A study on skin mobility according to joint movement: Variations in mobility according to joint motion range and correlation and influence with hydrica composition

Abstract

Background

Skin structures arranged in an advantageous structure for skin stretching to facilitate movement of the human body, and have structural functions to help the movement of the joints by changing the position of the skin, such as the stretch that occurs incidentally. Proper movement of the skin is required to be efficient owing to the nature of the skin that covers the entire human body with a single connected tissue layer.

Aims

The purpose of this study was to quantify the skin mobility that occurs during joint motion and to identify the correlation and influence with hydrica composition.

Materials & Methods

The subjects of this study were healthy people in their 20s–50s (20 male, 20 female), The movement of the skin marker attached to the skin was measured using X‐ray, and the hydrica composition was measured using Inbody S10.

Results

Experiments showed that the skin on the side at which the joint bends and wrinkles form moved away from the moving joint, while the skin on the side where the wrinkles spread out moved toward the moving joint. As the range of joint motion increases, the skin became more mobile (OR: 18.95 ± 5.91 mm, MR: 34.09 ± 7.87 mm, IR: 51.14 ± 8.73 mm, FF: 78.76 ± 12.24) (p < 0.05). As a result of regression analysis between the total amount of skin mobility and the factors of hydrica composition, it was found that the ABW (arm body water) affected skin mobility as B = 7.430 (p < 0.05, adjusted R ² = 0.119).

Conclusion

Based on the results of this study, it was revealed that directional movement of the skin appeared according to joint movement, and it was affected by body water.

1. INTRODUCTION

The skin has various functions, such as protection of internal organs, serves as a sensory organ, provides moisture maintenance, and achieves body temperature control. It plays an important role as a primary barrier to the external environment as well as part of the immune mechanism induced to prevent ultraviolet penetration and protect the human body from pathogenic microorganisms. The skin also contains the largest number of sensory receptors in the human body and is responsible for the transmission of a variety of sensory information. In addition, if there is a difference in temperature, it can control it through the expansion or contraction of capillaries and discharge waste through sweat. ¹ Wrinkle structures, such as sulcus cutis and crista cutis, are also arranged in an advantageous structure for skin stretching to facilitate movement of the human body and have structural functions to help the movement of the joints by changing the position of the skin, such as the stretch that occurs incidentally. ² Therefore, considering that the segmentation of the human body moves on various axes and faces, proper movement of the skin is required to be efficient owing to the nature of the skin that covers the entire human body with a single connected tissue layer.

Prior studies related to skin movement were mainly conducted on skin errors to increase the measurement reliability of three‐dimensional (3D) motion analyzers. ³ , ⁴ , ⁵ These studies aimed to measure the positional error of the skin markers of the 3D motion analyzer caused by the stretching or pushing of the skin during movement. Measured information is used as a proof of modification to improve the accuracy of measurement of motion analyzers.

A prior study on the movement of the skin according to joint movements showed that the skin moves with regularity. This finding was confirmed based on movement comparisons of the skin markers on the thigh, which faced each other, while the pelvis moved horizontally using a 3D motion analyzer. Furthermore, ultrasonic diagnostic devices were also used to observe the skin and underlying tissue gliding on superficial fascia. ² In addition, the possibility of treatment using skin movement was suggested based on a case study in which the skin increased the range of motion in patients with axillary web syndrome (using regular skin movements). ⁶ These results can be explained as evidence to support the skin's movement during human movements.

However, prior studies have only measured the relative direction of movement between skin markers, not the absolute value measured at the reference point for the movement of the skin that appeared with the body movement. ² , ⁷ No studies have actually measured bone location and skin movement in a visualized way, nor have any studies demonstrated differences in body composition that affect skin volume. To clearly present the movement of the skin, it is also necessary to present quantitative movement values and analyze the correlation and influence of individual body composition.

Therefore, the purpose of this study was to determine the direction and quantify the amount of skin movement during elbow bending using a digital radiography system, identify correlation and influences with hydrica composition, and present basic data on skin mobility. Based on this, we would also like to provide clinical basic data for a new therapeutic exercise intervention program for people who experience limited range of motion owing to severe pain and mobility in the skin system.

2. METHODS

2.1. Participants

This study was approved by the Kyungsung University Institutional Review Board (IRB approval number: KSU‐20‐04‐005). The study adhered to the principles of the Declaration of Helsinki. Informed consent was provided by all participants.

