Effect of flooring on standing comfort and fatigue.
This study investigated the influence of flooring on subjective discomfort and fatigue during standing and on potentially related objective measures. Participants stood for 4 h on each of 7 flooring conditions while performing computer tasks. During the 3rd and 4th h, floor type had a significant effect on a number of subjective ratings, including lower-leg and lower-back discomfort/fatigue and 2 of 4 objective variables (center of pressure weight shift and lower-extremity skin temperature). In addition, lower-leg volumetric measurements showed tendencies toward greater lower-extremity swelling on uncomfortable floors. The hard floor and 1 floor mat condition consistently had the worst discomfort/fatigue and objective ratings. Significant relationships were noted between the affected subjective ratings and objective variables. In general, floor mats characterized by increased elasticity, decreased energy absorption, and increased stiffness resulted in less discomfort and fatigue. Thus flooring properties do a ffect low-back and lower-leg discomfort/fatigue, but the result may be detectable only after 3 h of standing. Potential applications of this research include the reduction of work-related health problems associated with long-term standing.
Standing for long periods has been directly implicated in a number of health problems, particularly lower-extremity tiredness and discomfort, lower-extremity swelling and venous blood restriction, low-back pain, and whole-body tiredness. These problems are particularly prevalent among workers who stand for long periods in restricted areas, such as checkout supermarket workers, assembly and quality control inspection workers, and health care workers (see review article by Redfern & Cham, 2000).
Flooring may play a role in standing discomfort and fatigue, but studies of flooring have had mixed results. Fatigue is difficult to quantify. Some researchers have investigated localized muscle fatigue through electromyographic (EMG) spectral analysis and/or muscle force production decrements. In our study, we define fatigue as a perception of tiredness, either whole body or lower extremity, whereas we use discomfort to describe discomfort in specific body parts.
A number of investigators have found that flooring affects workers' discomfort/fatigue. Using subjective ratings, the majority of studies concluded that it is more comfortable to stand on soft (so-called antifatigue) mats than on hard concrete floors. However, other researchers have found no floor effect on discomfort/fatigue.
In an attempt to further understand the potential influence of mats on relieving discomfort/fatigue, a number of researchers have considered objective measures (physiological and biomechanical) thought to be related to fatigue. These measures include center of pressure (COP) displacement, leg and back EMG recordings, skin temperature, and leg and foot volumetric measurements. However, investigations concerned with the impact of flooring on all of these objective measures have also reported conflicting results. Thus the impact of flooring on discomfort/fatigue is still largely unknown.
In addition, the specific relationship between discomfort/fatigue measures and material characteristics of the floor (stiffness, elasticity, and energy absorption) has not been investigated. This is important in the design of the most effective mat (see review article by Redfern & Chain, 2000).
The goal of this study was to determine whether flooring has a significant effect on subjective and objective parameters thought to be related to discomfort/fatigue. A secondary goal was to investigate the relationship between these parameters and the mats' material properties.
Ten healthy participants (5 men and 5 women) with no history of lower-extremity or back problems participated in the study. Their ages ranged from 18 to 40 years (M 27 years, SD 6 years), weight from 65 to 107 kg (M 77.4 kg, SD 12.7 kg) and height from 157 to 185 cm (M 169.7 cm, SD 8.4 cm). Informed consent was given prior to participation. Standing habits of the participants were varied; 1, 5, 1, 1, and 2 participants spent less than 10%, 10% to 25%, 25% to 50%, 50% to 75%, and more than 75% of the working day standing, respectively.
Independent and Dependent Measures
The primary independent variable was floor type, with the dependent measures being both subjective and objective measures taken throughout the experiment (Table 1). Participants rated their level of discomfort and fatigue on a questionnaire using a CR10 Borg scale (Borg, 1998). These questionnaires were answered at the beginning of the session and at the end of each subsequent hour during the 4-h testing period. Participants rated their perceived hardness of the floor, overall fatigue, entire leg fatigue, and level of discomfort on eight specific body parts (upper and lower back, hips, upper legs, knees, lower legs, ankles, and feet).
