From The Ergoweb® Learning Center

Time and Physical Demands Analysis

Dan MacLeod, CPE, MA, MPH


By Dan MacLeod CPE
June 19, 2007


Time and Physical Demands Analysis combines biomechanics with time study, which provides a very practical approach to quantitative analysis.  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 are.
  • 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 is measured in foot pounds (or Newton-meters in the metric system), rather than a scoring system of arbitrary units.
  • The method combines key variables into a single graph:
    – 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.

The Time and Physical Demands Analysis is especially good in making before-and-after comparisons and for showing managers that physical demands that create risk for injury also waste time..  Note that this method 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 basic steps are:

  1. Videotape the task
  2. Review the video pausing each 0.5 second
  3. Calculate the loads on the shoulder and back for that frame (in these examples, the Utah models)
  4. Graph the results.

Example 1: Wasted time plus back strain

Before After

The task in this first example is a packing job in a manufacturing plant.  Before improvements were made , the task involved bending to lift products from a pallet on the floor (shown in photo above left), carrying them, sliding on a conveyor, packing three products, then walking back.  The improvements involved using pallet lifts plus changing the layout, which eliminated bending and other wasted steps.  Afterwards, the task was simply to pack (photo above right).

The graphs below show the results from the Time and Physical Demands Analysis of a single cycle of packing three products, both before and after the use of pallet lifts.  The “before” graph shows the steps of the job and the strain on the back and time involved for each step.  The “after” graph shows the equivalent once the pallet lifts were installed and the wasted steps eliminated.

The visual display of the graphs clearly shows both: (a) the high strain on the back and the time associated with bending and lifting, and (b) the reduction in both time and loads on the back that resulted from use of the pallet lifts.

The method also permits quantitative analysis:

Without pallet lift With pallet lift


Peak load on back 707 lbs.

185 lbs.


Average load on back 96 lbs.

74 lbs.


Time-weighted load on back




Time to complete 1 cycle

23 secs.

 12 secs.


Notes on units of measurement

— Technically, the load on the back is the compression force on the discs in the lower back, reported in pounds.

— The time-weighted loads are the estimated areas under the curve.  These loads provide the best measure of total strain, but are meaningful only in comparisons within the same task, thus units are eliminated.

— Loads on the shoulder (not shown above) are in foot-pounds.

Example 2: Time Savings from a Pallet Lift

This is a similar example from a distribution center showing how a pallet lift reduced cycle time for a simple lifting job by 14 – 20%, plus reduced the load on the spine by 66%. 

The graph below compares lifting a series of eight boxes, first with the pallet on the floor and then with the pallet lift.  The results are superimposed to help highlight the differences.  Each peak represents one lift.  The lower the peak and the less area under the curve, the less strain on the back.  The less horizontal distance at the base of each peak, the less time needed to make the lift.

 Once again, 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 results are:


Without pallet lift

With pallet lift


Peak load on back

856 lbs.

484 lbs.


Average load on back 

495 lbs.

196 lbs.


Time-weighted load on back
(eight lifts)




Time to complete (eight lifts)

25.5 secs.

22.0 secs.


Time to complete (full pallet)

6.5 mins.

5.2 mins.



Example 3:  Repetitive motions are a waste of time

Get large part Push buttons Place completed parts

This example is from operating a small power press in a manufacturing plant.  In this case, the focus was on the arms and graphs were created for both the left and the right arm.

In this case, the steps of the task do not appear very distinct in the graphs, since the arms are almost continually in motion.  Similarly, the peaks are not dramatically different from the valleys.  The primary value with this study is to show how much time is wasted because of the reaching — 93% of the cycle!  An “after” evaluation is not available, but improving the heights and lowering the buttons would yield obvious results.

Example 4: Comparison of three lifting techniques

Lift 1 (with carry) Lift 2 Lift 3 (high)

In this study, an employee lifted a heavy drum using three different techniques.  In Lift 1, he carried the drum to a dolly.  Lift 2, he moved the dolly to a better position, so the lift was direct.  Lift 3 involved lifting a drum over another drum.

This analysis shows the loads on both the shoulders and on the back.  Lift3 creates a whopping 1004 lb. load on the back.  Time differences that appear insignificant on the plant floor reveal themselves as meaningful with formal study — in this case, Lift 3 takes 40% longer.  As a final note, this method captures the strain from carrying the load in addition to lifting it, which is evident in comparing Lift 1 with Lift 2.

Lift 1 Lift 2 Lift 3 Lift 2 vs. Lift 3


Peak load

148 ft. lbs 185 ft. lbs. 271 ft. lbs. 32%

Time-weighted load

972 691 1381 50%


Peak load

626 lbs. 626 lbs. 1004 lbs. 38%

Time-weighted load

3235 2378 4192 43%


5 secs 3 secs 5 secs 40%

Example 5: Resolving a Labor-Management Dispute

This example is from a company that inspects and repairs boxes 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 stages: 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 precise “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.

Contact Dan for complete directions and a template to facilitate analysis: or (570) 242-4664.


*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 biomechanical models developed at the University of Utah for the shoulder and back provide the best system I have found for this use.