Biomechanical Risk Factors for Musculoskeletal Disorders: What’s New?

I teach EHS professionals about the biomechanical risk factors of musculoskeletal disorders (MSDs) and their associated threshold values, which are incorporated into common MSD risk assessment tools such as ACGIH’s threshold limit value (TLV) for hand-activity level (HAL), the NIOSH Lifting Equation and the Snook & Ciriello psychophysical guidelines for pushing, pulling and carrying activities.

All of these tools are meaningful and valid, yet most were developed in the 1980s and 1990s. More recently, there have been several excellent epidemiological studies that have added to our understanding of the risk factors associated with MSDs.

In 1997, NIOSH published a comprehensive review of high-quality epidemiological studies and determined that there was evidence of a causal relationship between several workplace risk factors and MSDs. The primary biomechanical risk factors included repetitive motion, excessive force, awkward postures, sustained postures and prolonged sitting and standing. These findings validated the approach of the risk assessment tools mentioned above.

More recent research can be incorporated with the numerous studies that examine the causal relationships between biomechanical risk factors of each body area and type of MSD. Here’s a summary:

Neck: Risk factors include awkward neck postures (both repetitive or sustained) and general heavy physical work, like forceful exertions and frequent lifting.

Low back: The risk factors include heavy physical work, such as heavy lifting and forceful exertions, repetitive lifting, excessive reaching, stooping and prolonged standing. Recently, Garg et al. (2014) showed a statistically significant exposure-response relationship between the NIOSH lifting equation peak lifting force (PLI) and the peak composite lifting index (PCLI), and low back pain.

Upper limb: Heavy physical work, forceful exertions, awkward postures, repetition and prolonged computer mouse use contribute to upper limb injuries (shoulder, arm, elbow, forearm, wrist and hand), such as tendinitis, lateral epicondylitis and carpal tunnel syndrome (CTS).

Shoulder: Physically strenuous work, heavy pushing and pulling, and repetitive tool use can influence shoulder injuries. Recently, Hoozemans et al. (2014) summarized high-quality cohort studies and showed there is strong evidence that pushing and pulling are related to upper-extremity symptoms, specifically shoulder symptoms.

Elbow/forearm: Risk factors include sustained gripping postures, forceful exertions, repetitive arm and wrist motions, prolonged computer mouse use and other repetitive work.

Wrist/hand: Risk factors include frequent awkward postures such as wrist flexion and extension, forceful exertions, heavy physical work, and prolonged computer mouse use. Recently, in a large multi-site, cross-sectional cohort study, Fan et al. (2015) found a strong dose-response relationship between jobs requiring high hand force and CTS, while wrist posture or repetition on their own were not significant contributors to CTS. Also, Kapellusch et al. (2014) demonstrated the ACGIH TLV for HAL is a strong predictor of CTS, as well as peak force and the hand activity level.

Lower limb: Two primary biomechanical risk factors are repetitive stair climbing and repetitive heavy lifting. Other risk factors for MSDs, such as meniscectomy, are prolonged kneeling or squatting.

What Has Changed?

The prevalence and economic burden of workplace injuries has changed significantly over the past few decades. MSDs are a leading cause of lost workdays and are associated with a large economic burden, according to the American Academy of Orthopedic Surgeons. According to the Liberty Mutual Workplace Safety Index, overexertion injuries (a type of MSD) are the leading cause of all non-fatal workplace injuries in the United States. Overexertion activities are defined as lifting, lowering, pushing, pulling, carrying, holding or throwing.

In 2013, overexertion injuries accounted for 24.4 percent of all non-fatal workplace injuries and amounted to $15.08 billion in direct costs. These direct costs amount to $290 million dollars per week or $41.4 million per day. The percentage of overexertion injuries has stayed relatively the same between 2007 and 2016. They accounted for 24 percent to 26.8 percent of all non-fatal workplace injuries per year in the United States.

However, the direct costs have escalated from $12.7 billion to $15.08 billion. Direct costs only include medical costs and indemnity payments; they do not include the indirect costs such as production time lost by the injured employee, fellow workers and supervisors; spoiled product; unhappy customers; cleanup time; schedule delays; training new employees; overhead costs; legal fees; or increased insurance costs. Indirect costs also are significant and can amount to 1.1 to 4.5 times that of the direct costs.

Our understanding of how the biomechanical risk factors interact also has changed. For most studies and for many years, it was assumed that each risk factor functions as an independent factor, without interdependence with other factors. Recently, Gallagher and Heberger addressed this assumption. Based on several studies, they eloquently determined that force and repetition are not independent, but they are interdependent. They showed that there is a strong interdependence between these two risk factors across a wide range of joint disorders and symptoms, including low-back disorders, CTS, hand/wrist tendinitis, wrist discomfort, lateral epicondylitis, shoulder tendinitis and shoulder and knee discomfort.