Quantitative physiological and biomechanical measurements were recorded regularly throughout the experiment. COP was recorded for periods of 10 minutes at a sampling rate of 20 Hz, using a force platform (Bertec Corp., Columbus, OH). The COP was analyzed to investigate the participants' lateral weight shifts (from one leg to the other) during the experiment. EMG signals were recorded for periods of 1 minute at a sampling rate of 1 kHz, using bipolar surface electrodes placed on two muscles of the right lower leg (tibialis anterior and soleus) and lower-back muscles (left and right erector spinae). Median frequency shifts of the power spectrum of the collected EMG data were evaluated to examine localized muscle fatigue. An infrared thermometer (Exergen Corp., Watertown, MA) was used to measure skin temperature over the leg muscles (soleus, tibialis anterior, quadriceps, and hamstrings) and elbow (control). Leg volume (up to the level of the tibial plateau) was measured before and after standing using a water tank displacement method.
Flooring Conditions and Material Testing
Flooring conditions included six different floor mats (Floors B-G in Table 2) as well as a hard floor control (Floor A) made of a steel plate covered with vinyl tile. A number of materials testing protocols were performed to quantify the material properties of each flooring mat. The results of these tests are summarized in Table 2. Three of the material property parameters (stiffness moduli, work lost, and 25% deflection load) were acquired through load-deformation tests (Figure 1). These data were collected using an Instron test at a head speed of 2.54 cm/min and a head cross-sectional area of 176.4 [cm.sup.2]. The slope of the load-deformation curves, which represent the stiffness moduli or firmness of the mats, were estimated at three load levels: 1000, 2000, and 4000 N. The area enclosed in the load/unload-deformation hysteresis curves, or work lost, was calculated. Another variable acquired from the load-deformation curves is the required load necessary to obtain a 25% deflection of the total thickness.
Another test, termed the drop-g test, measured the deceleration of a 4.54-kg weight dropped on the mat from a 3.15-mm height to estimate the rebound characteristics of the material. The deceleration of the weight at the first floor contact, termed drop-max-g, was recorded (Table 2). A final material test of the nonelastic properties was performed using a load-decay or relaxation method. This test was performed by compressing the mat to a fixed deformation corresponding to a load of 2000 N and holding this deformation for 2 min while measuring the load decay required to maintain this initial displacement (Table 2).
A full factorial experimental design was used such that all participants stood once on all seven floorings. Participants returned for a total of seven visits, each time to be tested on one floor. A minimum of 48 h between testing sessions was used to reduce interaction effects from session to session. The presentation order of the floors was randomized. All participants were supplied with the same brand and style of socks and hard-sole shoes throughout the testing. For each testing session, the floor condition was applied to the top of the force plate and to a runway 1.21 m wide and 6.10 m long, continuous with the force plate. Total testing time per session was 4 h. Participants stood on the force plate, located in front of a computer screen, and performed computer tasks requiring speed, attention, and memory. COP position, EMG recordings, and skin temperature were collected every 15 to 20 min. Every 30 min, participants were asked to walk back and forth on the runway for 2 min. Leg volume was measured befo re and after standing. At 1-h intervals, participants completed the subjective questionnaire that rated fatigue (i.e., perceived tiredness) and comfort. Thus, the questionnaires were answered at Hours 0 (beginning of session), 1 (end of 1st h), 2, 3, and 4 (end of session).
Prior to analysis, subjective ratings recorded on a CR10 Borg scale were transformed to a linear RPE Borg scale ranging from 6 to 15 (Borg, 1998). A statistical analysis of differences in discomfort/fatigue ratings among floors was performed within each hour period. This within-subject repeated-measures analysis of variance (ANOVA) investigating the flooring effect was performed for each subjective rating, with the independent variable being the flooring condition.
