University of Michigan based researchers Neal Wiggermann and W. Monroe Keyserling conducted an experiment to investigate the effects of select anti-fatigue mating on the perceived discomfort and weight shifting during prolonged standing. This article is a review of their methods and findings.
Standing for long periods occurs in a number of occupations, and is even becoming prevalent in office environments. Wiggermann and Keyserling report the following potential concerns that have been associated with prolonged standing in the scientific literature:
- discomfort in the feet, legs and lower back;
- increased risk of foot, leg and back pain;
- venous disorders; and
- preterm births.
After reviewing previous research that dealt with the question of perceived discomfort by users, they conclude that:
There is currently no method for predicting the effectiveness of a particular mat in mitigating discomfort.
The reason for this, they explain, is in part because there are currently no physiological explanations for differences in user perceived discomfort among flooring surfaces. This means that there are no agreed upon measures, other than subjective ratings, that can differentiate among various floor surfaces. Further complicating this question, research demonstrates large variability even in discomfort ratings, making even perceived discomfort a relatively weak predictor for which surfaces may be "the best."
Wiggermann and Keyserling go on to postulate that behavioral responses while standing, such as weight-shifting between the feet, may provide a more sensitive measure than subjective discomfort ratings, and they cite research that found:
- during standing, separate from the effects of flooring, center of pressure excursions were predictive of lower back pain;
- some significant differences were observed in lateral center of pressure shifts after 3 hours of standing, with greater shifts being associated with surfaces that also received higher discomfort ratings; and
- an increased frequency of postural changes over time while standing, though not among surfaces.
Another challenge in this type of research stems from a lack of flooring material properties being measured and reported in the research base. Without clearly defined and understood material properties, it's difficult to reproduce study results (a cornerstone of scientific method), and difficult or impossible to accurately predict the performance of other unstudied mats. They note that features like thickness, coatings, material additives, and geometry can "drastically alter their attributes."
Wiggermann and Keyserling report that:
… no study has measured properties a priori to allow strategic selection of mats that include a range of values representative of the population of commercially available mats.
They repeatedly cite the best previous research on this topic as coming from Cham and Redfern (2001). For example, they identify Cham and Redfern as providing the greatest detail on flooring material properties. One such property was "stiffness" and another was "work lost," both of which appeared to be associated with discomfort. Stiffness is a measure of a mat's resistance to deformation, or compression under loading; work lost is a measure of material absorbency. One study of shoe material properties found that stiffnss correlated with 'perceived levels of cushioning,' and Cham and Redfern found that "greater stiffness and lower work lost were associated with lower discomfort ratings."
However, my review of the Cham and Redfern (2001) paper found that they did not use the term "work lost," but instead described material properties of elasticity, stiffness and energy absorption. For example, Cham and Redfern state that:
The relationship between material properties of the mats and these discomfort/fatigue measures suggests that standing on a floor mat characterized by greater elasticity, greater stiffness, and lower energy absorption was “more comfortable” (decreased discomfort/fatigue ratings, COP weight shifts, and temperature changes). However, it is important to note that the range of material properties was restricted by the characteristics of the six mats that were tested. The optimal combination of material characteristics (e.g., stiffness, energy absorption, and elasticity) entails further research.
With this type of previous research as a backdrop, Wiggermann and Keyserling designed their study with two primary objectives:
investigate the effect of flooring on discomfort by evaluating anti-fatigue mats with material properties that are representative of a range of contemporary commercially available mats; and
measure several behavioral responses to prolonged standing and to determine how these responses were affected by flooring surface and how they correlated to discomfort.
Interested readers are directed to the full research article, cited below, for a detailed description of research methodologies. In summary, they include:
- participants included 5 males and 5 females (mean age 23.5 years; mean body mass was 67.4 kg (148.3 lb.));
- plantar surface pressure was measured using in-shoe pressure sensors;
- subjective discomfort ratings were also collected;
- 4 commercially available mats were chosen from a pool of 17 mats;
- stiffness and work lost material properties were measured for each mat;
- study participants attended 5 experimental sessions, at least 72 hours apart, and stood for 4 hours on the randomly ordered different mats, plus a linoleum-on-concrete control surface;
- shoes and socks were standardized;
- participants stood at an adjustable height table and were instructed not to support any weight except that of their forearms;
- task rotations included light typing, light assembly, and continuous monitoring;
- a 10 minute break was provided at 110 minutes, and participants were permitted to walk or sit as they wished;
- a discomfort survey was administered before the session start and at 55 minute intervals;
- 10 minutes of plantar pressure measurements were gathered during monitoring tasks, which occurred near the end of each hour of standing;
- the pressure data was used to calculate behavioral responses including weight-shifting, center of pressure excursions and weight distribution between feet.
Interested readers are directed to the full article for a review of statistical analysis methods.