The subjects of this study were healthy people in their 20−50s (20 male, 20 female), who had no history of disease or damage to the musculoskeletal, nervous, or skin systems of the dominant arm. If there was scar tissue on the skin between the joint being measured, the adjacent joint or the range of motion of the joint was reduced by more than 20% from the normal range, or did not meet the minimum function required for the experiment, the subject was excluded. The number of subjects for regression analysis for a drop rate of 10%, an effect size of 0.35, a significance level (α) of 0.05, and a power level of 80%, was equal to 40.

2.2. Design and procedures

For the experiment, the subjects were immediately laid down in a stable state after 15 min of rest and tested for body components (Inbody S10, Inbody Co., City, Korea). The subject was measured for body water after resting for 15 min in the supine position. Eight electrodes were placed on eight tactile points (thumbs, middle fingers, and ankles of both hands and feet, respectively) to perform the multisegmental frequency analysis. Next, a total of 30 impedance measurements were obtained at frequencies 1, 5, 50, 250, 500, and 1000 kHz, respectively, from different segments of the body, such as the right and left arms, trunk, and right and left legs, respectively. The subjects measured the lengths of the upper and lower arms and attached metal markers. From acromion to medial and lateral epicondyls of elbow joint was defined as upper arm, and the length was measured. In addition, from the midpoint of the styloid processes of radius and ulnar to the midpoint of the medial and lateral epicondyles of elbow joint was defined as the lower arm, and the length was measured. Skin markers were attached to the ventral and dorsal of each 1/3, 2/3 point of the upper arm and the lower arm. X‐rays were acquired from regions proximal to the elbow joint at full extension, outer range (1/4 bending), intermediate range (1/2 bending), inner range (3/4 bending), and full flexion positions. The C‐PACs program was used to analyze the movement distances of skin markers. The analysis reliability of the C‐PACs program was Cronbach α = 0.980, intraclass correlation = 0.960 (95% confidence interval 0.840−0.990) (p < 0.001). The positions of the eight markers calculated from the full flexion, 1/4, 1/2, 3/4, and the full flexion angle were recorded, and the movement distance was calculated based on the starting position (Figure 1).

FIGURE 1.

C‐PACs program.

2.3. Statistical analysis

The movement direction and total movement distance of each skin marker were presented as descriptive statistics, and the total movement distance of each skin marker according to the joint movement range of the forearm joint was compared by one‐way analysis of variance tests; postanalysis was performed by the Tukey's method. Multiple linear regression analysis was performed to verify the correlation and influence between body composition and skin movement distance. The statistics program used SPSS (version 22.0, IBM SPSS Inc., City, State, USA), and the significance level (α) in all statistical methods was set to 0.05.

3. RESULTS

3.1. General characteristics of subjects

The dominant arm of all subjects was on the right side, and the general characteristics of the subjects are as follows (Table 1).

TABLE 1.

General characteristics of subjects.

Contents	Subjects (n = 40)
Gender	Male = 20, Female = 20
Age (yrs)	† 37.85 ± 12.30
Height (cm)	166.89 ± 9.29
Weight (kg)	64.00 ± 11.68
TBW (ℓ)	35.38 ± 7.51
ABW (ℓ)	1.91 ± 0.57

Abbreviations: ABW, arm body water; TBW, total body water.

^† Mean ± SD.

3.2. Differences between male and female groups

In this study, there was a statistically significant difference in general characteristics and hydrica composition excluding age between male and female groups(p < 0.05), but no significant difference was found in skin movement direction, skin mobility according to joint movement, and skin mobility according to hydrica composition(p > 0.05). Therefore, the experimental results were analyzed as a group of normal adults.

3.3. Direction of movement and skin marker distance

The direction and distance of movement of each skin marker are presented using descriptive statistics. The initial positions of skin markers were defined as a negative (−) and positive (+) signs by setting the axis of the elbow joint as zero, the anatomically close side (proximal) was expressed using a positive sign, and the distant side was expressed by a negative sign. The movement direction and movement distance of the skin were presented as the total movement amount of each skin marker (Table 2, Figure 2).

TABLE 2.

Direction and movement distance of skin markers.

