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Course 711 - Introduction to Ergonomics

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Risk Factors Inherent in the Task

Risk Factors the Task Brings to the Job

In addition to considering the worker attributes that may increase the risk of injury, we must also analyze the risk factors the work task itself brings to the job. We look at the task variables in the workplace that may increase or decrease the risk of cumulative trauma disorders (CTDs) depending on its design and location.

In large measure, work processes are determined by the factors below:

Tools Equipment
Furniture Machinery
Materials Workstation layout

Design to Reduce Risk Factors

The objective in designing and locating these workplace objects should be to reduce the following tasks risk factors. There are several things you need to consider when designing a work station.

Required Force

lifting

Forcefulness is the amount of physical effort required by the person to do a task and/or maintain control of tools and equipment. The effort depends on:

  • Type of grip;
  • Object weight;
  • Object dimensions;
  • Body posture;
  • Type of activity;
  • Slipperiness of object;
  • Temperature;
  • Pinching;
  • Vibration;
  • Duration of the task; and
  • Number of repetitions.

Examples of work activities that exert force on the body include lifting, lowering, pushing, pulling, pinching, pounding, hitting, and jumping.

You are a lever!

Every time you lift, a lever action is accomplished that amplifies the compressive forces on the back. Take a look at the drawing below. Lifting 55 pounds at a distance from your shoulder (the fulcrum) of 22 inches will result in 550 pounds of compressive forces on the lower back.

Compressive forces on L5 / S1 disc exceeding 550 lbs. (250 kg.) causes four times the injuries than forces of less than 550 lbs. (The Joyce Institute, Principles and Applications of Ergonomics)

equation
Arm-Lever Equation
(Click to enlarge)

As you might guess, the greater the force required to do the work, the greater the degree of risk. Remember, too, that you must consider other risk factors (such as posture, velocity, repetition, and duration) in conjunction with force to accurately estimate the degree of risk of injury.

For instance, the likelihood of injury from pushing a 200 lb. cart would increase depending on the posture assumed while pushing. Common locations of injury due to exerting excessive force are:

  • Neck/shoulder;
  • Low back;
  • Foot/ankle; and
  • Wrist/hand.

Arm-Lever Equation Model

Here is a short video demonstrating the use of the Arm-Lever Equation.

Force-Related Conditions

Let's take a look at five force-related conditions that may increase your risk of injury:

1. Contact Trauma: When any part of your body presses against an external object, the resulting sustained force may cause too much mechanical stress on tissues. It is also possible for excessive mechanical stress to be produced from the impact shock of an object against a part of the body.

scissors

2. Grip: There's a right way and a wrong way to grip an object. This combination of force and posture, if not accomplished correctly, may result in harm to the hand. Two basic grips are used when handling tools, equipment or materials.

  • The Pinch Grip depends on the fingers to exert the force and manipulate the object. This grip strategy requires much greater muscle strength and, consequently, is more likely to cause an injury.
  • The Power Grip uses the muscles of the entire hand to apply force and manipulate objects. Consequently, it's the most effective, and safest, grip to use.

Force-Related Conditions (continued)

3. Static Exertion: Standing, sitting, or otherwise remaining in one posture for a long duration while you perform a task can increase the likelihood of injury. Static exertion combines force, posture, and duration to create a condition that quickly fatigues our muscles which increases the chances of acquiring a CTD. The greater the force, more awkward the posture, and longer the duration, the greater the risk.

Symptoms of Excessive Static Load

welder
Symptoms Possible Ergonomic Cause
Feet, legs ache, varicose veins Standing in one place too long
Lower back pain Trunk curved forward while standing or sitting
Shoulder and arm pain Arms outstretched, sideways, forward or upwards; Shoulders forced up due to position of work
Neck pain Head inclined too much backwards or forward
Forearm pain Unnatural grip, static grip too forceful
Wrist pain Repetitive hand or finger motion

4. Gloves: Have you ever worn thick leather gloves while trying to accomplish some kind of intricate manipulation of small objects? Try it, you will be frustrated. Whenever you wear gloves, more grip force is required to do a particular task. With the need for more force, increased risk of injury occurs.

5. Bulky clothes: Wearing bulky clothes, like heavy rain gear or protective clothing for removing hazardous waste, will increase the effort required by muscles to do work. Any time you increase the force necessary to complete a task, risk of injury also increases.

