Background
Bioelectrical impedance analysis (BIA) is a widely used, non-invasive field method for assessing body composition, which measures the electrical characteristics of human body either at 50 kHz (single-frequency BIA) or at several frequencies in the range 1–1000 kHz (multifrequency BIA and BIS = bioimpedance spectroscopy). Impedance (Z) is the opposition of the body to an alternating current, resulting from resistance (R) to the current that flows through tissue containing water plus electrolytes, and reactance (Xc), which is associated with the capacitive component of tissues (e.g. cell membranes and tissue interfaces) [
1]. In addition, phase angle (PhA), which is also stated as the arctangent of the Xc to R ratio, describes the angular shift (phase difference) between voltage and current sinusoidal waveforms; in humans the current reaches at regular intervals its maximum/minimum peaks after the voltage (positive PhA values) and this lag is most likely due to cell membranes and tissue interfaces [
1,
2].
Using BIA, total body water (TBW) and fat-free mass (FFM) can be estimated by means of predictive equations, which include BIA variables and almost always variables such as age, stature and weight. Alternatively, directly-measured raw BIA variables, such as PhA at 50 kHz or impedance ratio (IR = the ratio between Z at higher frequencies and Z al lower frequencies), have been gaining attention because they are considered indexes of water distribution (ratio between extracellular water-ECW and intracellular water-ICW), body cell mass (BCM), and cellular integrity [
2]. PhA and IR have been shown to be significantly associated with muscle strength and physical activity [
3,
4] and to vary between gender and with aging [
5,
6] in line with that is known about physiological changes in BCM and ECW/ICW.
In sport science the assessment of body composition has different applications such as identifying individual’s characteristics critical to performance, evaluating the effects of training programs, managing weight strategies in weight-category sports, etc. In this regard, BIA has been used in athletes as a field technique for estimating TBW and FFM. Indeed, there is still limited research and it is uncertain to what accuracy BIA may be used in athletes for single measurements or for tracking body composition changes [
7]. Even less attention has been paid to raw BIA data. A recent review has shown that Bioelectrical Impedance Vector Analysis (BIVA) of both R and Xc has yielded some conflicting results on the use if BIA for identifying dehydration [
8,
9]. On the other hand, at least in theory, the use of PhA or IR may be crucial in evaluating athletes’ body composition because it can provide useful data on the percentage of BCM in FFM (structural muscle quality) in both cross-sectional and longitudinal studies. A recent paper [
10] supported this view showing in 202 athletes that PhA significantly correlated with ICW and the ICW/ECW ratio. In this context, the purpose of this systematic review was to evaluate the variability of PhA among athletes and its relationship with sports performance. Additionally, we wanted to investigate whether PhA differs between athletes and controls or between different sports.
Methods
Search strategy
Two authors (ODV and MM) independently performed a literature search up to June 2019 of the electronic databases PubMed, Scopus, and Web of Science.
The following terms were used as search strategy string: (“bioelectrical impedance” OR “bioimpedance” OR BIA) AND “phase angle” AND (spor* OR athlet* OR “physical activity” OR fitness OR train*).
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [
11] were followed for performing the present review. Due to the study type (systematic review), ethical approval was not necessary according to local registration.
Eligibility criteria
The PICOS strategy was defined as follows: “P” (patients) corresponded to participants of any age, sex or ethnicity, “I” (intervention) designated regular physical exercise at amateur, elite and professional level, “C” (comparison) indicated no physical exercise or low physical activity, “O” (outcome) corresponded to PhA, and “S” (study design) indicated cross-sectional or longitudinal studies.
The following eligibility criteria were applied: a) studies on athletes following exercise programmes with or without a control group; b) papers published from inception to June 2019; c) full papers published in peer-reviewed journals or in relevant congress proceedings; d) studies evaluating body composition using BIA phase-sensitive devices and yielding overt data on PhA; e) studies written in English. No restriction was applied to age of participants and sample size.
Studies with the following criteria were excluded: a) non-healthy athletes; b) articles without full-text availability, opinion pieces, review articles and editorials.
Study selection and data extraction
Titles and abstracts from the electronic searches were screened independently by two authors (ODV and MM). The full texts of selected articles were checked by the same two authors to consider the fit with eligibility criteria. A third reviewer (LS) revised any differences in opinion to make a final decision.
An electronic database was designed to store all relevant data. Data were extracted separately by two investigators (ODV and MM), and in the event of disagreement LS cross-examined doubtful data. The following data were extracted: first author, year of publication, country of origin, study type (cross-sectional or longitudinal), study population (sample size, age, gender, period of data collection, and country of residence), type of sport/exercise, presence of a control groups, assessment method and when they were studied.
Risk of bias
Methodological quality was assessed using [
1] the Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies in observational studies [
2]; the Quality Assessment Tool for Before-After (Pre-Post) Studies With No Control Group in before-after (pre-post) studies. Both tools are recommended by the National Institute of Health, U.S. Department of Health and Human Services [
12], which were based on Evidence-based Practice Centers (AHRQ) criteria (Additional file
1: Table S1). The [
1] tool consists of 14 criteria and the [
2] tool of 12 criteria that are used to assess quality, including whether the population studied was clearly specified and defined, whether the outcome assessors were blinded, and an assessment of the participation rate. The criteria were classified as “yes”, “cannot be determined”, “not reported”, or “not applicable”.
Quality rates were good, fair, or poor as judged by two independent observers (ODV and MM) following the instructions given by the National Institute of Health and taking into consideration the number of positive responses. High risk of bias translates to a rating of poor quality. Low risk of bias translates to a rating of good quality.
