Time and Physical Demands Analysis

By Dan MacLeod CPE
www.danmacleod.com
June 19, 2007

Time and Physical Demands Analysis combines biomechanics with time study and is the most practical approach to quantitative analysis that I have found.  The original concept for this method was developed by Peter Holzmann, Ph.D., based on his own proprietary scoring system, which I found too complicated for practical use.  I subsequently tried the Rapid Upper Limb Assessment (RULA) scoring system, then realized that biomechanics provided an even better approach to characterizing physical demands for this purpose.

The method is valuable for a number of reasons:

  • It shows the physical demands for each step of a job, enabling you to see more clearly where the problems.
     

  • It includes time in a way that most other methods do not, enabling you to see how ergo problems usually take longer and interfere with efficiency.
     

  • The graphs are easily understandable by managers and decision-makers, providing you a good format around which to base a report or presentation.
     

  • Using biomechanics as the foundation of the method bypasses many arguments on how to combine the effect of different Musculoskeletal Disorder (MSD) risk factors, thus makes the results more scientifically accurate than other common measurement systems. 
     

  • The results are intuitive, i.e., the load on the shoulder measured in pounds or foot pounds (or Newton-meters in the metric system).
     

  • The method includes most of the key variables of concern:

  • Posture

  • Force

  • Motions (each motion is separately observable on the graph)

  • Magnitude of the load on the body

  • Duration of the load on the body

I have found the Time and Physical Demands Analysis is especially good in making before-and-after comparisons, in particular since it often shows how reducing the physical demands of a task reduce time requirements.  Note that it is helpful for studying jobs that involve full arm motions and bending of the back, although it is not yet fully applicable for the hand or fine movements of the forearm.

The method involves the following steps: videotape the task; review the video pausing each 0.5 second, calculate the loads on the shoulder and back for that frame using standard biomechanical models (in these examples, the Utah models), and graph the results.

Example 1: Wasted time and back strain

The following photos are taken from a video clip of one cycle of a packing job.  Each photo shows one of the four primary steps of the task, which involved picking up products from a pallet on the floor, then placing them in three shipping boxes.  The arrows show the corresponding section of the graph (the graph was generated from a full video clip; these photos are simply to help you see what the graphs depict.)

The full work cycle for this task took 23.5 seconds, as shown on the horizontal axes of the two graphs.  The loads on the arms and back are shown on the vertical axes, thus the higher the peak and the larger the area under the curve, the greater the strain.

Bending over to lift the product from the pallet (3.5 seconds) is very observable in the Back graph, as would be expected. 

Carrying the products to the conveyor (2.5 seconds) places a relatively small load on the back since the employee is now erect.  The load on the arms increases somewhat because the products are large, causing the arms to be more extended than when bending and lifting.

The next step is placing the products on the conveyor (4.5 seconds).  Since the weight of the products now primarily rests on the conveyor, the loads on the body are reduced.

Placing the products in three boxes (10 seconds) results in three blips on the graphs, as the arms are extended with the weight of the product in order to put it into the box.

The final step (3 seconds) is simply walking back to the pallet to start another cycle.  No photo is shown of this step, but the absence of any load other than the weight of the upper torso is clearly depicted on the graph.

The graph for the Back shows the issues most dramatically.  Clearly the strain on the back is associated with bending down to the pallet to lift the product. 

Most people who are involved with ergonomics would understand this point without needing to conduct a study.  However, it can be helpful to show managers and engineers this type of graph because it is easy to understand and the problems sort of point themselves out.

More importantly, the graph shows the time involved.  Packing the boxes (the only value-added step) took 10 seconds.  The remaining 13.5 seconds (57% of the work cycle) were wasted activities like walking and carrying.  If a pallet lift were used and moved adjacent to the conveyor, the wasted time would be eliminated along with unnecessary physical demands.  In fact, if set up properly, the employee could load two different products with less effort and less risk of injury than the current method.

Example 2: Time Savings from a Pallet Lift

This example from a distribution center shows how a standard pallet lift reduced cycle time by 14 – 20%, plus reduced the load on the spine by 66%. 

The graph below compares lifting a series of eight boxes onto the conveyor, first with the pallet on the floor and then with the pallet lift.  The sequence and orientation of the boxes were exactly the same.  The only difference is the height. The results are superimposed to help highlight the differences. 

Each peak represents one lift.  The lower the peak and the less area in the peak, the less strain on the back.  The less horizontal distance at the base of each peak, the less time needed to make the lift.

 The visual display of the graph clearly shows the benefits of the pallet lift and raising the boxes off the floor:

  • The peaks are lower, indicating less risk of injury

  • The area of the graph is less, also indicating less risk of injury

  • The time to perform the task is less, specifically eight trays are lifted using the pallet lift in the time it normally takes to lift seven.

Quantitative evaluation shows that the average load on the spine for these eight lifts without the pallet lift was 494.7 lbs. and with the pallet lift 166.1 lbs.  Thus, the load on the spine was 66.1% less.

The time needed to complete these eight lifts was reduced from 25.5 seconds to 22.0 seconds, thus a savings of 13.7%.  Additionally, a time study was performed on a full pallet-load of trays, which yielded a slightly larger time savings.  The time needed to unload a full pallet of trays was about 6.5 minutes without the lift and about 5.2 minutes with the lift, thus a savings of about 20%.

Example 3:  Resolving a Labor-Management Dispute

This example is from a company that inspects and repairs bags and other small containers for a major distribu­tion company.  The union had complained that the work was too fast and should be slowed down.  Management claimed that slowing down the work economically unfeasible.

This task involved two parts: inspecting and stacking the containers (A in the graph above), then straighten­ing and reorienting the stack and carrying it to a pallet on the floor (B above).  The argument had been about the time it took to perform step A.   However, the analysis clearly showed that part B was the demanding activity that was causing the exertion and fatigue.

Moreover, it was possible to eliminate all of part B by purchasing a pallet lift and stacking the containers directly onto the pallet.  This made about one-third more time available, both for more rest and for more production.  Thus more containers could be inspected with less effort and resulted in a win-win solution.

Caveat — “Safe” Limit

As a final note, I generally do not use this method — or any method — to determine if a job is “safe” or not.  I’m generally more interested in looking for ways to improve any job, since even if a particular situation has low risks for injury, there may still be inexpensive ways to make it more efficient and easier to do.  Furthermore, the scientific studies haven’t yet identified “safe” levels for loads on the shoulders, elbows, or wrists (although such evidence exists for the lower back).

However, this method may end up being a good tool for this purpose.  It provides a good way to capture a lot of information on the physical demands of specific tasks, which then can be compared to numbers of injuries associated with those tasks to help find out what is in fact “safe” and “unsafe.”

A good general rule for practical ergonomics is to play down measurement systems.   If the goal is to solve problems, rather than merely documenting them, it is much more valuable to get in the habit of simply watching videos of jobs and focusing on brainstorming improvements.

However, from time to time, you may need to quantify the physical demands of a task.  Numbers have power and you may need them to convince others of the need to take action.  Furthermore, sometimes the problems aren’t so obvious and you can benefit from a closer, more detailed look.