The Body as a Mechanical System
Functions of the Skeletal and Muscular Systems
2. Protection (the skull protects the brain and the rib cage protects
the heart and lungs)
3. Movement (muscles are attached to bone and when they
contract movement is produced by lever action of bones and joints)
4. Hemopoiesis (certain bones produce red blood cells in their
1. To produce movement of the body or body parts
2. To maintain posture
3. Heat production (muscle cells produce heat as a by-product and
are an important mechanism for maintaining body temperature)
The stability of body parts
The stability of the body parts depends on the shape of the base of support described
by the position of the feet. A is unstable, B is fairly stable in all directions, C is stable
anteroposteriorly, and D is laterally stable.
The Tent Analogy
The tent analogy- The skeleton is the tent pole, the muscles are
the guy ropes, and the soft tissues are the canvas.
Anatomy of the Spine and Pelvis Related to Posture
The lumbar, thoracic, and cervical spines and the pelvis (a) and sacrum (b). The
weight of the upper body is transmitted through the lumbar spine, the iliac bones of
the pelvis (c) to the hip joints (d) and legs.
Chapter 2: Fig 2.4
Function of (1) intervertebral disk and (2) facet joints. The disk resists the compressive load and the facets
resist the intervertebral shear force. (From Kapandji, I.A., The Physiology of the Joints, Churchill Livingstone,
Longman Group, Edinburgh, UK, 1982. With permission.)
Intervertebral disk and vertebral body
(a) In this view, the superior vertebral body has been removed to reveal the intervertebral disk
below. A is the nucleus pulposus, B is the annulus fibrosus, and C is the inferior facet joints at
the rear. (From Kapandji, I.A., The Physiology of the Joints, Churchill Livingstone, Longman
Group, Edinburgh, UK, 1982. With permission.) (b) Detail of the structure of the annulus
fibrosus. The annulus consists of a number of layers of cartilage. The fibers in the layers run
obliquely and in different directions somewhat like the layers of a cross-ply tire. The outer
layers run perpendicularly to each other. (From Vernon-Roberts, B., The Lumbar Spine and
Back Pain, III, Churchill Livingstone, Edinburgh, 1989. With permission.)
The pelvis as an arch. The pelvis viewed from the rear. A is the sacrum, B is the ilium, and
C is the ischium. The sacrum acts like a true keystone in this plane. (Redrawn from Tile,
M., Fractures of the Pelvis and Acetabulum, Williams & Wilkins, Baltimore, London, 1984.
View of the sacroiliac joint from above. A represents the ligaments, B is the sacrum, and C is the
pelvis. The ligaments act like the cables of a suspension bridge preventing the sacrum from
slipping forward. If the joint is deformed by loading, the ligaments can be pinched by bone causing
pain in the very low back usually on one side. (Redrawn from Tile, M., Fractures of the Pelvis and
Acetabulum, Williams & Wilkins, Baltimore, London, 1984. With permission.)
Relationship between sacral and lumbar angles.
Relationship between sacral and lumbar
angles. (a) Sacral angle and lumbar lordosis, as in standing.
(b) Moderate sacral angle and fattened lordosis as in sitting on
a chair with a backrest. (c) Minimal sacral angle and tendency
to lumbar kyphosis as in sitting on a low stool.
The Muscular System of the Pelvis
Schematic representation of the muscular system of the pelvis (sagittal view). When the abdominal or hip
extensor muscles shorten, the pelvis tilts backward. The result is a flattening of the lumbar spine to maintain the
trunk erect. When the hip flexors or erector spinae muscles shorten, the pelvis tilts forward. This is
accompanied by a compensatory increase in the lumbar lordosis.
Postural Stability and Adaptation
When the base of support is constrained, compensatory movements occur automatically to maintain postural
stability demonstrating that the ‘‘attitudinal as well as the righting reactions’’ are indeed involuntary. (a)
Balanced erect standing posture and (b) as the hip joints flex and the upper body moves forwards, the ankle
joints plantarflex to compensate and the lower body moves rearwards, maintaining balance.
Postural Stability and Adaptation
Postures can be stable but stressful if support of body mass depends on
soft tissues rather than bone.
Ligaments are able to resist high tensile forces, particularly if these forces
are exerted in the direction of their constituent fibers. They play a major role
in protecting joints by limiting the range of joint movement and by resisting
sudden displacements which might damage the joint. However, injuries can
occur if ligaments are exposed to sudden forces when prestressed by
extreme joint positions or by complex movements. This is one of the
reasons why ergonomists stress the importance of the posture of the hands,
wrists, elbows, and trunk when tools or controls are operated or when loads
Poor equipment design which forces the adoption of extreme joint positions
when holding an object predisposes the joint to injury.
Good design enables equipment to be used with the joints in the middle of
their range of movement.
