Shoulders and Knees are Also at Risk of Workplace Injuries due to Poorly Designed Carts
When we talk about pushing and pulling as potential causes of manual material handling (MMH) workplace injuries, we tend to focus on low-back injuries. This is not without good reason; back injuries and back pain are both prevalent and expensive. A study of the prevalence of back pain noted that about one in six individuals in the workplace has experienced back pain within the past two weeks. The Spine Research Institute of Ohio State University estimates the total cost of a lost-time back injury, including wage replacement cost for workdays lost to injuries, as ranging between US$40,000 to US$80,000. Back pain and injuries affect productivity as well; one study estimated that back pain annually limits productivity of affected individuals by about 10 percent. However, back injuries are not the only consequence of poorly-designed cart handling MMH tasks; shoulders and knees are also at risk; slips and falls while pushing or pulling carts are also of concern.
Although Less Frequent, Shoulder and Knee Workplace Injuries Can Cost Companies the Same or More than Back Injuries
The US Bureau of Labor Statistics (BLS) estimates the incidence rate of lost time back injuries as 15.2 cases per 10,000 workers per year worked; the prevalence of knee and shoulder lost time cases is lower, 7.6 and 7.2 cases, respectively. There are about twice as many lost-time back injury cases as shoulder and knee injury cases. However, the median lost days per case tell a different story. While the median number of lost work days for a back-injury case is 8 days lost, the median number of lost work days for a shoulder injury is 26 days and the median lost work days for knee injuries is 14 days. The cost of replacing daily wages for a median back injury case at the average US private sector hourly earnings rate of $27 per hour for eight days is roughly US$1,700, but increases to about $5,600 for a shoulder case and about $3,024 for a knee injury case. Taking the relative incidence and median days lost into account for each type of injury, the expected cost of lost wage replacement is roughly equal for backs and knees, but shoulders cost about 60 percent more due to the greater number of days lost. Although knee and shoulder cases are less common, the number of lost work days associated with them are roughly equivalent to the days lost to back injuries (knees), or much higher, as in the case of shoulders. A study of workers compensation costs in Ohio showed approximately equivalent costs between lost time cases involving the lumbar spine (low back) and those involving the shoulder.
Pushing and Pulling Heavy Loads May Be More Likely to Cause Shoulder Pain than Back Pain
An interesting study from the Netherlands found that individuals who reported that their job involved quite often or very often pushing or pulling loads greater than 50 kg (110 pounds) were roughly twice as likely to experience back pain than individuals in a control group whose job did not involve frequently pushing similar loads. However, for those same individuals whose jobs involved quite frequently or very frequently pushing or pulling loads greater than 50 kg were two to six times more likely to report shoulder pain than the individuals in the control group. A prospective study based on direct observation by the same authors reported similar findings.
Impact of Pushing/Pulling and Loading on All Parts of the Body
An operator exerts force on a cart through his or her hands. At the same time, an equivalent force is transmitted through the hands and arms, through the shoulder, down the back, through the legs and feet, to the surface. Each joint in this chain has its own unique ability to resist the forces and moments applied to it. For example, it is well-known that the end-plates between the spinal vertebrae and the spinal disks fracture under compressive loads of about 3,500 Newtons (786 pounds force), and that moments (force times a lever arm) about the spinal vertebrae manifest as compressive loads. Pushing and pulling carts applies compressive and shear loads to each link in the chain: hands to arms, arms to shoulders, shoulders to back, back to legs, legs to floor. The loading changes with posture; for example, Chaffin used a biomechanical model to demonstrate that different postures while lifting a battery from a truck and placing it on the floor created different loads to the shoulder and low back. The operators’ posture when picking the battery up exceeded the shoulder strength capability of about 65 percent of workers; the different posture used by the operator while setting the battery down on the floor exceeded the low back strength of a similar percentage of workers.
Moments about the shoulder joint are minimized when the push or pull force is applied to a cart at the same height as the shoulders and with the arms straight out. However, the standard recommendation is to place cart handles at about waist level, as that is where the maximum push strength can be developed. Pushing with that difference in height between the shoulders and the waist guarantees that pushing will create a torque, or rotational force, on the shoulder joint; similarly, moving carts up or down slopes requires the operator to push or pull up, which also creates a torque around the shoulder joint. Hopefully, the torques created do not exceed the shoulders’ strength capability. Unfortunately, the injury data suggest that is too frequently not the case.
How to Design Carts to Minimize the Risk of Back, Shoulder and Knee Workplace Injuries due to MMH Push/Pull Tasks
Carts should be designed so that the torques and forces applied to the joints by pushing and pulling carts match the capabilities of the individuals who will operate them. Tools such as the Liberty Mutual Manual Materials Handling Tables and the Ohio State University push, pull and turn tables are excellent resources for determining appropriate push, pull and turn forces to reduce the risk of back injuries.
A third tool, the Michigan 3-dimensional static strength models, offers guidance for acceptable forces for multiple joints, including the shoulders and the knees. Designers should incorporate the information from these sources in order to design carts with reduced risk of injury to both the shoulders and low back.
To learn more about designing better carts to reduce the push/pull workplace injuries risks, download the Guide to Workplace Ergonomics.
Tom Albin PhD is a licensed professional engineer (PE) and a certified professional ergonomist (CPE). He holds a PhD from the Technical University of Delft in the Netherlands. He is a Fellow of the Human Factors and Ergonomics Society.
Tom has extensive experience as a researcher, corporate ergonomist, and product developer. In addition, he has been active in the US and International Standards community. He is accredited as a US expert to several International Standards Organization working groups and is Vice-Convenor of the ISO committee revising the standards for input devices and workstation layout/postures. He chaired the committee that revised and published the American National Standard ANSI/HFES 100-2007 Human Factors Engineering of Computer Workstations and currently co-chairs the committee working on a new revision of that standard.