Comparative Study on Various Methods of Pressure in Compression Stockings

LI Jiecong()， KE Sicheng()， XIE Hong( )

School of Fashion Engineering， Shanghai University of Engineering Science， Shanghai 201620， China

Abstract： A series of self-designed and woven compression stockings were used in this pressure comparison experiment. In order to compare the differences of the garment pressure values exerted by the compression stockings with different structure parameters among three methods. The experiments were carried out with Flexi force sensors. The pressure value of nine pairs of compression stockings on five subjects and the model leg was collected, and the tensile force of the every section of these stockings was collected to calculate the pressure according to Laplace’s equation. The data analysis results show that the pressure values of the compression stockings obtained by comparing the three methods have great differences in various testing methods. There is a significant correlation between processing parameters and stitch density(SD). The pressure design of the compression stockings should be based on the actual wearing of the pressure.

Key words： compression stocking； pressure study； cylinder parametr(CP); elastic motor parrametor(EMP)

Introduction

Sport compression garments have been enjoying increased in more and more sport vents. They are used to help athletes to enhance their sport performance. Large amount of research have foucsed on the effect of compression garments on exercise function， fatigue and recovery[1-3]. Compression garments can effectively improve micro-circulation， promote peripheral circulation and blood flow[4-6]. Different compression stockings produce various compression effects.

Garment pressure is produced by the mutual effect between outer garment and skin of human. It is an important appraisal factor about their capacities and associated with motion performance and pressure comfort[3， 7-8]. Studies show that compression stockings should supply different pressures on body parts in varied sports[9-10].

According to the requirements of different human bodies and functions， the design and processing parameters are different[11-13]. The pressure generated by these garments is different， but it is rarely measured and monitored. At present， Europe， the United States， Japan and China have their own standard of the compression stocking[14]， and relevant scholars have various methods to test the pressure value.

In order to detect the pressure of compression products acting on human legs， many scholars conducted experimental tests on experimental subjects by direct measurements[15-17]， and obtained the pressure value of compression stockings on human legs by using different pressure test equipments[18-20]， and explored the effect of compression garments.

There are certain limitations in seeking test volunteers to conduct the pressure test experiments， which cost huge human and material resources. Therefore， the model leg is adopted and conducted by some people. It is convenient to use the model leg for testing in order to explore the changes of the influencing factors of the compression stockings[13， 18-19]， to compare the advantages and disadvantages of the testing equipment and to compare with the human leg test. Several compression stockings companies and some national standards have adopted the model leg as the test object.

It is also very common to use Laplace’s equation to calculate theoretical pressure value[18， 21-22]. This method requires scholars to have a solid theoretical foundation. The pressure can be calculated according to the tension of the hose of the compression stockings and the radius of curvature of the leg.

In the pressure test， the data obtained are the most authentic and reliable based on human legs under the direct method. But it will cost huge human and material resources. The model leg as the experimental object is used by most people because of less need of resources. The indirect method is based on Laplace’s equation. After being obtained the tension force of the fabric and the curvature radius data of the legs， the theoretical pressure value can be directly calculated without measuring and consuming resources.

Previous studies on the use of garment pressure have mainly focused on the single method and apply the results to all aspects. However， the difference of garment pressure value of compression stockings on lower limb among the model leg， human leg and Laplace’s equation is often neglected.

The purpose of this study is to compare the differences of the garment pressure values exerted by the compression stockings with different structure parameters among the model leg， human leg and Laplace’s equation. One of the aims of the study is to assess the difference of establishing standard in various compression stockings. This study is of great significance to the study of pressure in compression stockings.

1 Experiments

The series of experiments in this paper included measuring fabric properties，e.g. tensile strength of the fabric， pressure values under compression stockings worn by five subjects， and pressure values under compression stockings on the model leg.

1.1 Materials and instruments

Nine samples of compression stockings were knitted on an MP 615 stocking machine made by Santoni with a cylinder diameter(CP) of 95.25 mm and 200 needles. They were prepared in our lab under the controlled processing parameters. All stockings were plain structure. All yarns were purchased commercially. Surface yarn was pure cotton(29.16 tex)， ground yarn was polyamide covered yarn(3.33 tex spandex yarn covered by 7.78 tex polyamide yarn)， the mouth yarn was nylon(15.56 tex) and elastic ribbon was 120 spandex yarn. The parameters of machine adjustment include CP and elastic motor parameter(EMP). Adjust the CP to change the size of the knit stitches and their closeness and coordinate the EMP to adapt the tension of stockings. When adjust the machine parameters， we need to ensure that the parameters are in the right range. The CP ranges from 350 to 650. There will be many holes in the knitting process if it is lower than 350. The fabric density will be too small to be formed and the yarn will be easily tangled to cause downtime to affect knitting if it is higher than 650. The EMP ranges from 650 to 2 000 r/min. The elastic will be easy to break if the EMP is lower than 650 r/min. But the elastic will be easy to loose and drop