For the first 2 h, this analysis found no significant differences (p > .05) in discomfort/fatigue ratings among floors. However, differences in ratings among floors became significant during the last 2 h of testing for all the segmental/joint discomfort/fatigue ratings of the lower extremity and lower back (p < .05). It should be noted that discomfort was a cumulative feeling over time; however, these differences in discomfort/fatigue ratings among flooring conditions were not detectable at a statistically significant level in the 1st and 2nd h of the testing session. Standing on the hard floor (A) consistently yielded the highest (worst) discomfort/fatigue ratings for all subjective ratings (Figure 2). In addition, Mat F had the highest ratings among the mats for most ratings. Differences in other ratings, such as upper-back discomfort and overall fatigue, were not significant across flooring conditions, even at Hours 3 and 4.
Further post hoc Tukey comparisons tests (Figure 2), conducted only for variables on which flooring had a significant effect, revealed significant differences among certain floors or group of floors, whereas the performance of other flooring conditions was comparable. For example, significant differences in whole-leg fatigue ratings were found between the hard floor (A) and Mats B, D, E, and C and between Mats F and C; however, whole-leg fatigue ratings were comparable within each of the following groups of floors: A, F, and G; F, G, B, D, and E; and G, B, D, E, and C.
Pearson correlation coefficients (r between .4 and .9) indicated that during Flours 3 and 4, increasing elasticity (i.e., increasing drop-max-g; decreasing work lost and load decay) was associated with a decrease in discomfort/fatigue ratings, whereas an increase in the energy absorbing capability of the material (i.e., decreasing drop-max-g; increasing work lost and load decay) yielded higher discomfort/fatigue ratings. In addition, with increasing stiffness, there was a decrease in discomfort/fatigue measures.
Center of pressure (COP). COP measurements were used to compute the number of lateral weight shifts as participants stood on the floors. The rationale was that a greater amount of weight shifting may indicate an attempt to reduce increasing body stress and fatigue. A body-weight shift was defined as a change in the lateral COP beyond 10% of the total distance range seen for the trial. Thus, each time the participant shifted weight from one leg to the other, a weight shift was counted. For example, given that the maximum COP position is +x cm to the right and that the minimum COP position is -y cm to the left, each time the COP position crosses the reference lines +5% of x or -5% of y is counted as one weight shift. For the first 3 h there were no significant differences in the number of weight shifts among flooring conditions. However, in Hour 4, the number of weight shifts was significantly different (p.=04) among floors, with Floor F (mat) and the hard floor (A) being associated with the highest number of COP weight shifts (Figure 3). Tukey comparison tests found comparable COP performance among a number of floors.
Material properties of the mats were correlated with COP weight shifts (r between .5 and .7). Increased stiffness and drop-max-g and decreased work lost and load decay were associated with significant decreases in COP weight shift. Thus, within the range of tested floor mats, the results indicated reduced weight shifts with increased elasticity, decreased energy absorption, and increased stiffness.
Leg skin temperature. A statistical analysis of differences in normalized skin temperature changes was performed with the independent variables including the flooring condition and the muscle group over which the measurement was made. During the 4th h of the experiment, the change in the normalized (to the elbow control temperature) skin temperature (over the soleus, tibialis anterior, quadriceps, and hamstrings) was significantly affected by floor conditions (p < .0001). This was true only for the 4th h of the experiment. In addition, these differences significantly varied among muscle groups (p < .001). The muscle group associated with the highest normalized skin temperature change (averaged over all floors) was the soleus (0.54[degrees]C), followed by the tibialis anterior (0.34[degrees]C), the quadriceps (0.20[degrees]C), and the hamstrings (0.11[degrees]C). Although the degree of temperature change differed among muscles, the variation across floors followed the same tendencies. Thus the normalized tempe rature changes were averaged over all muscles within each floor to describe the overall effect.
