Skin biomechanical properties and leg volume in aging healthy adults

Abstract Background In adults ageing is accompanied by changes in limb volume and skin biomechanical properties. Objectives To explore the relationship between body size, aging, skin biomechanics, and leg volume, V Leg and to define predictive equations linking leg volume with these properties. Methods Sixty‐two healthy adults (Age 18–80 years, M:F 45:55) were recruited. Anthropomorphic measures were recorded along with V Leg (via circumferential method) and skin tone, stiffness, and elastance (via tonometry). Regression analysis was performed to define relationship between the measured parameters. Results In healthy adults bilateral V Leg were the same regardless of leg or sex, 5791 ± 1363 for females and 6151 ± 1203 mls for males. V Leg was positively correlated to body weight, where V Leg (mls) = 1058 + 69 × Wt (kg) for females and V Leg (mls) = 539 + 65 × Wt (Kg) of body weight for males. Skin surface biomechanical properties varied with sex, leg volume, and location on the leg with the malleolus exhibiting the stiffest surface. Conclusion The study shows that anthropometric measures change with sex and leg size are multifactorial and body weight, sex, and skin condition as important determinant factors of leg volume.


INTRODUCTION
Often anthropomorphic studies of healthy adult legs measure limb muscle mass, adipose tissue content, and limb circumference but rarely address the issue of limb volume or the skin covering the limb. The ability of the leg to increase in volume is dependent on the skin's ability to stretch. In studies that have investigated limb volume various methods have been used and do not provide predictive equations related to body size or mass. The earliest measures of limb volume were achieved by simply measuring the water displaced following immersion of the leg or even the whole body in a water filled vessel. 1  sophisticated methods were developed, such as water-displacement plethysmography, which was used to measure small changes in leg volume that are associated with altered blood supply to the limb. 2 In terms of assessing limb muscle mass alone, the circumference of the leg muscle at set anatomical points proved sufficient to provide a good estimate. The evolution of ever more sophisticated technology now allows muscle mass (and limb volume) to be measured using electrical impedance, or with digital imaging technology such as X-ray and Dual-energy X-ray absorptiometry (DEXA) scanning, with magnetic resonance imaging (MRI) and ultrasound scanning being the most recent techniques pioneered. 3 In the healthy leg, bone and muscle The aim of this study was to establish the anthropometric relationships between skin biomechanics and limb volume in healthy adult males and females of differing age. Further aims were to derive predictive equations of the relationship between volume and body properties and to compare limb properties between the distal and proximal leg. All participants while seated underwent bilateral leg volume assessment using the standard tape measure method used in lymphoedema studies. 5 Measurements were made utilizing a metric nonstretch tape measure (Medi Ltd, Germany). Leg circumference was measured at several points starting just above the malleolus and then at 4 cm intervals ascending the leg to the knee (the distal leg), and then from above the knee up to the upper thigh (the proximal leg). 5,6 A portable hand-held myotonometer (Myoton AS, Tallinn, Estonia) was used to quantitatively assess the skins mechanical and viscoelastic properties. 9 This palpation device uses a preloaded probe to briefly compress the skin and then follows the decay of the natural oscillatory response to the applied load. 10 The myotonometer provides three measures, two mechanical properties, tone (Hz) which reflects the oscillation frequency characteristics of the skin in its passive state, and stiffness (N/m), or dynamic stiffness a measure of the skins resistance to the applied external force (load) deforming the skin from its initial shape. An increase in these parameters signifies an increase in the tone and stiffness of the skin, an undesirable characteristic of skin. 11 The third property is elasticity a measure of the skin's ability to recover its initial shape after the removal of an applied load, the device records the skin's speed of movement (acceleration) at the point of maximum depression, this parameter is termed the logarithmic decrement, and the measure is inversely related to elasticity, thus a decrease in the logarithmic decrement parameter reflects an increase in elasticity. 12 The myotonometer was applied to four anatomical sites on each leg; immediately above the malleolus, the mid-calf (akin to the distal portion of the leg), the knee, and finally the mid-thigh, the proximal portion of the leg, resulting in eight measures per person ( Figure 1).