to cause the automatic shutdown due to the slow motor speed if it is higher than 2 000 r/min. If the EMP is slow， the length of elastic is less and there are more coils per unit length in compression stockings. When the rubber is stretched， the stockings wrap tightly around the calf. According to the setting range of parameters， different gradients are set to explore the pressure difference of samples under different parameters. The knitting parameters are shown in Table 1.

Table 1 Knitting parameters in stocking machine

1.2 Structural parameters of compression stockings

The structural parameters of the samples are mainly measured by stitch density(SD) which is the total number of needle loops in a given area(25 cm2). The figure is obtained by using Y115B mobile pick glass(Jie Lian Instrument Co. Ltd.， Wenzhou, China). The number of courses in 5 cm and the number of wales in 5 cm are counted by the same method， then the SD is figured by multiplying the number of courses by the number of wales.

Table 2 Details of compression stockings

1.3 Pressure test

Each sample was laid flat for 20 h at normal temperature(about 20 ℃)， and then laid for 4 h according to FZ/T 73031-2009 in standard laboratory conditions((20±2) ℃ and (65±4)% humidity). The measurement of garment pressure was taken by use of Flexi force A201 sensors. The sensor consists of two layers of polyester film. Silver conductor is laid between the polyester films， and apply a special layer of pressure-sensitive semiconductor material. They have many advantages such as thin (0.203 mm)， lightweight (203 mm)， accurate(±3%). The sensor is lightweight and flexible， and it can measure the pressure between various contact surfaces and is suitable for various test environments. The range of the sensor is 1 pound and the unit of system output value is Newton. According to Formula(1)， the value unit is converted to Pa. The whole equipment consists of flexi force sensors， bluetooth receiver and a computer screen display software.

(1)

wherePrepresents pressure(Pa)，F(xiàn)represents force(N)， andSrepresents pressure area(m2).

1.3.1Participants

The subjects did not exercise intensely the day before the experiment and there was no lactic acid accumulation in the legs to ensure that the leg muscles remained in normal condition. Before experiment， the subjects wearing flat shoes and standing naturally determined the ankle circumference (B)， the muscle transition of achilles tendon and leg (B1)， the maximum circumference of the calf (C)， the position of the first tibia protuberance (D)， and measured the circumference of each part. During the experiment， the subjects wore various numbers of pressure stockings to the natural standing state for that the stockings were not stretched too loose or too tight. In order to place the sensor， the pressure of the compression stockings was measured from welt to ankle. First， test the pressure value of the tibia， next the maximum circumference of the calf， then the change of achilles tendon and calf muscle， and finally the ankle position. After each sensor was placed， the stockings were lifted to the natural state， and the stockings were kept in the natural state when each part was measured. After the sensor was stabilized， operator started to record for 5 s. For all the samples with CP are 350， the loop length was too short to coverDposition with the length， so the pressure of the other three positions was compared.

Five healthy female college students voluntarily participated in the study. An anthropometric criterion was presented in Fig. 1. The smallest position of the calf was located to identify the ankle(positionB) and mark it. When the calf exerts， there is a clear dividing line between the muscle and the achilles tendon， thus determining the positionB1. Keep calf relaxed when testing pressure value. Use visual method combined with soft ruler to find the maximum position of leg circumference， so as to determine positionC. The joint of the tibia with the knee is swollen and has obvious darts， thus determining the positionDand marking it. The sizes of the five subjects are similar， and the mean values are shown in Table 3.

Fig. 1 Test positions of model legs

PositionB/cmB1/cmC/cmD/cmSubjects20.68±0.9728.09±0.9933.62±1.6931.87±1.44Model leg23.6127.8332.5032.03

1.3.2Modelleg

Based on the human leg test data， we designed a model leg with similar size. The respective circumferences fromBtoDare 23.6， 27.8, 32.5 and 32 cm. The leg was made from hard material evenly， with a thin layer of fabric. We enlisted volunteers whose calf circumferences was similar to the model. The leg was fixed on the iron frame， and the compression stockings were worn on the model leg evenly. The method of measuring the pressure value was the same as above.

