Biomechanics

Rebecca W. Boren, Ph.D.IEE 437/547Introduction to Human Factors EngineeringNovember 16, 2011

Occupational Biomechanics

Plays a major role in studying and analyzing human performance and musculoskeletal problems.

Human musculoskeletal system is a mechanical system that obeys the laws

of physics – Newton’s Laws.

One technique for studying biomechanics is the use of a pressure

platform.

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Disclaimer: this is not an endorsement for Fitflop shoes,merely an illustration.

The main functions of the musculoskeletal system are

• To support and protect the body and body parts

• To maintain posture and produce body movement

• To generate heat and maintain body temperature.

Musculoskeletal system is comprised of

• Bones• Muscles• Connective tissue

– Ligaments

– Tendons

– Fascia

– Cartilage

Bones

• Bones form the rigid skeletal structure.• Two or more bones are linked with each

other at joints• Synovial joints (e.g. shoulder, elbow)

– Most joints are in this category.– No tissue exists between the two bones. – The joint is highly lubricated with synovial fluid.

• Fibrous joints (e.g. bones of the skull).• Cartilaginous joints (e.g. spine)

Joints can be classified by range of motion

No-mobility joints (e.g. seams of the skull)

Joints can be classified by range of motion

Hinge joints

(e.g. elbow, knee)

Joints can be classified by range of motion

Pivot joints

Joints can be classified by range of motion

Ball and socket joints

Connective Tissue

• Tendons are dense fibrous connective tissues that attach muscles to bones and transmit the forces exerted by the muscles to the attached bones.

• Ligaments are also dense fibrous tissues, but their function is to connect the articular extremities of bones and help stabilize the articulations of bones at joints.

• Cartilage is a translucent elastic tissue that can be found on some articular bony surfaces and in some organs such as the nose and the ear.

• Fascia covers body structures and separates them from each other.

Connective Tissue

• Tendons are dense fibrous connective tissues that attach muscles to bones and transmit the forces exerted by the muscles to the attached bones.

Connective Tissue

• Ligaments are also dense fibrous tissues, but their function is to connect the articular extremities of bones and help stabilize the articulations of bones at joints.

Connective Tissue

• Cartilage is a translucent elastic tissue that can be found on some articular bony surfaces and in some organs such as the nose and the ear.

Connective Tissue

• Fascia covers body structures and separates them from each other.

Fascia is a three dimensional, web like, network of connective tissue which wraps all of the muscles, bones, organs, and brain.

Muscles

• Muscles make up about 40-50% of body weight• Muscles consume almost half of the body’s

metabolism, which supplies the energy for maintaining body posture , producing body motion, generating heat, and maintaining body temperature.

• Composed of bundles of muscle fibers, connective tissue, and nerves.

• Connective tissue provides a channel through which nerves and blood vessels enter and leave the muscle.

Muscles

Muscles

• Muscle activities are regulated by motor nerve fibers, which transmit impulses from the central nervous system to the muscles.

• Each motor nerve fiber regulates a group of related muscle fibers.

• Motor nerves fire to activate the muscles as an all or nothing function.

Structure of Skeletal Muscle

Muscle Activity

• No device to measure tension within the muscle directly. We can measure the amount of force or torque it exerts. We can measure electrical activity of the muscles (electromyography or EMG).

• In performing physical work, excessive loading can cause musculoskeletal problems such as bone fracture and muscle fatigue.

EMG (Electromyography)

• EMG can be a useful tool in the evaluation of work performance.

• Ergonomic analyses often include use of EMG when comparing the specific MS stress in given muscles associated with various work positions or activities.

EMG setup

Measuring muscle activity in the flexor muscle.

Static and Dynamic Strength Measurement

• To measure static elbow flexion strength as a function of the angle of elbow rotation.

• To measure dynamic elbow flexion strength as a function of the velocity of flexion.

• To compare maximum dynamic strength to maximum static strength at a given angle.

Motion Analysis

• Motion of limbs and the hammer used as a representative task.

• Calculate the angles of deviation.

• Calculation the relative position, velocity, and acceleration of the hammer.

The force plate system is used both independently to measure parameters such as static balance as well as in conjunction with the motion capture system in order to determine lower extremity kinetics such as ground reaction forces and moments.

Click here for a demonstration.