Marker	Total distance (n = 40)
LFv	† −13.70 ± 9.40
LNv	−56.38 ± 13.94
UNv	29.17 ± 9.83
UFv	5.51 ± 9.18
LFd	.91 ± 11.88
LNd	13.81 ± 9.19
Und	−20.79 ± 11.75
UFd	−24.94 ± 13.68

Abbreviations: LFd, lower, far, dorsal marker; LFv, lower, far, ventral marker; LNd, lower, near, dorsal marker; LNv, lower, near, ventral marker; UFd, upper, far, dorsal marker; UFv, upper, far, ventral marker; UNd, upper, near, dorsal marker; UNv, upper, near, ventral marker.

^† Mean ± SD.

FIGURE 2.

Location and direction of skin markers.

3.4. Skin mobility according to the range of joint motion

The differences in the total distance moved by the skin according to the movement range of the elbow joint were 18.95 ± 5.91 mm in the outer range, 34.09 ± 7.87 mm in the middle range, 51.14 ± 8.73 mm in the inner range, and 78.76 ± 12.24 mm in the full flexion. These differences were significantly different (p < 0.05). Based on postanalysis, skin movements for each joint motion range were compared, and there were significant differences in all ranges (p < 0.05) (Table 3). In particular, the rate of increase in skin movement tended to be high at the maximum joint bending position (Figure 3).

TABLE 3.

Skin mobility according to joint range of motion during elbow flexion (unit: mm).

Range of motion	Distance	F	p‐Value
OR (a)	† 18.95 ± 5.91bcd	325.94	0.00*
MR (b)	34.09 ± 7.87acd
IR (c)	51.14 ± 8.73abd
FF (d)	78.76 ± 12.24abc

Abbreviations: abcd, p < 0.05 between each variable; FF, full flexion; IR, inner range (3/4 range of FF); MR, mid range (1/2 range of FF);

OR, out range (1/4 range of FF).

^† Mean ± SD.

^* p < 0.05.

FIGURE 3.

Skin mobility according elbow flexion.

3.5. Correlation and influence between skin mobility and hydrica composition

Multiple linear regression analysis was performed to confirm the correlation and influence between skin mobility and body composition. For the analysis, the subject's characteristics and body composition indicators were set as independent variables, and the total skin movement distance was set as the dependent variable. In the analysis process, variables, which were not suitable for the regression equation, were excluded, and the arm body water was used for the analysis. As a result of the analysis, it was determined that the regression model, which yielded F = 5.111 (p < 0.05), was suitable. In addition, 11.9% of the explanatory power was reflected by the adjusted R ² = 0.119. Arm body water was B = 7.430 (p < 0.05), and it was affecting skin mobility. Given that the sign of B was positive (+), it can be said that the total distance moved by the skin increased by 7.430 when the arm body water content increased by 1 (Table 4). This may be expressed by the regression equation of Y = 64.569 + 7.430X ¹ (X¹: Arm body water).

TABLE 4.

Results of multiple linear regression analysis.

	UC	SC
Variable	B	SE	β	t(p)	TOL	VIF
(Constant)	64.569	6.540		9.873***
ABW	7.430	3.286	.344	2.261*	1.000	1.000
F(p)	5.111*
adj. R2	0.119
Durbin‐Watson	2.257

Abbreviations: ABW, arm body water(X₁); SC, standardized coefficients; TOL, tolerance; UC, unstandardized coefficients; VIF, variance inflation factor.

^* p < 0.05.

^** p < 0.01.

^*** p < 0.001.

4. DISCUSSION

Aging of the skin is affected by external factors, such as sunlight, and by internal factors, such as natural, physiological, and chronic aging. In general, aging caused by force majeure and genetic factors is defined as the uniqueness of the aging process, and is caused by changes in the endocrine system, chronic diseases, and the law of gravity. ⁸ In women, reduced estrogen causes skin changes, such as increased wrinkles. ⁹ However, the purpose of this study was to present general skin mobility in normal adults. For this reason, the study was designed to have the same men and women proportions, and were distributed evenly in their 20−50s. The yielded low‐measurement errors, high repeatability, and high‐measurement consistency. ¹⁰ Based on these findings, it was determined that the method used to verify the mobility of the skin in this study was sufficiently reasonable as a measurement tool to achieve the research objective. According to the conducted experiments, skin movement was shown to move away from the axis of the joint from the side at which skin wrinkles were formed when the forearm joint was bent. Conversely, the side at which the skin wrinkles spread, appeared to be closer to the axis of the joint. In the previous study, the skin movement during flexion and extension of the shoulder joint was confirmed using the skin reflection marker of the motion analyzer. At this time, it was revealed that the skin on the side at which wrinkles were formed moved away from each other around the joints when the joints are flexion. Conversely, the skin on the side where wrinkles were spread gathered toward the joints. ¹¹ In addition, the same method of study was used to experiment with skin movement in the legs that occurred during pelvic movement, and similar results were obtained. ² These findings showed regularity and reported this as the directional law of skin movement that occurs during motion. ¹² Skin movement in this study was also consistent with the regularity presented in previous studies. It was determined that during joint flexion, skin gathered or pulled beyond its viscoelastic limits, and that some type of active and passive insufficient occurred, which resulted in passive movement.