1.3.3Tensiletest

The first two experiments are generally direct measurements which are in common adopted. Many scholars also use Laplace’s equation to calculate the theoretical pressure value. On the compression stockings， knitting samples of 5 cm width was taken at four respective positions for stretching. The stretching quantity corresponded to the stretching force. The stretching force was converted into the tension of the fabric. The leg circumference was considered as a circle， and the shape variable of compression stockings was converted to the radius of curvature. Lastly， calculated the pressure value using Formula(2).

(2)

wherePrepresents pressure(Pa)；Trepresents tensile force(cN/cm)；Rrepresents curvature radius(cm).

The pressure on the leg depended not only on the tension， but also on the curvature radius of the leg. In order to explore the pressure was produced by tensile deformation of specimen. Based on the application of Formula(2) in garment pressure， the tensile strength test was carried out， and the corresponding pressure was obtained by measuring the tensile forceT. The tensile test was carried out under the condition of (20±2)℃ at room temperature and(65 ±3)% humidity. The data are as follows.

2 Results and Discussion

2.1 Relationship between parameters

The relationship between SD and processing parameters was carried out in two steps：(1) partial correlation analysis between processing parameters and SD；(2) linear-regression analysis for describing the relationship between them.

Firstly， partial correlation test was done using SPSS 21.0 software to determine the correlation significance of variables(Table 4). Secondly， the linear-regression analysis was used for searching the relationship among CP， EMP and SD. Significance levels and the interaction between processing parameters are analyzed and shown in Table 5. In addition， the expected cum prob chart is shown in Fig. 2.

Table 4 Correlations among CP， EMP and SD

Table 5 Effects of CP and EMP on SD

Note:*Means statistically significant;Frepresents value inF-test;Prepresents probability

Fig. 2 Normal P-P plot of regression standardized residual dependent variables

By Using SPSS 21.0 analysis software， we can get the correlation and relationship among processing parameters(including CP and EMP) and SD. In these compression stockings， there is a significant correlation between them(p<0.05). There is Formula(3)at the welt and Formula(4)at the ankle obtained according to the same method.

Welt： SD=-2.636×CP-1.575×EMP+7 454 (R=0.968),

(3)

Ankle： SD=-3.449×CP+7 225.556

(R=0.861).

(4)

2.2 Pressure data of subjects

Pressure data of all parts of the legs of five subjects wearing compression stockings were collected， as shown in Fig. 3.

(a) Pressure comparison under CP of 350

(c) Pressure comparison under CP of 650

In Fig. 3(a)， the CP of the three sets of samples is 350， and the samples with EMP of 2 000-650 r/min， 1 500-650 r/min， and 1 000-650 r/min correspond to the sample number 1, 4 and 7 respectively， and the pressure value fluctuated from 2 kPa to 5 kPa. The reduction of the EMP differentials produced a greater pressure， with the pressure of the compression stocking of sample 7 being the highest， the pressure produced by sample 4 being second， and the pressure produced by sample 1 being the least. Figures 3(b) and 3(c) had the same rules. Sample 1 gradually showed an increasing trend of pressure values fromCtoB， and the pressure value atBhad the maximum pressure value， which was consistent with the gradient distribution of the compression stockings. The pressure values of samples 4 and 7 atB1 toBalso showed the same trend， and the pressure generated atCwas significantly greater than that pressure atB1， because the stretching of the two stockings atCproduced a larger pressure. The smaller the CP of the stocking， the smaller the stitch length； and the smaller the degree of stretch ability when subjected to an external force， the greater the pressure generated.Ctest position was the largest part of the calf circumference， the compression stocking stitch structure was tight， and the tensile strength was the largest atC， so the pressure generated was the largest. At B and B1 positions， the compression stockings had a smaller deformation and a lower pressure.

In Fig. 3(b)， the SD of the three sets of samples is 500， and the samples with the EMP of 2 000-650 r/min， 1 500-650 r/min， and 1 000-650 r/min correspond to the sample number 2, 5, 8. Respectively， the range of pressure value is between 1.0 kPa and 3.5 kPa. Compared with Fig.3(a)， the overall pressure value is smaller， and the pressure order from large to small of the sample is 8, 5, 2. FromDtoB， there is a tendency for increasing pressure， and the pressure at B is the greatest.

