Relationship between Lower-Limb Bioelectrical Impedance Indicators of Hydration Status and Muscle Function in Aged Population: A Cross-Sectional Study

Research Article

J Fam Med. 2023; 10(2): 1327.

Relationship between Lower-Limb Bioelectrical Impedance Indicators of Hydration Status and Muscle Function in Aged Population: A Cross-Sectional Study

Mateu Serra-Prat1,2*; Isabel Lorenzo3; Mateu Cabré4; Jessica Martínez5; Elisabet Palomera1; Pau Ferrer6; Emili Burdoy7; Eulogio Pleguezuelos8

1Research Unit, Consorci Sanitari del Maresme, Spain

2Centro de Investigación Biomédica en Red de Enfermedades Hepáticas y Digestivas (CIBEREHD), Instituto de Salud Carlos III, Spain)

3Information Management Unit, Consorci Sanitari del Maresme, Spain

4Department of Internal Medicinem Consorci Sanitari del Maresme, Spain

5Dietetics and Nutritional Unit, Consorci Sanitari del Maresme, Spain

6Tecnocampus Fundation, Spain

7Department of Primary Care, Consorci Sanitari del Maresme, Spain)

8Department of Rehabilitation, Hospital of Mataró, Consorci Sanitari del Maresme, Spain

*Corresponding author: Mateu Serra-Prat Research Unit, Hospital de Mataró, Carretera de Cirera s/n, 08304 Mataró, Barcelona, Spain. Tel: +34 93 741 77 30 Email: [email protected]

Received: March 06, 2023 Accepted: April 15, 2023 Published: April 22, 2023


Background: Reduced Intracellular Water (ICW) has been related to poor muscle strength and functional capacity in aged populations. We aimed to evaluate hydration status and intracellular hydration of lower-limb lean mass in aged population and the relationship with thigh muscle function.

Methods: Design and population: Cross-sectional study of community-dwelling ≥70 year old subjects.

Measurements: Lower-limb hydration status was assessed by bioimpedance analysis and thigh muscle function was assessed by isokinetic evaluation. Frailty was established according to Fried criteria, sarcopenia according to EWGSOP-2 criteria, and physical performance according to the Timed Up-and-Go (TUG) test.

Results: Assessed were 117 individuals (mean age 75.0 years, 50.4% women). Women had lower Fat-Free Mass (FFM), Total Body Water (TBW), and Phase Angle (PhA) values than men. Age was negatively correlated with ICW (r=-0.187; p=0.045), the ICW/FFM ratio (r=-0.192; p=0.039), and PhA (r=-0.311; p<0.001). Extension and flexion strength and power of the knee were positively correlated with Total Water (TW) (L), FFM (kg), and the protein compartment (kg) of the lower-limb in both men and women, but not with TW as percentage of weight, ICW as percentage of TW, and the TW/FFM and ICW/FFM ratios, while the TUG test was negatively correlated with ICW as a percentage of TW.

Conclusion: Knee flexion and extension strength, power, and work increase as thigh FFM, TBW, and protein compartment values increase. No correlation was observed between intracellular hydration and muscle function, but correlation was observed with performance. As plausible hypotheses, further studies are required to test for these correlations.

Keywords: Dehydration; Intracellular water; Muscle function; Aged; Isokinetic test; Bioimpedance analysis


Water is the main component of the human body and an essential nutrient for life and health [1,2]. Water is distributed in the body into intracellular and extracellular compartments and flows from one compartment to the other according to osmotic pressure through transmembrane protein channels called aquaporins. Plasma or extracellular osmolarity must remain between 285-295 mOsm/L of water, as any increase in these values will induce intracellular dehydration, affecting cell structure and function [3]. It has been suggested that cell volume may be a metabolic signal, such that cell swelling favours anabolism and cell contraction, catabolism, and protein degradation [4,5]. Cells have different mechanisms to compensate for plasma hyperosmolarity (>295 mOsm/L) and restore osmotic balance, such as ion transporter mechanisms, osmolyte synthesis, aquaporin gene expression induction, and cytoskeleton rearrangement [3,6]. However, osmotic balance is regulated in the body mainly by arginine vasopressin acting at the renal level, favouring water reabsorption and urinary concentration. Hyperosmotic stress has been related to an inflammatory response [7], diabetes, insulin resistance, metabolic alterations, chronic renal failure, cardiovascular diseases [8], and with muscle function, especially in young adult athletes [9]. There is also growing evidence on the effect of hydration status on muscle function in aged populations.

