Occupational Ergonomics Basics
Avoiding Serious Back Injuries
Whether you’re at a traditional office or working from a home office, understanding the essentials of ergonomics can save you from a serious back injury. At one point or another, you may need to pick up a printer or another heavy item, especially after hours of sitting down at your desk. Knowing the basics of occupational ergonomics is important to keeping yourself safe.
To illustrate this, let’s consider a simple example. Assume that a worker employed by a parcel delivery company is required to load products onto a truck in a loading bay. The products come in boxes of different sizes (small to relatively large) that weigh between 30–40 pounds. The boxes have different degrees of fragility, and the worker needs to determine how fragile each box is by looking on the box for some type of warning. Fragile boxes must be stacked in more secure areas. The boxes are all stacked up in the loading area prior to being moved inside the truck. For health reasons, the worker must wear gloves, and may also have to wear extra clothing during cold weather seasons.
The available space for loading products inside the truck is relatively high, and the worker is supposed to make use of all available space. Small stools are used to step on in order to place boxes in difficult-to-reach areas. Hand carts are used to move several boxes at a time into the inside of the truck. Due to tight shipping deadlines, there may be time pressure associated with getting the truck loaded. There may also be up to three workers performing loading operations within a truck at the same time.
The worker performs this job over an 8-hour day. Several 10-minute breaks are taken in the morning and in the afternoon, and the worker takes a ½-hour break for lunch.
Musculoskeletal Injuries
The National Institute for Occupational Safety and Health (NIOSH) is the U.S. federal agency responsible for conducting research and making recommendations to prevent work-related injuries and illnesses. According to NIOSH, each year about a half a million U.S. workers suffer some form of musculoskeletal injury related to overexertion. Two of the most commonly reported problems are upper-extremity cumulative trauma disorder (UECTD) and lower back pain.
The cause of the injury could involve physical and psychological factors, as well as organizational factors and individual characteristics. For example, people who do repetitive tasks involving lifting, bending, and twisting, especially those who do so under stressful conditions, are more at risk of suffering from musculoskeletal problems than others. Additional factors such as an individual’s built, cold weather, vibrations, awkward postures, and having to wear extra equipment or clothing might also contribute to the problem.
When it comes to material handling, such as lifting boxes, the lower back is often considered the most vulnerable section of the musculoskeletal system. The lumbar spine is connected to the pelvis and is where most of the weight bearing and body movement takes place. Also, the muscles and bones of the lower back are the farthest from the load being handled. That strain in addition to the weight of the upper torso creates a considerable amount of load on the lower back, particularly at the disc between the fifth lumbar and the first sacral vertebrae, known as the L5-S1 lumbosacral disc.
Initial Observations and Assumptions
Let’s start by making a few observations and assumptions:
- The parcel delivery worker labors for eight hours a day, taking a lunch break of a half hour, and several 10-minute breaks in the morning and in the afternoon.
- Lifting involves small to large boxes, ranging in weight from 30 to 40 pounds.
- Since all of the available space in the truck must be used, including difficult-to-reach areas, the worker at times may have to step on a small stool to place a box. This deviation from the neutral position of the joints involved is considered an awkward posture.
- Boxes are stacked up in the loading area prior to being moved inside the truck. The worker uses a hand cart to move several boxes at a time inside the truck. Because more than one worker may be involved, it is very possible that boxes are passed from one worker to the other while being carried and placed on the cart (frequent torso twisting during the lift)
- Some boxes are more fragile than others, and there is also a tight shipping schedule. These factors can possibly be a source of psychological stress to the worker.
- The worker must wear gloves for safety reasons, and may also have to wear extra clothing when it’s cold. The cold temperature is bad for lifting and the extra gear will require more energy.
- The parcel delivery worker is a male of average weight and height.
Low-Back Biomechanical Modeling
Biomechanics studies the effects of internal and external forces on the mechanics of living organisms. Biomechanical models are mathematical models that allow us to use physics, mechanical engineering, and the mechanical properties of the human body to predict how much stress is placed on certain musculoskeletal components.
Biomechanical modeling is based on Newton’s Three Laws of Motion:
- Every object in a state of uniform motion tends to remain in that state unless an external force is applied to it.
- Force is proportional to the acceleration of a mass. The relationship between an object’s mass m, its acceleration a, and the applied force F is F = ma.
- For every action there is an equal and opposite reaction.
A body or body segment is said to be in static equilibrium when it is not in motion, that is, when the sum of all external forces acting on it is zero, and the sum of all external moments acting on it is zero. In physics, moment refers to the amount of force applied to a rotational system at a distance, known as the moment arm, from the axis of rotation.