Biomechanical Models

• Mathematical models that represent the mechanical properties of the human body.

• The musculoskeletal system is analyzed as a system of mechanical links; and, the bones and muscles act as a series of levers.

• Allows one to predict the stress levels on specific musculoskeletal components quantitatively.

• Serve as an analytical tool to help job designers identify and avoid hazardous job situations.

The designer must fit the physical dimensions of the products & workplaces

to human characteristics.

The consequences of a mismatch.

• Worker assumes awkward positions.

• May not be able to use the product at all.

• Worker is at risk for injury.

• The two most prevalent injuries are low back pain and upper-extremity cumulative trauma disorders. Incorrect hand position.

Work-related Problems: Low-back Pain

• One of the most costly and prevalent work-related musculoskeletal disorder in industry.

• Accounts for one-third of all workers' compensation payments.

• Prevalent in two common situations– Manual material handling– Sedentary work environments requiring a

prolonged static sitting posture.

Work-related Problems: Low-back Pain

Manual material handling

Improper LiftingProper Lifting

Seated Work and Chair Design

Low-back pain caused by prolonged static sitting posture.

Designers must pay particular attention

• Lumbar support should be adjustable.

• Arm rests support the body weight and reduces the load on the spine, if they are adjusted correctly.

Backrest Inclination Angle

• Backrest inclination angle (1100 is preferred over 900 because it retains the body’s natural state of lordosis).

Reducing Back Stress

• Other factors can help reduce back stress, but are not as critical (tiltable seat surface, properly adjusted seat height, use of cushioned seat surfaces, and adequate leg space)

• Slouching is OUT. It causes kyphosis.

• Workers must take frequent breaks.

Upper-extremity cumulative trauma disorders (CDTs)

• Also called repetitive motion injuries or repetitive strain injuries (RSIs).

• Can be even more costly than low-back problems.• Examples

• CDT’s of the fingers: caused by vibrating hand tools; can cause numbness, tingling, or pain when the hands are exposed to cold. Also trigger finger and gamekeeper’s thumb.

• CDTs of the hand and wrist (carpal tunnel syndrome)

• CDTs of the elbow: tennis or golfer’s elbow.

Carpal Tunnel Syndrome

Signs and Symptoms

• CTS is characterized by pain, numbness and tingling sensations in the proximity of the median nerve’s entrance into the wrist.

• Onset is gradual• Localized pain and numbness of thumb and first 2 1/2

digits• As disorder progresses, pain worsens at night and

grip strength is diminished

Carpal Tunnel Release Surgery

• Carpal Tunnel bounded by carpal bones on bottom and transverse carpal ligament on top

• Surgeon transects ligament to increase size of carpal tunnel

• Relieves pressure on median nerve

Carpal Tunnel Release Surgery Types

Open Endoscopic

Carpal Tunnel Release Surgery

Carpal Tunnel Syndrome

• Surgery is not the only option and may be ineffective or cause further damage.

Avoiding Carpal Tunnel Syndrome

• Fit your workstation to your size.• Use proper posture.• Avoid awkward wrist positions.• Take frequent breaks.• Stretch and strengthen muscles.• In extreme cases avoid

keyboarding altogether.• Wear a wrist brace at night, not

while you are working.

Other Upper-Extremity Injuries

• Excessive and repetitive use of tendons can cause inflammation of tendons.

• One example is lateral epicondylitis (tennis elbow).– Can also be caused by painting with a brush or

roller, running a chain saw, and using many types of hand tools.

Other Upper-Extremity Injuries

• Shoulder pain can be caused by rotator cuff injuries, swimmer’s shoulder, or pitcher’s arm.

Other Injuries

• Vibration injuries: whole body, hand, finger, or shoulder.

Hand Tool Design

• Keep a neutral position• Do not bend at the wrist• Shape tool handles to assist grip• Provide adequate grip span• Provide finger and gloves

clearance

Keep a neutral hand position

Avoid these movements

• Flexion/extension

• Radial/ulnar deviation

How to Avoid or Minimize RSIs from Computer Work

• Avoid awkward wrist positions!

• Relax your muscles while you work!

• Fit your workstation to your size

• Use proper posture

• Take frequent breaks

• Stretch and strengthen muscles

• In extreme cases avoid keyboarding altogether

Questions?