Starting in adulthood, water content of the body progressively declines, mainly due to a relative reduction in Fat-Free Mass (FFM) and Muscle Mass (MM), but also attributed to chronic low-intensity dehydration in aged populations [10]. Muscle composition changes with ageing, so that a decrease in the ratio Muscle Cell Mass (MCM)/Skeletal Muscle Mass (SMM) and a relative expansion of ECW and extracellular space is observed in advanced ages [11]. Bio-Impedance Analysis (BIA) derived hydration parameters have shown to be useful markers of muscle performance [12]. Some authors have reported that an increased total body Extracellular Water/Intracellular Water (ECW/ICW) ratio is an independent predictor of poor muscle strength and decreased gait speed in the elderly [13-15]. Other authors have suggested that cellular dehydration, as measured by the total body ICW/FFM ratio, is associated with poorer muscle strength and functional capacity and a higher risk of frailty [16]. Moreover, when considering only specific group of muscles, other authors have observed that ICW/total water in the lower limbs is a predictor of dorsiflexion and plantar flexion strength [17]. These results, however, need to be corroborated and confirmed, especially with more accurate and precise measurements of muscle function and for the same specific muscle groups.

The objective of this study, based on the hypothesis that muscle function depends on hydration status, was to evaluate hydration status and intracellular hydration of lower-limb FFM in a community-dwelling aged population and to determine their relationship with thigh muscle function.


Study Design and Population

An observational cross-sectional study was conducted of community-dwelling subjects aged 70 years and older. A random sample was pre-selected from the database of inhabitants ascribed to 3 primary care centres in the XX region (COUNTRY). Pre-selected subjects were invited by telephone to a visit with their primary care physician, who informed them about the study, checked eligibility criteria and, if they agreed to participate, obtained their signed consent. Exclusion criteria included active malignancy, neuromuscular disease, dementia, serious mental illness, life expectancy of less than 6 months, bilateral knee or hip prosthesis use, and in palliative care or institutionalized. Recruitment took place between January and March 2020, but the study was stopped due to the COVID-19 pandemic, so all assessments were postponed to between July and October 2021. The local ethics committee approved the study protocol (code CEIm CSdM 65/19).

Data Collection

Body composition and hydration status was assessed by BIA, a non-invasive, fast, reproducible, validated, and widely accepted method of analysis. BIA involves application of a low-intensity alternating electrical current at different frequencies and measurement of the opposition to the passage of this current by biological tissues.

Impedance reflects 2 vectors: Resistance (R), i.e., opposition to the flow of electrons, is inversely proportional to Total Body Water (TBW), and reactance (Xc), i.e., cell membrane opposition to the passage of electrical current, reflects the integrity of the cell membrane. BIA provides estimates, for the whole body and for each of its 5 segments (the limbs and trunk), of Fat Mass (FM), Fat-Free Mass (FFM), and Muscle Mass (MM) (in kg and as a percentage of body weight), TBW or Total Water of the limb (TW) (in litres and as a percentage of body or limb weight), and Extracellular Water (ECW) and Intracellular Water (ICW) (in litres and as a percentage of TBW). It also provides estimates of the protein compartment (in kg) and the Phase Angle (PhA), which is an indicator of cell membrane functionality and selective permeability.

zll no intense exercise in the previous 24 hours, no alcohol consumption in the previous 8 hours, strict fasting in the previous 2 hours, a toilet visit prior to the evaluation, and no evident clinical signs of dehydration. Used as indicators of right leg hydration and of cellular hydration were, respectively, the percentage of TW with respect to right leg weight and to FFM (TW/FFM), and the percentage of ICW with respect to TW and to FFM (ICW/FFM).