Discussion
BIA is applied in athletes as a field technique to estimate body composition, being useful in sport science for single measurements or for tracking body composition changes [
7]. On the other hand, raw BIA variables, such as PhA or IR, are commonly related to ECW/ICW ratio, BCM, and cellular integrity [
2]. In addition, an association between muscle strength and PhA has been observed in various pathophysiological conditions (for instance,1–3), suggesting that raw BIA may be useful in assessing muscle quality.
In this context, only a few papers have so far evaluated raw BIA variables in athletes. A recent systematic review examined the applications of BIVA in sports and exercise, a methodology giving information on hydration status by analyzing the length of bioimpedance vector and its inclination [
9]. The authors concluded that the current technique, called “classical BIVA”, is not fully reliable to identify dehydration in individual athletes. The review by Custodio Martins et al. [
47] explored the use of different BIA-derived estimates of body composition in athletes, adding a concise, preliminary view on PhA, a raw BIA variable that has been considered in recent years for assessing body composition in various pathophysiological conditions [
1‐
3].
In this systematic review, we aimed to extend previous information on PhA values as measured in athletes by focusing in depth on different issues of interest. Thirty-five papers were selected according to inclusion and exclusion criteria. In almost all cases single-frequency BIA has been performed (on the whole body). Although it is well known the standardization of measurement conditions is essential for obtaining accurate and reproducible BIA data, most of selected studies did not give enough details in this respect, in particular on the length of time since the last training session (a critical aspect especially in the case of strenuous exercise).
One might expect that training, especially muscle strengthening, should affect not only muscle function but also BCM and muscle cell mass. The first question in this study sought to determine whether PhA differs between athletes and control subjects. Surprisingly, only few papers have so far addressed this issue, sometimes in small groups of athletes. A very marked increase in PhA was observed in bodybuilders [
13] (+ 17.8% on the average), female dancers [
16] (+ 9.6%), male dancers [
18] (+ 12.0%), cyclists [
18] (+ 11.4%) and marathon runners [
19] (+ 9.7%).
Thus, these findings suggest that muscle strengthening causes a greater increase in PhA compared to endurance training. Indeed, contrary to expectations, Meleleo et al. [
17] reported that PhA was significantly lower in competitive vs. non-competitive children, suggesting that the effects of training on PhA may be different in childhood.
As far as main individual’s characteristics are concerned, in the general population PhA increases with age in both genders until late adulthood and then decrease in the elderly [
22‐
26], with a between-gender difference that becomes greater through adolescence [
48,
49] and with mean values in adult age consistently higher in males than females [
5,
6].
The papers selected for gender diversity are in line with the aforementioned findings, with no difference in young adolescent judo athletes [
21] and significant higher values in adolescent/adult male compared to female athletes [
20]. Similarly, four out of five selected papers reported an age trend in various sports [
22‐
25], whereas a single paper found the opposite, with higher PhA in adolescent male than adult male road cyclists [
26]. It should be noted that differences in years of practice and training programmes may influence changes with time.
A key point of the present review was to evaluate whether and to what extent PhA differs between different sports and performance levels. Overall, the selected papers have provided inconsistent and puzzling findings, possibly because of inappropriate study design (for instance, in selecting subjects) or small sample sizes. The variability of PhA was high, as indicated by large standard deviation values [
27‐
29]. Variations between sports emerge but no definite conclusions could be drawn on endurance vs. resistance training or recreational vs. competitive sports, although some results suggest indirectly that PhA increase with muscle-strengthening activities [
20].
Turning to athletes of the same sport, two studies [
26,
31] demonstrated that PhA was higher in soccer players and cyclists with a better performance level, whereas another one did not find differences between a stronger and a weaker volleyball teams [
30]. Thus, it could be argued (but not definitely demonstrated), that the relationships between PhA and performance level may vary in different sports and are possibly influenced by the criteria used to assess performance level. Interestingly, changes emerge also for the same sport when athletes differ depending on their physical characteristics. For instance, among cyclists PhA was lower for climbers compared to sprinters and all-rounders [
26].
Overall, in order to interpret variability of PhA, a single study [
33] indicated that PhA is influenced by ACE or VDR gene polymorphisms, in line with their involvement in a variety of performance-related functions. In addition, another study has shown that mean PhA was higher in white than black football players [
32], which may not surprising given that differences in body composition due to ethnicity are well known [
50].
Finally, longitudinal evaluation of body composition may offer, at least in theory, relevant information on the changes in body composition and hydration due to training or untraining, which might be associated with physical performance. Unfortunately, the papers selected for the present review [
14,
17,
25,
36‐
46] have given inconsistent results. A comprehensive view of the issue cannot be formed because they considered different athletic disciplines and had very different experimental protocols (sometimes with small experimental groups).
Conclusions
This systematic review aimed to summarise the current knowledge on the evaluation of BIA-derived PhA in athletes. Of note, two recent studies strongly support the idea that PhA is an index of ECW/ICW ratio or BCM [
10,
45]. PhA increases with age and is likely to be higher in males. Unfortunately, it is still uncertain to what extent PhA varies between different sports and changes with training/untraining. It can be argued that for a given sport much more data should be collected in a systematic way and for a period of time appropriate in order to determine changes and trends. This is even more crucial in the case of intervention studies.
From a practical point of view, at the present time the measurement of PhA is a promising approach to evaluate muscle quality in groups of athletes, for instance detrained compared to well-trained subjects. On the other hand, further studies are needed to specify the most appropriate measurement conditions and to assess to what extent PhA may be a reliable index for identifying individual’s characteristics critical to performance, evaluating the effects of training programs, managing weight strategies in weight-category sports, etc.
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