At one time or another we have all heard that the main ergonomic risk factors are Force, Posture, and Repetition. Of course, you could argue that Vibration, Temperature, or a number of other secondary risk factors should be included. But the general consensus - for the purpose of most ergonomics training courses - has long been that Force, Posture, and Repetition are the primary ergo risk factors. But I’m beginning to question this. We may need to modify this list to read Force, Posture, and Duty Cycle.
For the last few months, I’ve been exploring more contemporary force-based ergonomic evaluation models (notably Jim Potvin’s MAE Equation). My knowledge of ergonomics has also been influenced by reviewing research related to muscle fatigue, and doing some developmental work of my own in the area of muscle fatigue modeling. In light of this new (to me) knowledge, I’m beginning to conclude that Repetition, basically the number of times we do something, is simply overrated. Of much more significant is Duty Cycle, the percentage of the time that we actually apply the force.
Most ergonomist and industry practitioners are well aware of the need to consider Duration (how long a force is applied). But the typical thinking here is to do this when the force duration is deemed “significant” - say 6 seconds or longer. This is problematic. Allow me to explain . . .
Repetition implies doing something a number of times (say 5 times per cycle). This can also be expressed as frequency (# times you do something over a period of time . . . as having a frequency of 5/min). For force evaluation purposes, of more relevance and usefulness than repetition is knowing the percentage of the cycle over which the force is applied. This is called Duty Cycle. Duty Cycle is much more meaningful than repetition alone; and Duty Cycle is a measure that inherently includes repetition.
A hypothetical example: Let’s assume a force of some unknown magnitude is applied 5 times over a 25 sec cycle. In Case 1, the force has a duration of 2.5 sec. In Case 2 the force duration is 0.5 sec.
Case 1: 5 repetitions of 2.5 sec over a 25 sec cycle
Case 2: 5 repetitions of 0.5 sec over a 25 sec cycle
Lets assume that we don’t know anything else, and assume that the person applies the force in a neutral posture.
In both cases the number of repetitions is 5. Also, in both cases the frequency is 0.2 repetitions per second (0.2/sec). Operating from a repetition-based view of ergonomics analysis, we would conclude that the 2 cases are equivalent. This is absolutely not the case! Case 1 and Case 2 are VERY different. In Case 1, the force is applied for a much greater duration of the 25 sec cycle (5X more!). Duty Cycle provides insight into these differences.
Duty Cycle can be defined as the total force duration divided by cycle time. See Duty Cycle calculations below.
Case 1 Duty Cycle = (5 x 2.5 sec) / 25 sec = 0.5
Case 2 Duty Cycle = (5 x 0.5 sec) / 25 sec = 0.1
The Duty Cycle in Case 1 (DC = 0.5) is 5X greater than Case 2’s Duty Cycle (DC = 0.1). In Case 1 the force is applied for a total duration of 12.5 sec (5 x 2.5 sec), while in Case 2 the force is applied for a total duration of 2.5 sec (5 x 0.5 sec). It is intuitive that Case 1would result in greater physical stress and result in a higher degree of fatigue than would Case 2.
Let’s see how Duty Cycle (DC) impacts acceptable forces. Using Jim Potvin’s simplified MAE Equation (MAE = 1 - DCExp0.24) and calculating a force acceptable for 75% of females and approximately 100% of males, the following “acceptable forces” were calculated for Hand Grip.
Case 1 Acceptable Hand Grip = 8.4 lbs
Case 2 Acceptable Hand Grip = 23.2 lbs
Given equivalent conditions for cycle time, posture, and repetition; the longer total force duration, Case 1 had an acceptable force of roughly 1/3 of the value calculated as acceptable for Case 2. Force duration matters, even for relatively short duration force exertions. The longer the duration, the lower the acceptable force.
When using Duty Cycle in force evaluation calculations, repetition (or frequency) alone is often of little significance. Repetition is basically a “quick and dirty” measure . . . as in counting how many times something occurs. Of greater practical value is knowing the Duty Cycle.
When evaluating jobs, the percentage of time force is applied (Duty Cycle) is often of greater significance than the number of times the force is applied (expressed as repetition, or frequency).
Exercise caution when applying ergonomic risk assessment tools or evaluation methods that do not directly (or at least indirectly) include Duty Cycle. This is very important! Just entering a repetition or frequency range (for example “5-10/min”) may not account for Duty Cycle, the % of time the force is applied.
Force duration is not only important for evaluating longer duration forces (say forces with a duration > 6 seconds), but also important for evaluating shorter duration forces (even forces with a duration < 1 sec).
The spirit of the Saturn Blog is to provide practical, useful & insightful content for the busy ergonomics practitioner . . . expert and non-expert alike. I wish you the best applying ergonomics.
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