Muscle function was assessed by the isokinetic test (System 4 Pro device from PRIM), which allows objective measurements of muscle strength, work, and power. It allows the muscle to be exercised at constant speed throughout the range of joint movement. The isokinetic evaluation system has 3 elements: a goniometer, which measures the arc of movement; a tachymeter, which indicates the speed of movement; and a dynamometer, which measures strength at any given moment. The resistance produced by the system is always proportional to the exercised strength, with the corresponding muscles developing maximum tension and fibre activation. Strength (N), total work (J), and power (W) measurements were made for flexion and extension at the knee joint.

Sarcopenia was established according to the definition and criteria established by the European Working Group on Sarcopenia in Older People -2nd revision (EWGSOP-2) [18]. Sarcopenia is defined as low muscle strength measured according to grip strength (<27kg in men and <16kg in women using the handheld JAMAR dynamometer) and low muscle quantity measured as the Appendicular Skeletal Mass (ASM)/height² (<7.0Kg/m² in men and <5.5Kg/m² in women according to BIA). Sarcopenia is rated as severe if physical performance is also poor (gait speed ≤0.8m/s) [18]. Dynapenia was considered when low muscle strength was present (considering the mentioned grip strength cut-off points) accompanied with normal muscle mass.

Frailty was established according to the Fried criteria [19], which consider that a person suffers from frailty if presents at least 3 of the following criteria: weight loss, exhaustion, poor physical activity, weakness, and low gait speed. Physical performance was assessed according to the Timed Up-and-Go (TUG) test, which consists of getting up from a standard armchair, walking 3m, turning, walking back, and siting down again. The time to complete the task is measured and ten seconds or less is considered as normal mobility, and more than 20 seconds indicate the need for assistance. Other study variables included sociodemographic data (age, sex), comorbidities, and chronic medication use.

Statistical Analysis

For sample size estimation, correlations between BIA and isokinetic parameters were main analyses considered. Accepting an alpha risk of 0.05 and a beta risk of 0.10 in a two-sided test, 113 subjects are necessary to detect as statistically significant a correlation coefficient of 0.3. Continuous variables were described using mean and Standard Deviation (SD) values, and categorical variables were described using percentages. Pearson (r) or Spearman (rs) correlation coefficients were used to assess the relationship between hydration status indicators and isokinetic measurements, and also the relationship between other numerical variables and isokinetic measurements. The t-test or Mann-Whitney U-test was used to assess the relationship between dichotomous variables and muscle function parameters, depending on application conditions. Normality was assessed by the Kolmogrov-Smirnov test. Multivariate linear regression analysis was performed to adjust the effect of hydration indicators on muscle function for age. As body composition and muscle function differ for men and women, all analyses were performed separately by sex, and analyses were also performed for the 2 age groups of 70-79 years and ≥80 years. Statistical significance was set to p<.05.


Main Sample Characteristics

Of the initial 237 candidate participants who attended the recruitment visit, 117 who completed baseline BIA and isokinetic test assessments constituted the study sample. Mean (SD) age was 75.0(4.1) years, 50.4% were women, 25.6% lived alone, 62.1% had an educational level of primary education or lower, 55.2% never smoked, 15.4% had no outdoor life, 7.0% were frail, 3.4% were sarcopenic (all rated as severely so), 17.9% presented dynapenia, and 6.0% had fallen in the previous 3 months. The main comorbidities were arterial hypertension (62.9%), arthritis (57.8%), dyslipidaemia (49.6%), gastro-oesophageal reflux (19.8%), diabetes (15.5%), peripheral artery disease (15.5%), cancer (14.8%), and depression (14.8%), and mean medication consumption was 4.37(3.02) drugs.

Body Composition and Muscle Function by Sex

Table 1 describes the main body composition parameters (including hydration indicators) for the full sample, by sex, and by age group. Differences in body composition between men and women were significant, with women having higher FM and lower FFM, TBW and PhA values than men. Men lost more TBW and gained more FM than women as they aged. Age was negatively correlated with ICW (r=-0.187; p=0.045), the ICW/FFM ratio (r=-0.192; p=0.039), and PhA (r=-0.311; p<0.001).