In Fig. 3(c)， the SD of the three sets of samples is 650， and the samples with the EMP of 2 000-650 r/min， 1 500-650 r/min， and 1 000-650 r/min correspond to the sample number 3, 6, 9. Respectively， the range of pressure value is between 0.5 kPa and 3.0 kPa. Compared with Figs. 3(a) and 3(b)， the overall pressure value is the smallest， because the three pressure stockings have larger CP and the longest stitch length. When stretching， there is enough length among the stitches for stretching. In the case where the EMP are the same， the stitch can be stretched over a wide range， so that the pressure generated under the same stretching is minimized.

In general， the achilles tendon and bone at the ankleBare more prominent， and the fat and muscle content are less. According to the Laplace’s equation， under the same stretching， the smaller the radius of curvature， the greater the pressure generated. In the experiment， the experimental subjects have a uniform leg， the ankle is full of elliptical shape， the leg muscles are relatively strong， and the curvature of the achilles tendon and calf muscles is small due to its unique structure. In addition， the pressure distribution of the compression stocking is related to the design parameters and the muscle distribution of the human calf. Subjects with more developed leg muscles exert greater pressure on wearing compression stockings.

Comparing the three figures in Fig. 3， in the case of the same CP， the smaller the EMP， the smaller the length of the elastic fed； the smaller the stretchability and the greater the pressure generated. When the CP is 650， the influence of the stitch on the pressure is the smallest， and the change of the EMP shows that more obvious gradient about the pressure value changes， and the error range of the pressure value is also the smallest. Under the same EMP， the smaller the CP， the greater the pressure generated by the fabric. Because the smaller the CP is， the smaller the stitch length is， and the more tight stitch closeness of the fabric， the smaller the stretchable amount between the fabric stitches， and the greater the stretching force.

2.3 Pressure data of model leg

Pressure data of all parts of the model leg wearing compression stockings were collected， as shown in Fig. 4.

(a) Pressure comparison under CP of 350

(b) Pressure comparison under CP of 500

(c) Pressure comparison under CP of 650

Different parameters correspond to the sample number are the same as participants’ leg. The pressure value measured on the leg model is larger than that of the human leg. That is because the surface layer of the human body is muscle or fat， soft and has a certain viscoelasticity. The deformation of the compression stocking is smaller than that of the model leg. The value atBon the model leg is much larger than the test value on the human leg because the jaw size of the model leg is larger than the human leg.

In Fig. 4(a)， sample 1 produced a smaller pressure gradient at different locations， with a minimum pressure of about 3 kPa atCand a pressure of about 6 kPa atB. Sample 4 produced a large pressure gradient at different locations， with a pressure atCabout 3 kPa and a pressure of about 9 kPa atB. The pressure of sample 7 is much larger than that of samples 1 and 4， and the pressure range is between 8.7 kPa and 12.5 kPa. That is because the CP and the EMP in sample 7 are the smallest at the same time， the elongation between the fabric stitches is poor， and the tensile force is larger. In Fig. 4(b)， the samples number 2, 5, 8 have the same pressure distribution trend and gradual increasing pressure from welt to ankle， with the lowest pressure atDand the maximum pressure atB. It can be seen that the change in the EMP causes a change in the pressure distribution. In Fig. 4(c)， the three samples have the same pressure distribution trend that the pressure is increasing fromDtoB. It can be seen that the samples with EMP of 2 000-650 r/min and 1 500-650 r/min have a lower fabric density and a more uniform pressure gradient distribution. While the EMP of 1 000-650 r/min has higher density and larger tensile force. More sensitive to dimensionally induced deformation.

In horizontal comparison， the smaller the CP， the greater the density of the fabric； and the greater the tensile force， the greater the pressure generated under the same tensile condition.

There are two differences between the experiment of human leg and model leg. One is the different stretch size of the compression stockings， the other is the leg material. Compared with Figs.3 and 4， direct measurement was used in both experiments. Both have the same pressure distribution trend， increasing pressure from the welt to the ankle. The maximum pressure is at the ankle. Compression stockings with EMP of 1 000-650 r/min produced the maximum pressure value. However， the pressure on the human leg is obviously less than that pressure on the model leg. Because the human leg has soft tissue such as skin and muscle， which can relieve the pressure of the compression stockings to some extent. And the model leg material is hard， so the pressure value is larger.

2.4 Laplace’s equation

The shape variables and corresponding tension of compression stockings in the tensile test are shown in Tables 6 and 7.

The pressure value is calculated according to Formula(2)， and is plotted to obtain Figs. 5 and 6.