The greatest changes in the normalized temperature were recorded during standing on the hard floor (Figure 4). Post hoc analyses of the normalized change in skin temperature revealed comparable performance between the hard floor (A) and Mat F. A number of pronounced differences were found: between the hard floor (A) and Mats C, G, E, D, and B; between Mat F and Mats G, E, D, and B; between Mats C and D; and between Mats G and B. Just as for COP and subjective discomfort/fatigue, skin temperature increases were reduced with increased floor stiffness and drop-max-g and decreased load decay and work lost.
Leg volume. Volumetric measurements were made before and after the standing period of 4 h. An analysis of variance found no statistically significant changes in lower-leg volume as a function of flooring condition. However, despite this nonsignificant flooring effect, the results showed some interesting trends. First, lower-leg volume increased on all floors, with the largest mean increase (181 [cm.sup.3]) observed on Mat F (the worst-ranked mat for subjective questionnaire rating), the hard floor (A; 179 [cm.sup.3]), and Mat D (178 [cm.sup.3]). The smallest change (35 [cm.sup.3]) was noted for Mat G.
Electromyography. Localized muscle fatigue under isometric contractions is known to lead to shifts in the EMG power spectrum toward the lower end of the frequency spectrum. This effect can be investigated by examining the changes in the median frequency of the EMG power spectrum. An ANOVA was performed on the median frequency to determine potential time and floor effects on this parameter. No statistically significant changes in the median frequency occurred with time. In addition, no statistically significant floor effect on EMG spectral change differences was found. These results apply to all muscle groups.
Correlation between Subjective and Objective Results
A correlation analysis was performed to investigate relationships between subjective (discomfort/fatigue ratings) and objective variables thought to be related to standing discomfort/fatigue. The independent variables were the discomfort/fatigue rating and the participant variable, to account for intrasubject variability, whereas the objective measure of interest was used as the dependent variable. The correlation analysis between subjective ratings and lateral weight shifts (for Hours 3 and 4) revealed a significant logarithmic relationship between the number of COP weight shifts and a number of discomfort/fatigue measures, including overall fatigue, whole-leg fatigue, hip discomfort, upper-leg discomfort, and ankle discomfort (p < .05). In each case, an increase in the number of weight shifts was associated with an increase in the discomfort/fatigue rating (see Figure 5). Thus the data suggest, in general, that as participants become increasingly tired, there is an overall increase in the number of weight shifts while standing.
A similar analysis of variance was used to investigate the relationship between discomfort/fatigue ratings and normalized temperature change. Statistically significant relationships were found between the normalized temperature change and several subjective ratings, including lower-back discomfort, hip discomfort, lower-leg discomfort, ankle discomfort, and foot discomfort (p < .05). In general, increases in the discomfort/fatigue ratings were associated with increases in the normalized temperature change. (Figure 5 shows an example for the temperature change averaged over all muscles.) No significant relationships were reported between discomfort/fatigue ratings and other objective measures, including leg volume and EMG results.
Results of this study suggest that flooring condition played a role in standing comfort quantified using subjective discomfort/fatigue ratings and a number of objective measures (lateral COP weight shifts and normalized skin temperature). Table 3 shows the ranking of floor conditions for each significant dependent variable from best (1) to worst (7). It is interesting to note that the trends across all variables were similar: The rankings in Table 3 show that Floors A (hard floor) and F consistently performed worse (largest increases in subjective ratings, COP weight shifts, and normalized skin temperature) than all other floors. It also appears that Floors E and C had generally good performance; however, this is not as clear as the negative performance of Floors A and F. EMG and volumetric measurements were not significantly different among flooring conditions, although interesting tendencies suggested that leg swelling was greatest for the less-comfortable floors (F and A).
Another important finding was that duration of standing during investigations of discomfort/fatigue is an important factor. This study found that the impact of flooring on standing discomfort/fatigue measures may not become pronounced enough to allow detection of statistically significant differences among flooring conditions until the 3rd and 4th h of standing.