Vibration

Segmental Vibration

When handling vibrating tools for a prolonged duration, vascular insufficiency in the hand and fingers can also result in interference with sensory receptor feedback. If a worker can't "feel" the grip properly, he or she may compensate by applying more force than is necessary to hold and handle an object. Segmental vibration has also been linked to carpal tunnel syndrome.

Check out this short audio clip by Dan Clark of the theSafetyBrief.com. Vibration White Finger is an injury from using vibrating tools over extended periods. Also known as Hand-Arm Vibration Syndrome, it can cause permanent damage. The white fingers name comes from low blood flow due to the collapse of vessels. Nerves and muscles are also injured with excessive vibration.

Whole Body Vibration

When the whole body is subjected to vibration, as most commonly experienced by truck drivers, there is an enhanced risk of injury, especially to the lower back.

Whole-Body Vibration usually results from two types of forces acting on the worker.

  • A non-cyclical force over a very short period of time (instantaneous shock load). A vehicle striking an obstacle or a sudden drop into a hole may produce these shock loads. If these shock loads are sufficiently great, the operator may be thrown from his seat or struck by objects flying around in the cab.
  • The most common whole-body vibration forces are not sudden, but rather occur over a period of time. For example, this type of vibration is usually created by large vehicles as they travel over highways and especially terrain.

Duration of exposure plays a large role in the effects of vibration forces. Vibration forces delivered over time are more difficult to define than the instantaneous damage caused by high shock loads. The body will respond significantly to vibrations in the range of .5 to 80 Hz (Hertz, cycles/second). Certain parts of the body are resonant at various frequencies in this range. The body's response to vertical vibration will be greatest between 4 and 8 Hz, while vibrations between 2.5 and 5 Hz generate strong resonance in the vertebra of the neck and lumbar region. Vibrations between 20 and 30 Hz set up the strongest resonance between the head and shoulders. Prolonged exposure to these vibration forces in these frequency ranges may create chronic stresses and sometimes even permanent damage to the affected organs or body parts.

Repetition

boxer

Repetition is a measure of how frequently we complete the same motion or exertion during a task. The severity of risk depends on the frequency of repetition, speed of the movement or action, the number of muscle groups involved, and the required force. Repetitiveness is influenced by machine or line pacing, incentive programs, piece work, and unrealistic deadlines. For instance, an experienced worker packing apples (piece work) may complete many more similar exertions or movements than a new worker. Unfortunately, he or she may be performing at such a rapid rate that they may be developing a CTD. Repetition alone is not an accurate predictor of injury. Other factors like force, posture, duration, and recovery time must also be considered.

High Risk Repetition Rates by Different Body Parts

From Kilbom... [1994]. Repetitive work of the upper extremity; Part II: The scientific basis for the guide. Int J Ind Erg 14:59 - 86.

Body Part
Repetitions Per Minute
Shoulder
More than 2 1/2
Upper Arm/Elbow
More than 10
Forearm/Wrist
More than 10
Finger
More than 200

Caution: Do not judge the risk of MSDs solely on the basis of repetition. Much depends on force and the postural factors that reflect the intensity of each action. High repetitiveness, when combined with high external forces and extreme postures, probably represents the highest risk of MSDs.

Check out this short audio clip by Dan Clark of the theSafetyBrief.com. Swinging a hammer all day can give you MSD. So can using a computer keyboard and mouse. Repetitive motions, awkward postures and using heavy force can all cause MSDs. Find relief from these tasks to avoid prolonged injury.

Duration

grinder

Duration is a measure of length of time of exposure to a risk factor. Of course, the assumption is that the longer the duration of exposure, the greater the risk of injury. Duration may be measured in seconds, minutes, hours, days, weeks, months, and even years.

As with most individual risk factors, duration must be considered along with other person, task, and environmental risk factors such as the physical conditioning of the worker, posture, force, weight, temperature, stress, etc.

Recovery Time

Recovery time is a measure of the rest (or low stress activity) period available to the muscle group between similar exertions. Recovery time is important in preventing muscle fatigue because oxygen and metabolites are allowed to rejuvenate while uric acid and other waste products are removed from the muscle group. Recovery time needed will lengthen as the duration of the task increases.

Posture

posture

Posture is the position of the body while performing work activities. Awkward posture is a deviation from the ideal working posture of arms at the side of the torso, elbows bent, with the wrists straight. Awkward postures typically include reaching behind, twisting, working overhead, kneeling, forward or backward bending, and squatting. If the posture is awkward during work, there is an increased risk for injury. The more the joint departs from the neutral position, the greater the likelihood of injury.