The human skin is composed of several layers of nonlinear and unequal viscoelasticity. ¹³ Therefore, it is very difficult to accurately identify the mechanical properties of the skin. ¹⁴ However, prior studies of the cross section of the skin microscopically indicated that sulcus cutis and crista cutis can provide more fluidity than the viscoelasticity of the skin itself against external tension. ¹¹ , ¹⁵ Given that sulcus cutis and crista cutis are finite structures, fluidity is bound to decrease as the range of joint exercise increases. Therefore, it is believed that the increase rate of movement distance increases as it goes to the end of the range of joint motion. Eventually, it was determined that at the beginning of the range of joint motion, structural flow of the skin was provided, which resulted in less prominent movement of the skin, and led to increased mobility of the skin as the range increased.

The skin exhibits a combined mechanical response reflected by the elastic solid and viscous fluid responses, commonly and collectively known as its viscoelastic property. ¹⁶ Elasticity at this time means the nature of the shape being restored to its original state by physical external force, and viscosity means the resistance that occurs internally in situations, where the shape is changed by external force. ¹⁷ The mechanical properties of the skin are determined by various factors. It is also determined mainly by extracellular materials, and by fibroreticular structured with collagen fibers and elastic fibers, and is also known to be affected by skin thickness and moisture content. ¹⁸ , ¹⁹ The decrease in water volume is related to the decrease in extracellular substances (e.g., proteoglycans and hyaluronic acid), which reduces skin viscoelasticity. ²⁰ , ²¹ This fact justifies the regression results generated in this study to verify the correlation and influence of skin mobility and body water.

In this study, we used a digital radiography system to identify the correlation between skin mobility and body water in the arm. The skin system is anisotropic and can move in different directions depending on the area, even if the same tension was applied. It is related to the direction of the cleavage line of skin. ²² If tension is applied perpendicular to the cleavage line of skin, there is a considerable skin movement, whereas if it is parallel to the skin cleavage line of skin, there is reduced skin movement. ²³ However, it was impossible to predict the impact of the area, where the experiment was conducted because the skin line does not exhibit specific directionality. It is also believed that owing to the nature of the inspection equipment, the movements from the other planes contributed as errors in the measured values owing to the limitations that could only be measured on a single plane. Nevertheless, in this study, the mobility of the skin was consistent with movement. This means that the mobility caused by joint movements was greater than that given by the anisotropy of the skin. In future studies, it is thought that if the anisotropy of the skin is considered, it could lead to the generation of more advanced data on skin mobility. Despite the high correlation between body water volume and age, there was no correlation between age and skin mobility in the group of subjects evaluated in this study. Although age and body water are correlated, it is thought that the change in age did not significantly affect skin mobility because it includes various changes in body elements. Therefore, if the number of subjects is increased and the width of the age group is widened, outcomes will be more meaningful.

This study demonstrated that the skin has mobility in certain directions depended on the movement of the joints. It also revealed that the mobility of the skin was affected by body water. Therefore, based on these results, it is suggested that intervention considering the direction and characteristics of skin mobility is necessary in postoperative recovery patients, and in patients with burns and edema, especially those with mobility restrictions owing to skin system disorders. ²⁴ , ²⁵ Furthermore, the results of this study are expected to serve as fundamental data for the study of skin mobility owing to changes induced based on the movement of the human body.

Choi SH, Lee BJ, Lee SY. A study on skin mobility according to joint movement: Variations in mobility according to joint motion range and correlation and influence with hydrica composition. Skin Res Technol. 2023;29:e13288. 10.1111/srt.13288

DATA AVAILABILITY STATEMENT

All the data for this study will be made available upon reasonable request to the corresponding author.