A side view of the spine, from the base of the skull downward, is shown in Figure 2.4. The bones of the spine progressively increase in size from the top to the bottom of the spine, corresponding to an increase in carrying capacity for the lower bones. The individual bones of the spine are called vertebrae and are divided into four different categories, depending on their location. That is, there are seven cervical vertebrae, counting from the top of the spinal column downward; 12 thoracic vertebrae in the middle region; five lumbar vertebrae below the thoracic vertebrae, in the next lower region; and a fused group of sacral vertebrae, just above the coccyx, or tailbone.
The specific vertebrae are numbered in terms of their location (i.e., C1–C7, T1–T12, and L1–L5). Most back problems related to the spine involve the vertebrae of the lumbar region, illustrated in Figure 2.5. The vertebrae are separated by disks, which have a fibro-cartilage exterior and a liquid center. These disks serve as shock absorbers between the bony ver- tebrae in the spine. Over the course of a day of standing, these disks are under compres- sion, causing the spine to shrink about 1.25 cm. Those disks also begin to degenerate after middle age. Since the disks occupy about 25% of the spinal length, and disks compress as the person ages, older people tend to grow shorter.
Several muscles and ligaments run along the spine. One set of ligaments runs from the skull to the tailbone, along both the front and back of the spine. Individual vertebrae are joined by elastic ligaments and muscles that help the spine maintain its natural curvature and permit limited bending in multiple directions. Rotations of about 90° in the upper part of the spine and about 30° in the lowest part are also possible.
Cervical (7)
Thoracic (12)
Lumbar (5)
Cauda equina
Coccyx Sacrum
Vertebra Intervertebral disk
Figure 2.4 Side view of the spine.
The abdominal muscles in front of the spine create tension that tends to curve the body forward. Tensing the erector spinae muscles behind the back counteracts these muscle forces.* The latter muscles connect to bony, rearward projecting, protuberances on individual vertebra. Other muscles also connect to the spine, including the trapezius muscle of the upper shoulder and the latissimus dorsi of the lower back. The separate mus- cle attachments to particular vertebrae provide the spine a great deal of mobility. This happens because each vertebra is separately controllable. Needless to say, determining the sequence of muscle activations needed to reach a particular posture or make a particular movement is a complicated control problem.
Back disorders cost industry a great deal of money. Low back pain is a special concern because it often disables a person for an extended period. Some cases have resulted from moderately poor posture over long time periods. More often this condition is a result of lifting heavy loads or lifting in a poor posture. Unexpected load shifts and slips while exerting force are especially likely to cause back strains and other injuries. In some cases, a disk may be ruptured, resulting in severe pain due to pressure of the disk’s liquid center on the spinal cord.
*The fact the muscles and ligaments in front and behind the spine are both exerting tension forces can result in large compression forces on the spinal column.
Intervertebral disk
Vertebra
Vertebral body
Nucleus Intervertebral
disk
Intervertebral foramen
Nerve root (A)
(B)
Facet joints
Ligamentum flavum
Posterior longitudinal
ligament Vertebral foramen
or spinal canal Anterior
longitudinal ligament
Figure 2.5 View of the lumbar region of the spine—(A) side view and (B) cross section.