They also found that tasks that expose individuals to:

• Low force (at either a low repetition or high repetition) only nominally impact the risk of joint injury.
• High force and low repetition pose a low risk of joint injury, and
• High force and high repetition significantly increase the risk of joint injury.

We also should consider the impact of another risk factor in this relationship: awkward posture. Based on the force-joint posture relationship, certain changes in postures, such as awkward hand/wrist postures, increase the force requirements and the risk of injury. So awkward posture and high repetition will increase the risk of joint injury, while neutral posture and high repetition will have little impact on injury.

While new research helps us clarify some issues surrounding workplace risk factors for MSDs, we’ve learned the primary biomechanical risk factors linked to MSDs have stayed the same. They include heavy physical work, heavy lifting, awkward postures, repetitive work and prolonged work. However, the dominant and most important biomechanical risk factor appears to be heavy physical work, otherwise known as forceful exertion.

What Does It Mean? 

Incorporating newer findings with existing research can be seem overwhelming, so let’s take a look at what this all means.

First, we have a very good understanding of the biomechanical risk factors that lead to the development of MSDs for several body areas and joints. They include heavy physical work (forceful exertions), heavy lifting, awkward postures, repetitive work and prolonged work. Second, there are complex relationships between risk factors, meaning there is some interdependence. Third, some risk factors, such as forceful exertion and awkward postures, seem to play a larger part in the development of MSDs.

So, make sure to use a valid MSD risk assessment tool that covers all of the main body areas, that focuses on forceful exertions and awkward postures, and that also allows assessment of sustained work and repetition.  

About the author: Blake McGowan, managing consultant and research specialist for Humantech, has 15 years of experience helping companies implement and sustain effective workplace ergonomic initiatives. His clients have achieved best-in-class results that improve human performance, increase productivity, enhance product quality and reduce injury and illness rates and costs. He received a bachelor of science degree in Kinesiology and a master of science degree in biomechanics from the University of Waterloo in Waterloo, Ontario. He has achieved recognition as a certified professional ergonomist (CPE), is a member of the Human Factors and Ergonomics and the American Industrial Hygiene Association (AIHA), and is an officer of the AIHA Ergonomics Committee.

References

American Academy of Orthopedic Surgeons. 2008. The Burden of Musculoskeletal Disorders in the United States: Prevalence, Societal, and Economic Cost. Executive Summary. Rosemount, IL.

da Costa BR and Vieira ER. 2010. Risk Factors for Work-Related Musculoskeletal Disorders: A Systematic Review of Recent Longitudinal Studies. American Journal of Industrial Medicine. 53: 285-323.

Fan ZJ, Harris-Adamson C, Geer F, Eisen EA, Hegmann KT, Bao S Silverstein B, Evanoff B, Dale AM, Thiese MS, Garg A, Kapellusch J, Burt S, Merlino L, and Rempel D. 2015. Associations between Workplace Factors and Carpal Tunnel Syndrome: A Multi-Site Cross Sectional Study. American Journal of Industrial Medicine. 58: 509-518.

Garg a, Boda S, Hegmann KT, Moore JS, Kapellusch JM, Bhoyar P, Thiese MS, Merryweather A, Decker-Schaefer G, Bloswick D, and Malloy EJ. 2014. The NIOSH Lifting Equation and Low-Back Pain, Part 1: Association with Low-Back Pain in the BackWorks Prospective Cohort Study. Human Factors. 56(1) 6-28.

Garg A, Waters T, Kapellusch J, and Karwowski W. 2014b. Psychophysical Basis for Maximum Pushing and Pulling Forces: A Review and Recommendations. International Journal of Industrial Ergonomics. 44: 281-291.

Hoozemans MJ, Knelange EB, Frings-Dresen MH, Veeger HE, and Kuijer PP. 2014. Are pushing and pulling work-related risk factors for upper extremity symptoms? A systematic review of observational studies. Occupational and Environmental Medicine. 71(11):788-95

Kapellusch JM, Gerr FE, Malloy EJ, Garg A, Harris-Adamson C, Bao SS, Burt SE, Dale AM, Eisen AE, Evanoff BA, Hegmann KT, Silverstein BA, Thiese MS and Rempel DM. 2014. Exposure-Response Relationship for the ACGIH Threshold Limit Value for Hand-Activity Level: Results from a Pooled Data Study of Carpal Tunnel Syndrome. Scandinavian Journal of Work, Environment, and Health. 40(6): 610-620 

Liberty Mutual Research Institute for Safety. 2016. Liberty Mutual Workplace Safety Index. Executive Summary. Hopkinton, MA.

National Institute for Occupational Safety and Health (NIOSH). 1997. Musculoskeletal Disorders and Workplace Factors: A Critical Review of Epidemiologic Evidence for Work-Related Musculoskeletal Disorders of the Neck, Upper Extremity, and Low Back. DHHS (NIOSH) Publication No. 97-141.