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:
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.
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:
Examples of work activities that exert force on the body include lifting, lowering, pushing, pulling, pinching, pounding, hitting, and jumping.
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)
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:
Here is a short video demonstrating the use of the Arm-Lever Equation.
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.
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.
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
|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.
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.
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.
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 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.
From Kilbom... . Repetitive work of the upper extremity; Part II: The scientific basis for the guide. Int J Ind Erg 14:59 - 86.
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 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 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 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:
Below are some rules to follow for sitting workstations:
Below are some rules to follow for standing workstations:
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.
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.
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.
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.
Take a look at this interesting video on proper lifting presented by the Canadian Centre for Occupational Health and Safety (CCOHS).
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.