In the case of the parcel delivery worker, we could obtain a quick estimate of the stress being placed on his lower back by applying a simplified single-segment planar static model. This model consists of analyzing an isolated segment of the body with the laws of mechanics to identify this stress being placed on the corresponding joints and muscles. In this case, we will consider only two factors: the weight of the box being lifted and its position relative to the center of the spine. We need to determine the moment on the back muscle and the compression force on the L5/S1 disc.
Using the low-back biomechanical diagram above as a reference, the formula given is:
which says that when a person with an upper-body weight of W(torso) lifts something that weighs W(load), the combination of the load and the upper torso create a combined clockwise rotational moment clockwise moment. This force of rotation would then need to be counteracted by a counterclockwise rotational moment, produced by the back muscles with a moment arm or distance of about 5 cm:
Assuming the following is true:
Based on the second condition for static equilibrium (the sum of all external moments acting on it must be zero), the formula to apply is:
so we have:
which tells us that the back muscle force is equal to the combined weight of eight times the load plus four times the torso weight, a typical effect of many lifting tasks.
= (150 x 8) + (490 x 4) = 3,160
A back muscle force, therefore, of 3,160 N may exceed the capacity of some people, the normal range being between 2,200 to 5,500 N according to estimates by H.F. Farfan (Mechanical disorders of the low back, Philadelphia, PA: Lea and Febiger, 1973).
To figure the compression force on the L5/S1 disc, we use the first condition of static equilibrium (sum of all external forces acting on it is zero). This can be estimated at:
To keep things simple, we can ignore abdominal force, and consider only the angle between the horizontal plane and the sacral cutting plane found perpendicular to the disc compression force. This gives us:
Assuming this angle is 60°, a person with a torso weight of 490 N lifting a load of 150 N gives us the following level of disc compression:
This amount of disc compression, although not excessive, slightly exceeds the accepted criterion of 3400 N, which is the compressive force at the L5/S1 disc that defines an increased risk of low-back injury[1]. As mentioned earlier, other possible factors could also increase the risk of injury. For example, if the work activity is repeated over time without sufficient rest of the area involved, it could eventually lead to problems with the lower back. Repeated compressive force is often responsible for low back injuries such as disc herniation, and spine damage in general.
We could have more accurately determined the reactive forces and moments at the L5/S1 disc using a multi-segment model, taking into account factors such as abdominal pressure, degrees of torso twisting, and biomechanical differences between males and females during lifting, but the single-segment model allowed us to quickly assess the situation.
NIOSH Lifting Equation.
The NIOSH Lifting Equation is a tool that ergonomists and occupational safety and health practitioners use to identify and analyze some risks associated with lifting demands on the lower back. The goal is to prevent or reduce the occurrence of low back pain and injury among workers. NIOSH first came out with the equation in 1981 and then revised and expanded it in 1991 to increase the types of task variables the tool could assess.
The lifting equation is based on three criteria:
- Biomechanics: 3.4k N is the compressive force at the L5/S1 disc that defines an increased risk of low-back injury. The biomechanical limit of 3.4k N was actually established as part of the NIOSH lifting equation on the basis of epidemiological data and cadaver data.
- Psychophysics: The Maximum Acceptable Weight of Lift (MAWL) is the amount of weight a person chooses to lift for a particular task. A lifting task perceived as acceptable is less likely to cause low-back pain and injuries.
- Work Physiology: Some activities will use more muscle groups than others and therefore require more energy. Metabolic energy expenditures should be below predicted levels. The task at hand should not exceed the worker’s capacity to produce energy. This criterion sets the limit of maximum energy expenditure for a lifting task at 2.2 to 4.7 kcal/min.
Based on these criteria, measurements are recorded and computed both at the origin and destination of a lift.
Task Variables
The task variables involved in the NIOSH Lifting Equation include the object weight, the hand location, the vertical distance, the angle, the frequency (lifts per min), time, and object coupling:
W refers to the average weight of the object.
H is the horizontal distance in centimeters between the hands and the midpoint between the ankles.
D is the vertical distance in centimeters between the origin and destination of the object being lifted.
A is the angle of asymmetry or the torso twisting that takes place when the object is not directly in front of the worker.
C is the coupling of an object, considered good if the worker’s fingers wrap completely around the object or its handles, fair if only a few fingers can grasp the object or handles firmly, and poor if only a few fingers or fingertips are partially under or around the object.
F refers to an average number of lifts per minute.
For the purposes of this discussion, let us assume the coupling is good (the worker can firmly hold the box) and we have measured and recorded the values shown in the table below. Let us also assume it is a perfect day and the boxes all weigh exactly 35 pounds, the average between 30 and 40 (note that 35 pounds is approximately 15.88 kilograms).
