Ergonomics

What is Ergonomics?

Ergonomics is the study of the kind of work you do, the environment you work in, and the tools you use to do your job. It encompasses the design of tools, machines, systems, tasks, jobs, and environments for safe, comfortable and effective human use. 

Physical Labor Ergonomics

There are 3 primary ways to sustain a trauma induced musculoskeletal injury.  The mechanisms of occur due to changes in the magnitude, time, and direction of physical stress to the individual and their tissues.  (Magnitude is the amount of stress on a tissue at any given moment in time. Time is the duration, the number of repetitions, and the rate at which stress is applied to the tissues. Direction is the line of force acting on that tissue.)  Modification to these factors can either lead to tissue damage but may also improve the body’s function and tolerance to stress if managed appropriately.

Three Primary Ways to Sustain A Musculoskeletal Tissue Injury

1. A High-Magnitude Stress Applied for a Brief Duration

Example: Falling off a ladder

2. A Low-Magnitude Stress Applied for a Long Duration

Example: poor posture while driving a forklift

3. A Moderate-Magnitude Stress Applied to the Tissue Repeatedly

Example: repeated poor lifting mechanics 

Step 1: The first thing to consider when working with a patient that has a work-related injury is to figure out what type of stress was applied to the patient in the above examples.  If the injury is a High-Magnitude Stress Applied for a Brief Duration, then diagnosing the source of the injured tissue (i.e. bone, muscle, tendon, ligament, etc.) and the extent to which the tissue is injured (ex. Bone fracture or bone bruise, torn muscle or muscle contusion, etc.) is the most important first step.  Failure to accomplish this may lead to poor outcomes and further tissue trauma/injury.

Step 2: If it was not a High-Magnitude Stress Applied for a Brief Duration, then consider if your body is trained for the work environment.  A person transitioning from working in a warehouse lifting objects has now moved up the corporate ladder and has a desk job that requires 8 hours of sitting a day.  This could cause strain to tissues from not having sufficient tissue adaptation for sustained long periods of sitting.  

The opposite scenario can be true; such as, the person who graduated from school and must work in a mailroom lifting mail, sorting it, and delivering it may not have the trunk and lower extremity strength and endurance to transition to 8 hours of physical labor.

Step 3: Finally, you need to consider is if the individual’s mechanics has a role in the musculoskeletal injury.  This will usually entail either the Low-Magnitude Stress Applied for a Long Duration or a Moderate-Magnitude Stress Applied to the Tissue Repeatedly types of tissue injuries mentioned earlier.  If the symptoms occur from a Low-Magnitude Stress Applied for a Long Duration such as sitting posture, then refer to the Office Ergonomics Handout for proper sitting ergonomics.

Assessment and Treatment for Physical Labor Ergonomics

Standing Posture – Low Magnitude Stress Applied for a Long Duration

This would be important for individuals that have to stand for long periods such as cashiers, front desk hospitality workers, etc.

Proper posture consists of having your feet hip width apart with feet slightly in external rotation about 10 degrees, knees not fully locked out, anterior pelvic tilt of 15 degrees, mild lordotic lumbar curve, the apex of the thoracic spine in line with the sacrum if you were to draw a line straight down from the thoracic spine, medial border of the scapulae about 3 inches from the spinous processes, scapulae in about 30 degrees of internal rotation, mild cervical lordosis, neutral upper cervical flexion/extension, and alignment of the external auditory meatus, acromion, greater trochanter, and lateral malleolus.  See the right most posture image for an example.

Modification of:

• Magnitude
  • Using a back brace intermittently throughout the day until muscle endurance improves
  • Standing with feet wide to reduce work of the hip muscles to hold them upright
  • Alternating a foot on a foot stool to reduce the activity of the muscles of each leg
• Time
  • Break up the standing position with periods of sitting or walking.
  • Build up endurance with standing by starting with 15-30 minutes at a time
• Direction of Physical Stress
  • Change the position of the pelvis with increasing pelvic anterior or posterior tilt throughout the day to change the stress on the spine

Lifting Mechanics – Moderate-Magnitude Stress Applied to the Tissue Repeatedly

Proper lifting mechanics entail having a wide base of support, keep the object close to your center of mass (underneath you), keep your back straight as you flex through your hips and knees.  Tighten your stomach muscles and pelvic floor when you initiate lifting the object off the surface it is resting on.   Keep items in the “Power Zone”.  Keep the object close to the body between mid-thigh and mid-chest height.  Twisting, turning, and bending the back should be avoided, as well as, lifting more than 50 pounds.

• Magnitude
  • Reduce the size of the object
  • Reduce the height of the object that you have to lift it to
  • Get assistance for heavy objects
  • Complete this in a slow controlled manner as a fast lift will increased the force demands on the tissues
  • Use a back support or lift assist device
  • Place an item on a cart to roll it
• Time
  • Break the task into segments instead of tackling it all at once 
• Direction of Physical Stress
  • Avoid lifting too far outside your center of mass
  • Avoid twisting while lifting
  • Pull or push instead of lift

Upper Extremity Activities Such as Hammering or Using Wrench – Moderate-Magnitude Stress Applied to the Tissue Repeatedly

Make sure that the tool you are using is appropriate for your size and weight for the activity.  Try to keep the activity between shoulder and waist height and directly in front of you.  Initiate the movements from your trunk and shoulder and avoid gripping too hard.

• Magnitude
  • Shorten the lever arm by gripping higher on the hammer or farther down the shaft of a wrench
  • Add an extension to a wrench for a better mechanical advantage (optimal length is >100mm for more force)
  • Increase the grip size of the handle to provide a better purchase on the tool (30-50mm in diameter for power and 8-16mm for precision)
  • Place a robber or gel sleeve on the tool to reduce the vibration of the hammer and reduce the stress at the bones and joints when using a wrench
• Time
  • Break up the task to other job requirements to reduce the repetitive stress and trauma to tissues
• Direction of Physical Stress
  • Keep the tool within shoulder width and between waist to shoulder height

References:

Canadian Centre for Occupational Health and Safety. Hand Tool Ergonomics – Tool Design. https://www.ccohs.ca/oshanswers/ergonomics/handtools/tooldesign.html 

Mueller MJ, Maluf KS. Tissue adaptation to physical stress: a proposed “Physical Stress Theory” to guide physical therapist practice, education, and research. Phys Ther. April 2002; 82(4): 383-403.

Occupational Safety & Health Administration. Materials Handling: Heavy Lifting https://www.osha.gov/SLTC/etools/electricalcontractors/supplemental/principles.html#power

Wetzel EM, Haynes W, Holley PW. (2017). Evaluation of self-modifications too hand tools in the construction industry. 53^rd ASC Annual International Conference Proceedings.