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.
The significant relationships found between subjective discomfort/fatigue ratings and objective variables (COP weight shifts and skin temperature) confirmed that long-term standing fatigue is not only a subjective feeling but is also quantifiable with physiological and bio-mechanical variables. In this study, increases in discomfort and fatigue were associated with increases in COP weight shifts and leg skin temperature, both of which were significantly affected by the flooring condition. A fundamental physiological question remains unanswered: Why are increases in skin temperature and variations in other physiological measures, such as leg swelling, associated with feelings of discomfort and fatigue? The answers to such basic physiological questions could provide better insight regarding the mat characteristics that can best reduce discomfort/fatigue.
Finally, we will discuss possible reasons for disagreement in the findings across studies (Redfern & Chain, 2000). Most previous studies have used 2 h or less of standing time; thus, based on the findings of this study, we believe that limited exposure accounts for most of the conflicting results regarding both subjective and objective measures across studies. In spite of limited exposure times, the majority of investigations (including the present study) have found that standing on a floor mat is more comfortable and results in less lower-extremity fatigue than standing on a hard surface. However, there were differences in some of the specific body part discomfort ratings. For example, flooring affected low-back discomfort in our study. This result is similar to those reported by Redfern and Chaffin (1995) and Rys and Konz (1989) but are contradictory with the studies by Konz, Bandla, Rys, and Sambasivan (1990), Krumwiede, Konz, and Hinnen (1998), and Rys and Konz (1988). For the subjective results, the dur ation of exposure is probably the main explanation for these differences.
Present results regarding COP variables agree with findings of Madeleine, Voigt, and ArendtNielsen (1998). However, Hansen, Winkel, and Jorgensen (1998) and Zhang, Drury, and Woolley (1991) found no such floor/foot interface effects on COP measures. In this study, the floor effect on COP weight shifts was significant only during the 4th h of the experiment, which again suggests the necessity to have a 4-h minimum exposure time.
The relatively young and healthy participant population recruited in this study is believed to be responsible for the insignificant flooring effect found on leg volume measurements. When comparing volumetric measurements across studies, disagreements are partly attributable to the wide variance in the age of participants tested across investigations. Another possible reason is the method used to assess leg swelling. For example, some studies measured direct leg/foot volume (Hansen et al., 1998), whereas others recorded calf circumference (Madeleine et al., 1998).
The skin temperature results of this study were similar to those reported by Konz et al. (1990) but contradictory to findings of Brantingham, Beekman, Moss, and Gordon (1970), who suggested that higher calf temperatures result from standing on a softer surface. Other studies of temperature (Hansen et al., 1998; Madeleine et al., 1998) have found no floor/foot interface effect on skin temperature. Reasons for changes in skin temperatures during long-term standing are multifactorial. First, skin temperature regulation is a complex physiological phenomenon related to edema formation and local blood flow during standing. Second, in addition to the short exposure time, the type of thermometer used to measure skin temperature may have been another source of error in the data. Using a contact thermometer (thermistor) might have altered skin temperature recordings because of heat exchange effects between the skin and the transducer. Finally, in this study we used a control skin temperature reading (at the elbow) to remove any systemic changes in skin temperature possibly attributable to changing room conditions. This procedure, which had not been used in other studies, is believed to be particularly important in a noncontrolled environment.
EMG spectral shifts are more commonly seen for dynamic tasks characterized by relatively high muscle contraction levels over short periods (e.g., lifting tasks). For long-term standing, the muscle forces are significantly lower and of much longer duration. We believe this factor is responsible for our EMG findings (not sensitive to flooring condition), which were in accordance with results of some investigators (Cook, Branch, Baranowski, & Hutton, 1993; Zhang et al., 1991) and contradicted by others, including Kim, Stuart-Buttle, and Marras (1994) and Madeleine et al. (1998), who reported significant floor/foot interface effects on EMG measures (leg and/or back). These discrepancies in the EMG results may be explained by two factors: (a) The procedure of collecting EMG data during a specific percentage of a maximal voluntary contraction (Kim et al., 1994) causes fatigue in itself and possibly masks "standing fatigue" effects; and (b) skin impedance changes over time, making EMG signals unreliable over long p eriods.