Recommended Weight Limit
The NIOSH lifting equation provides a recommended weight limit (RWL) as follows:
where
RWL is the recommended weight that nearly all healthy workers could lift over a period of time for up to eight hours without the risk of getting injured.
LC is the load constant, a fixed factor of 23 kg.
HM is the horizontal multiplier. As the horizontal distance between the load and the spine increases, disc compression force increases. HM, for example, is calculated as 25/H [cm] at the start and end of the lift[1].
VM is the vertical multiplier. NIOSH recommends lifting a load from 30 inches above the floor as the best originating height.
DM is the distance multiplier. As the vertical distance of lifting increases so does physical stress
AM is the asymmetric multiplier. Asymmetric lifting or twisting of the torso while lifting is considered more harmful than symmetric lifting.
FM is the frequency multiplier, referring to the lifts per min and work duration (lifting frequency). Let us assume it is 1 box per minute.
CM is the coupling multiplier or the quality of grabbing and lifting the load (e.g. presence of handles or couplings to help grip and lift the load).
Based on the formula given, we can determine the multipliers and compute the RWL as follows:
Lifting Index
After the RWL is calculated, it is compared with the actual weight (W) of the load to figure the Lifting Index (LI):
So we have:
The LI is an estimate of the stress level associated with a lifting effort. LI is calculated at the origin and destination of a lift, and whichever one is greater is considered the lift’s “stress Level.” As LI increases, so does the risk for developing low back pain. According to NIOSH, an LI equal to or less than 1.0 does not pose a risk whereas an LI > 3.0, for example, poses a significant risk to most workers. In this case, because the stress level is at 7.12, the parcel delivery worker is at a high risk of suffering a lower back injury.
The parcel delivery worker will be able to perform his job only if he has enough energy to support the job activities, namely continually loading 30–40 lbs onto a truck. The maximum rate of energy-expenditure that an individual can achieve for a few minutes is called the aerobic capacity or short-term maximum physical work capacity (MPWC). Another term used to describe aerobic capacity is VO(2 max), which denotes that person’s ability to process oxygen, based on the lung’s capacity and the pumping action of the heart to deliver oxygen from red blood cells to the working muscles.
For 8-hour shifts, NIOSH recommends capacity limits of 5 kcal/min for males and 3.5 kcal/min for females. For a lifting task, the maximum range is from 2.2 to 4.7 kcal/min. If the worker expends 30 to 40 percent of his maximum aerobic capacity, at the end of an 8-hour shift he will most likely experience whole-body fatigue. His exhaustion will be even greater if the energy spent is higher.
Assuming the job was not redesigned (i.e. using robotic equipment or a special forklift to load the truck), the right work-rest schedule can help keep the worker’s energy-expenditure at an acceptable level. The formula provided to determine the proper work-rest schedule is:
where
PWC is the physical work capacity, in this case 5 kcal/min (for a male).
E(job) is the required energy-expenditure to perform the job
E(rest) is the energy-expenditure rate at rest.
Assuming the worker were expending 5.5 kcal/min, given the job demands outlined earlier, we could calculate the required rest period as:
This would roughly mean that for every 34 minutes of work, the delivery worker would need about a 26 minute break to avoid expending excessive energy. Since this, of course, is not practical, the job would most likely have to be redesigned (e.g. introducing devices such as adjustable carts, ramps, rollers, etc.) so the workers consume much less than 5.5 kg/min to perform their job.
The NIOSH lifting equation seen earlier told us that the parcel delivery worker was at a high risk of suffering a lower back injury. This applies to the daily wear and tear. However, an accidental injury can also occur for several reasons:
- Since the work involved is physically demanding, with time, it could cause tissue damage, which makes the worker more susceptible to falling. Also, excessive or repetitive loading can cause small bone fractures which will not repair if the body does not have enough time to rest and recover.
- Over time, joints wear down with excessive use and can fail. Connective tissues may also damage, which can lead to strains and sprains.
- Since the worker must at times stand on a small stool to place a box in a hard-to-reach area, this awkward posture can easily cause the worker to slip or fall.
These factors all contribute to the possibility of an accident occurring while the worker is performing his job. Less importantly, some of the boxes are marked fragile, which means the the product could damage if dropped in the case of an injury.
Although this example covers the scenario of a parcel delivery worker, if you work from home, you should also take measures to prevent a back injury, especially if you’re sitting for long hours and then standing to pick up something heavy. Get your joints moving. Change your work-break schedule, stand up often and stretch, and also eliminate possible hazards that may be lurking in your office.