Statistical analyses
Descriptive statistics were performed to show means and standard deviations in the participant population, leg volume and skin

Body composition
Females (n = 34) had a lower body weight (on average 16 kg less) and were smaller (on average 0.13 M less) than the male participants (n = 28) ( Table 2). Ratio measurements such as the BMI and the Waist to Hip (W-H) ratio were no different between the sexes ( Table 2). The female leg length was shorter by a mean of 6 cm, while the leg volumes (the average of the left and right leg volume) did not differ significantly at around 6053 ± 1286 mls ( Table 2). While the volume of the distal region was larger than the volume of the proximal region volume there were no sex differences (Table 2). There were no differences between the right leg volume (F:M 5987 ± 1436 ml: 6188 ± 1183 ml, p = 0.556) and left leg (F:M 5956 ± 1308 ml: 6114 ± 1227 ml, p = 0.629) volumes between the sexes.
The leg volume increased with body weight (Figure 2 and Table 3).
In males there were also relationships between leg volume with age and height, with volume decreasing with age and increasing with height (Table 3). Forward stepwise regression analysis including weight, height, and age showed that in both sexes weight alone was sufficient to predict leg volume (p < 0.001).

Skin mechanical properties
The skins mechanical properties tone and stiffness measured at the four anatomical points ascending each leg were not related to the participant's age, weight, or height. The only exception being the relationship between body weight and skin stiffness at the knee (stiffness ((N/m) = 92.04 − 0.06 × weight, p = 0.02). Skin elasticity decreased with age at three anatomical sites, but no relationship existed between body weight or height (Table 4, Figure 3).  In males the three mechanical properties at the knee correlated with age, height, and weight in most comparisons (exceptions being elastance with weight and height) ( Table 5).

Relationship between limb volume and skin viscoelastic properties
In both sexes limb volume and skin biomechanics were correlated.

DISCUSSION
This is the first study to report the relationship between lower limb volume and skin biomechanical properties measured by the myotonometer. It is important in that it shows that these skin properties change with volume and that myotonometry is a useful tool in this assessment.
The study population is representative of the general UK population, matching the national mean heights and weight of 1.62 ± 0.14 and 1.76 ± 0.17 meters (mean and SE shown) and 72.1 ± 034 and 85.4   18 The current study shows that regardless of sex, leg volume was positively related to body weight (Table 4), increasing by around 69 for females and 65 for males with every kg increase in body weight (Table 4). The increase in volume in this study is more likely due to additional adiposity rather than muscle (as in elite athletes) or fluid (as found in those with lymphoedema). 3,19 In males, the increase in volume was related to height, with those of greater height having the larger volumes; whether this was due to longer legs could not be determined. A loss of male leg volume with age is most likely due to general loss of muscle mass; this condition is known as sarcopenia and is characterized by the degenerative loss of skeletal muscle mass, quality, and strength in older persons. 20 The viscoelastic properties of skin differs between anatomical sites and the sexes ( Table 4). The skin at the malleolus had a higher tone, stiffness, and elastance than the other three sites, in both sexes. Female skin exhibited a lower tone, stiffness, and elastance than male skin.
Other studies using the myotonometer have shown similar differences with anatomical sites, such as the shin and the calf, being 657 and 380 N/M, respectively. 21 In females the skin elastance also increased with age, becoming softer ( Figure 3). 11,12 Skin viscoelastic properties at the four anatomical sites are associated with leg volume at the malleolus and knee, with the tone decreasing with increasing volume (Figure 4). Only in females was skin tone altered in the proximal region as well. Skin stiffness was also related to volume at the knee site in both sexes; although only in the females was there a relationship between the proximal site and volume ( Figure 5).
At no sites was the distal skin parameters related to volume. This is the first study to report the relationship between lower limb volume and skin biomechanical properties measured by the myotonometer. A larger sample population would allow a closer inspection of the relationships between leg volume and body characteristics, as aged-matched, height-matched, and weight-matched participants could then be compared.

CONCLUSIONS
In adults, there is a positive correlation between body size and leg volume. This relationship is further influenced by age and sex. Leg volume alone is independent of sex but is best predicted by body weight, volume increasing with body weight. Skin biomechanical properties (such as tone, stiffness, and elastance) are influenced by sex and leg volume.
Different regions of the leg exhibit different skin properties according F I G U R E 5 Relationship between skin stiffness (N/m) and leg volume (mls