Listed below are some specific postures that may be associated with increased risk of injury:

  • Wrist flexion or extension that occurs regularly is associated with a greater risk of carpal tunnel syndrome (CTS).
  • Ulnar deviation of the wrist of greater than 20 degrees increases the risk of pain and other disease.
  • Shoulder abduction or flexion of greater than 60 degrees for more than one hour a day increases the risk of acute neck and shoulder pain.
  • Hands working at or above shoulder level can result in increased risk of tendinitis and various shoulder diseases.
  • The greater the angle of flexion of the neck, the more quickly potentially severe neck and shoulder pain results.
  • Bending at the lower back while working increases the likelihood of low back disorders.

Below are some rules to follow for sitting workstations:

  • The angle between the upper arm and the forearm should be 70 to 135 degrees.
  • The angle between the upper and lower leg should be 60 to 100 degrees.
  • The angle between the torso and the thigh should be greater than 90 degrees.
  • The feet should be placed flat on the floor.

Below are some rules to follow for standing workstations:

  • The work surface should be 2 - 4 inches below elbow height for precision work.
  • The work surface should be 4 - 6 inches below elbow height if lifting light weights and working with bins, tools, and other materials.
  • The work surface should be 6 - 16 inches below elbow height if lifting heavy weights.

Twisting and Force

Twisting in the middle of a lift amplifies the forces on the lower back. Imagine placing a tomato between the palms of your hands and applying direct pressure. It might take a great deal of force to burst the tomato. However, give the motion a twist while applying the pressure and it will take far less pressure to burst the tomato. (If you perform this exercise in class, you better wear an apron!)

Twisting while lifting, pushing, pulling, lowering, or raising may have the same effect on the back. Consequently, twisting while taking any of these actions exposes the back to much greater risk of injury.

Velocity/Acceleration

Angular velocity is nothing more than the speed at which the body part moves. Acceleration is the rate of change of the speed at which the body part moves. For example, a drummer's hand/wrist may move very rapidly back and forth while drumming. The faster that back and forth motion occurs, the greater the risk of injury. Other factors like the weight of the object being moved will also increase risk.

Heavy Dynamic Exertion

Work requiring heavy physical exertion places a greater demand on the body's cardiovascular system to provide the necessary oxygen and metabolites to muscle tissue. If a muscle group's demand for metabolites and oxygen cannot be met, that muscle group will become fatigued, increasing the risk of injury. When the muscle is tired and sore, it's fatigued, and injury may more likely occur. If the entire body becomes fatigued, an actual cardiovascular failure may occur.

Last Words

We've had quite a discussion of the risk factors associated with the task. I hope you have gained some information that will help you better consider ergonomic risk factors while conducting job hazard analysis.

Video

Take a look at this interesting video on proper lifting presented by the Canadian Centre for Occupational Health and Safety (CCOHS).

Instructions

Before beginning this quiz, we highly recommend you review the module material. This quiz is designed to allow you to self-check your comprehension of the module content, but only focuses on key concepts and ideas.

Read each question carefully. Select the best answer, even if more than one answer seems possible. When done, click on the "Get Quiz Answers" button. If you do not answer all the questions, you will receive an error message.

Good luck!

1. The amount of effort in performing a task depends on a number of factors. _____ is not one of those factors.

2. According to the Joyce Institute, compressive forces on the L5/S1 exceeding 550 lbs. causes _____ times the injuries than forces of less than 550 lbs.

3. All of the following are common locations of injury due to exerting excessive force, except _____.

4. What condition exists when any part of your body presses against an external object and the resulting sustained force has caused too much mechanical stress on tissues?

5. Which of the following is a grip strategy to manipulate objects that requires much greater muscle strength?

6. This form of exertion combines force, posture, and duration to create a condition that quickly fatigues our muscles which increases the chances of acquiring a cumulative trauma disorder (CTD).

7. If a worker can't "feel" the grip properly, he or she may compensate by applying more force than is necessary to hold and handle an object.

8. This term describes a measure of the rest (or low stress activity) period available to the muscle group between similar exertions.

9. Awkward postures typically include all of the following, except _____.

10. _____ is nothing more than the speed at which the body part moves and _____ is the rate of change of the speed at which the body part moves.


Have a great day!

Important! You will receive an "error" message unless all questions are answered.