Table 6 Average data of the deformation and tension for five subjects

Table 7 Average data of the deformation and tension for model leg

(a) Pressure comparison under CP of 350

(b) Pressure comparison under CP of 500

(c) Pressure comparison under CP of 650

(a) Pressure comparison under CP of 350

(b) Pressure comparison under CP of 500

(c) Pressure comparison under CP of 650

Figure 5 shows the average pressure value of different parts of the five subjects calculated by the Laplace’s equation. Figure 6 shows the pressure value of different parts of the model leg calculated by the Laplace’s equation. The two figures have the common characteristics， namely the compression stockings. The pressure value atDandBare large， and the pressure value atCandB1 are small. The pressure value in overall situation decreases as the CP increases， that is because the stockings whose CP is set to 350 are close-knit and have much stretch. The effect of tensile force on pressure is greater than that radius of the model leg.

In Fig. 5， the CP are 350， 500 and 650， respectively， and the pressure value range from 5.0 kPa to 8.0 kPa， 3.2 kPa to 5.5 kPa， 2.8 kPa to 4.5 kPa， which are much larger than the human leg pressure value. In Fig. 6， the CP are 350， 500 and 650， respectively， and the pressure value range from 5.0 kPa to 8.0 kPa， 3.3 kPa to 5.0 kPa， 2.4 kPa to 4.0 kPa， which are quite different from the model leg pressure value. Comparing with Fig. 6， the overall difference of Fig. 5 is small and has a strong similarity. After the knitted fabric is stretched， the variation of tensile force corresponding to the tensile deformation is very small due to its inherent characteristics. The morphological variables of the four parts tested in the experiment are the smallest atBand the largest atC. Due to the gradient design of EMP， the variation of tensile force in each leg of the hose is not synchronous with that of the shape variable. After dividing by Formual (2)， the pressure value increases and is larger than the direct measurement.

There is a big difference between the stretching of human leg and model leg. Compared Fig. 3 with Fig. 5， the pressure value calculated in Fig.5 is much larger than the actual measured value， and the pressure distribution trend is different. The comparison results in Fig. 4 and Fig. 6 show similar results. In direct measurement， the variation of the pressure value of the compression stockings is related to the deformation of the stocking tube and the subcutaneous tissue. Whereas the theoretical calculation method is mainly related to the performance of the fabric.

Not only the pressure value but also the pressure distribution is different. Overall， the pressure variation ranges among Figs. 3-6 decrease as the CP increase. Because the smaller CP of the compression stockings， the denser the fabric， the greater the tension of the stitches in the unit sizes during stretching， and the greater pressure generated. In addition， Figs. 5 and 6 have the maximum pressure atDunder the EMP of 2 000-650 r/min. The larger the EMP， the slower the feeding speed of the elastic； and the shorter length woven into the unit time， the greater the deformation of the rubber per unit length during stretching， which result in greater pressure.

Comparing Fig. 4 with Fig. 6， the maximum value in Fig. 6 is significantly smaller than that value in Fig. 4. Because Fig. 4 is a hard model leg， the sensor has contact surface properties， and Fig. 6 is only a theoretical calculation， so there are big differences.

3 Conclusions

This paper studies the weaving of compression stockings and the pressure changes between different test objects. The relationship between weaving parameters and finished product attributes of compression stockings can be known by statistical and analysis. The EMP is slow， the length of elastic is less, and there are more coils per unit length in compression stockings. When the rubber is stretched， the stockings wrap tightly around the calf.

The number of wales is directly affected by CP. As the CP is set to 300， the number of wales are more than 650. The number of courses is directly affected by EMP. As the EMP is set to 2 000 r/min， the number of courses is less than 1 000. The number of courses(ankle) is directly affected by EMP(at the ankle). The EMPs at the ankle are the same， and the number of courses is similar. There is significant correlation between processing parameters and SD.

The comparison among pressure value of the compression stockings obtained by three methods (the human leg， the model leg and the Laplace’s equation)， has great differences in tests. Laplace’s equation has great advantages in theoretical calculations， simple and convenient， but its value does not represent the true pressure value when the human leg wears compression stockings. The model leg has general advantages in testing， but due to the limitation of the material， the measured pressure value is also large， and it can not accurately represent the pressure value of the human leg dress. If the compression stockings work equally well in different groups of people， it is necessary to fully consider the differences among people. The pressure design of the compression stockings should be based on the actual wearing of the pressure. If necessary， it is best to customize compression stockings.