Based on the relatively small-size, healthy, and young participant population used in this study, the results reported here suggest evidence of significant effects of material properties of flooring conditions on standing comfort assessed using both subjective and objective parameters. EMG data were found not be sensitive enough to detect local muscle fatigue attributable to standing. A number of recommendations can be suggested for future research: (a) using a minimum 4-h exposure time, (b) testing a wider range of material properties, (c) investigating the effect of age and other anthropometrical characteristics on discomfort/fatigue measures, (d) studying theoretical relationships between feelings of discomfort/fatigue and physiological measures, and (e) investigating the effect of flooring on performance of typical industrial tasks.
This study was supported by the 3M Company. We would especially like to thank Gordon Altshuler, Ph.D., at 3M for providing material properties data.
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TABLE 1 Dependent Variables Objective Variables Subjective Variables from Questionnaires Right soleus EMG Perceived hardness of floor Right tibialis anterior EMG Overall body fatigue Right quadriceps EMG Overall leg fatigue Right hamstrings EMG Upper-back discomfort Right erector spinae EMG Lower-back discomfort Left erector spinae EMG Hip discomfort Lateral weight shifts from Upper-leg discomfort center of pressure Skin temperature over right soleus Knee discomfort Skin temperature over right Lower-leg discomfort tibialis anterior Skin temperature over right Ankle discomfort quadriceps Skin temperature over right Foot discomfort hamstrings Leg volume up to tibial plateau TABLE 2 Material Properties of Floor Mats Stiffness Modulus (N/mm) Floor Thickness Work Lost Type (mm) 1000 N 2000 N 4000 N (N H mm) B 7.1 864 940 6733 1408 C 7.1 1896 3180 4463 303 D 11.2 720 1380 3040 992 E 15.6 534 1252 2444 3348 F 16.9 349 936 2252 3392 G 14.6 563 893 1814 1600 Floor Load Required for Drop-Max-g Load Type 25% Deflection (N) Acceleration (g) Decay (N) B 1249 6.66 269 C 3993 8.49 437 D 1176 5.98 462 E 179 4.72 982 F 130 3.06 686 G 1602 4.88 468 TABLE 3 Ranking of Floor Conditions for Each Dependent Variable from Best (1) to Worst (7) Floor Dependent Variable A B C D E Whole-leg tiredness 7 4 1 3 2 Lower-back discomfort 7 5 1 2 3 Hip discomfort 7 4 2 6 1 Upper-leg discomfort 7 4 3 5 1 Knee discomfort 7 1 3 6 2 Lower-leg discomfort 7 4 2 5 1 Ankle discomfort 7 4 1 5 3 Foot discomfort 7 5 1 4 2 Center of pressure (COP) weight shifts 6 1 5 2 3 Mean normalized skin temperature change 7 1 5 2 3 Total (higher rank = worse flooring) 69 33 24 40 21 Floor Dependent Variable F G Whole-leg tiredness 6 5 Lower-back discomfort 6 4 Hip discomfort 5 3 Upper-leg discomfort 6 2 Knee discomfort 5 4 Lower-leg discomfort 6 3 Ankle discomfort 6 2 Foot discomfort 6 3 Center of pressure (COP) weight shifts 7 4 Mean normalized skin temperature change 6 4 Total (higher rank = worse flooring) 59 34
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|Author:||Cham, Rakie; Redfern, Mark S.|
|Article Type:||Statistical Data Included|
|Date:||Sep 22, 2001|
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