TABLE 2.3 Risk Factors for Back Disorders
Includes job dissatisfaction, avoidance of pain, expectation of sickness payments, and stress
Transitional (extra) vertebrae and spina bifida
Pain is more severe when it occurs
Lateral curvature of the spine
Evidence for greater risk in obese people and in people with reduced lumbar lordosis
Participation in sports High-level participation in gymnastics, weight lifting, soccer, and tennis associated with increased risk of back pain; degenerative disk
disease in those who start at a young age. Recreational sports may protect adults
Evidence of a small but significantly increased risk in smokers
Increased risk in pregnancy especially among younger, overweight people
Pain associated with osteoporotic fractures
Bed rest in first 2 days of an acute episode seems to lengthen recovery. Encouragement to return to work as soon as possible shortens
recovery time, even if there is still pain
Psychological factors Depression and stress increase the risk of chronic pain. Negative beliefs and “fear avoidance” coping strategies also play a role. A lack of
formal education, living alone, being divorced or widowed all increase the risk
Chronic pain, musculoskeletal pain in other parts of the body, self-reported disability, and systemic health complaints associated with
increased stress increase the risk
TABLE 2.5 Psychosocial Risk Factors for Back and Neck
1.Psychosocial factors influence the transition from acute pain to chronic pain disability and have predictive value
2.Psychosocial factors are associated with the onset of pain
3.Back pain disability depends more on psychosocial factors than on biomedical factors or biomechanical exposures
4.There is no simple “pain prone” personality and the role of personality traits is unclear
5.Cognitive factors are related to pain development and disability:
•Fear avoidance facilitates pain development
•Passive, rather than active, coping strategies facilitate pain development
•Over-reaction (catastrophizing) of back pain enhances pain disability
•Depression, anxiety, and stress increase pain disability
•Self-perceived poor health increases pain disability
Specific ergonomics risk factors (Carter and Birrell, 2000) for priority redesign are:
Repetitive heavy lifting .
Lifting, pulling, pushing of objects over 11 kg .
Lifting objects from the ground in a twisted position .
Jobs with high physical demands .
Exposure to whole body vibration .
Prolonged sitting at work (>95% of the workday) .
Non-neutral trunk postures held for >10% of the work cycle in repetitive jobs
The main risk factors for musculoskeletal injury in the workplace are force,
posture, repetition rate, and fatigue and their external (task) counterparts are
load, layout, cycle time, and work organization (shifts and rest periods)—all of
which can be modified by ergonomic redesign.
Biomechanics of Spinal Loading
The ergonomic risk factors for back pain in Table 2.4 have a common
feature— exposure increases the compression on the lumbar spinal
motion segments (Figure 2.4). The mechanism by which spinal
compression causes injury is likely to be
fracture of the vertebral end plates which are the weak link in the chain
(and not, as is sometimes supposed, the intervertebral disks).
The U.S. National Institute for Occupational Safety and Health (NIOSH)
has specified an action limit for spinal compression of 3400 N. What this
means is that work tasks that impose a compressive load greater than
3400 N on the lumbar motion segments are deemed to be hazardous and
in need of redesign.
Spinal Compression Tolerance Limits
The spinal compression tolerance limit (SCTL) is the maximum compressive load
that a specified motion segment can be exposed to without failure.
In practical situations, manual handling tasks can be evaluated using
biomechanical models to estimate the compression load. If the estimated load
exceeds the SCTL, then the tasks must be redesigned, either by reducing the load
or the load moment.
Several factors reduce the SCTL and therefore increase the risk of injury. SCTL is
greatest in 20–29 year olds declining by 22% in the next 10 years, 26% in the next
10, and 42% in the next 10. At 60 years of age or more, the SCTL has declined by
Female SCTLs are approximately 67% of male values.
SCTLs are lower when spinal motion segments are loaded in complex ways, as
when compression and bending are combined. A hyperflexed lumbar spine has a
lower SCTL than a flexed lumbar spine. In a flexed position, however, the
compressive load on the thicker part of the disk (the anterior annulus) is increased
and the load on the facet joints is reduced.
Physical activity seems to strengthen both the vertebral bodies and the
Women, older workers, and those unaccustomed to lifting should not be expected
to carry out forceful exertions at work.
TABLE 2.7 NIOSH Hazard Evaluation Checklist Risk of Back Pain in Manual Tasks
1.1Does the load handled exceed 23 kg?
1.2Is the object difficult to bring close to the body because of its size, shape, or bulk?
1.3Is the load hard to handle because it lacks handles or cut-outs or does it have slippery surfaces or hard edges?
1.4Is the footing unsafe? For example, are the floors slippery, inclined, or uneven?
1.5Does the task require fast movement, such as throwing, swinging, or rapid walking?
1.6Does the task require stressful body postures such as stooping to the floor, twisting, reaching overhead, or excessive lateral
1.7Is most of the load handled by only one hand, arm, or shoulder?
1.8Does the task require working in environmental hazards, such as extreme temperatures, noise, vibration, lighting, or airborne
1.9Does the task require working in a confined area?
2.1Does lifting exceed five lifts per minute?
2.2Does the vertical lift distance exceed 1 m?
2.3Do carries last longer than 1 min?
2.4Do tasks that require large sustained pushing or pulling forces exceed 30 s duration?
2.5Do extended reach static holding tasks exceed 1 min?
Note: “Yes” response indicates risk of low back pain. The larger the number of “yes” responses, the greater the risk.
The Regression Equation to Calculate SCTLs
Ayoub and Mital (1997) quote SCTLs of 6700 N for people under 40 years of
age and 3400 N for people over 60.
The regression equation below can be used to calculate SCTLs for a given
lumbar motion segment
CS = -13331.2 - (73.7 x Age) - (962.6 x Sex) + (403 x LMS) + (79.8 x BW )
where CS = compressive strength Age = age in years
Sex—use 1 for male and 2 for female
LMS=lumbar motion segment (L1-L2=44, L2-L3=45, L3-L4=46, L4-L5=47,
BW =body mass (kg)
Static Strength Prediction Programme
Body diagram for pain rating. These are often included in questionnaires for use in ergonomic surveys. Pain
is often rated on a 10-point scale where 1 1⁄4 mild discomfort and 10 1⁄4 the pain could not be worse
Purchase answer to see full attachment