At this point in their article, Wiggermann and Keyserling begin using the term "soft" as a characteristic of the mats, reflecting the stiffness of the mats (ie., the lower the mechanical stiffness, the softer the mat).
- only the lower leg discomfort ratings were significantly influenced by mats across all 4 hours of the study;
- discomfort in other body regions were only significant for measurements taken after the 4th hour;
- low "over-all leg" discomfort ratings was significantly associated with the 2 softest mats when compared to the linoleum-on-concrete control surface;
- the hardest and softest mats were significantly associated with low discomfort in the lower leg when compared to the linoleum-on-concrete control surface;
- only the softest mat was significantly associated with low discomfort for the feet when compared to the linoleum-on-concrete control surface;
- there were no statistically significant differences between any of the mats when compared to the linoleum-on-concrete control surface for the lower back and knees, though the control did receive higher discomfort ratings after 4 hrs of standing;
- there were no significant discomfort effects for the overall body, buttocks or thighs;
- stiffness and work lost material properties correlated only for the foot, where increasing stiffness and decreasing work lost corresponded to increased discomfort.
For weight-shifting behavior:
- the number of weight-shifts increased with statistical significance with each successive session, and with elapsed hours of standing (duration);
- the hardest mat showed significantly more weight-shifts than did the soft mats;
- weight-shifting was positively correlated, though somewhat weakly, with foot, lower back and overall discomfort;
- stiffness and work lost material properties showed no effect on weight-shifting.
For weight-bearing between the left and right foot:
- in 51% of the observations participants stood with more than 80% of their weight on a single foot;
- in 35% of the observations they stood relatively balanced with 40% to 60% of their weight on each foot;
- percentage of single foot stance increased with standing duration and was positively correlated with discomfort ratings;
- stiffness and work lost material properties showed no effect on weight-bearing.
For center of pressure excursion:
- medial-lateral and anterior-posterior excursions increased with discomfort;
- soft mats showed significantly less medial-lateral excursion distance than did the hard mat or linoleum-on-concrete control surface;
- the softest mat showed significantly less anterior-posterior excursion distance than did the hard mat or linoleum-on-concrete control surface;
- the next softest mat showed significantly less anterior-posterior excursion distance than did the hard mat;
- stiffness and work lost were both predictive of anterior-posterior excursions.
What This Might Mean for Ergonomists
This study, like others before it, showed that a hard standing surface can create significantly higher discomfort ratings than softer surfaces — no surprises there. On the other hand, this study did not find significant discomfort differences between the 4 commercial mats tested (the mat brands and models were not revealed by the researchers), even though some previous studies have. Wiggermann and Keyserling suggest this might be due to the fact that each of the 4 mats used in this study were all "contemporary commercially successful mats," and that less effective mats have failed in the marketplace. They also pointed out that none of the mats used in this study "bottomed out" — meaning they are so easily deformed and therefore compress so much that they, in effect, became harder surfaces. Mats that bottomed-out were identified in previous studies as leading to more discomfort than those that do not bottom-out.
All of the behavioral responses to the mats showed positive correlations with discomfort, but their statistical significance was week, making it impossible to be certain. Even so, the researchers believe that weight-shifting in particular shows promise as an indicator of discomfort because of its "likely connection to physiological mechanisms for discomfort." They also note that center of pressure excursions could prove to be a useful measure, since their study "showed a consistent trend with respect to flooring stiffness, with increasing stiffness corresponding to larger [anterior-posterior] excursions."
Wiggermann and Keyserling also recognize that subjective discomfort ratings are a challenging measure because they vary so much and can be affected by individual differences among the participants. Their calculations indicate that they would need to conduct a study with 105 participants to gain the statistical power to better detect a difference between mats using discomfort ratings. Likewise, they would need 49 participants to better understand weight-shifting differences, and 25 participants to better understand center of pressure excursion differences.
Wiggermann and Keyserling conclude their article with these interesting statements:
… these findings do suggest that for standing workstations, the selection of mats can be based more on criteria such as safety, durability, and cost and less on perception of comfort.
The results also show that while mats reduce discomfort, the effect of hours spent standing is much greater than the effect of flooring surface. This means that eliminating standing work, using sit/stand stations, or rotating seated and standing tasks will provide greatest comfort to the worker, regardless of flooring surface.
Primary Reference (the article reviewed above)
Neal Wiggermann and W. Monroe Keyserling (2012). Effects of Anti-Fatigue Mats on Perceived Discomfort and Weight-Shifting During Prolonged Standing. Human Factors, first published online on November 27, 2012 as doi:10.1177/0018720812466672. At the time of this writing this article was available to HFES members and other Sage Publications subscribers at: http://hfs.sagepub.com/content/early/2012/11/26/0018720812466672.full
Cham R., Redfern M. (2001). Effect of flooring on standing comfort and fatigue. Human Factors, 43, 381–391
This article originally appeared in The Ergonomics Report™ on 2012-12-03.