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Cristian Andrés Nuñez-Espinosa, cris-
tian.nunez@umag.cl
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DOI 10.7717/peerj.18061
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2024 Mabe-Castro et al.
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Associations between physical fitness,
body composition, and heart rate
variability during exercise in older
people: exploring mediating factors
Diego Mabe-Castro1,2, Matías Castillo-Aguilar1, Matías Mabe-Castro1,3,
Ruby Méndez Muñoz1, Carla Basualto-Alarcón4,5and
Cristian Andrés Nuñez-Espinosa1,3,6
1Centro Asistencial Docente e Investigación, Universidad de Magallanes, Punta Arenas, Chile
2Departamento de Kinesiología, Universidad de Magallanes, Punta Arenas, Chile
3Escuela de Medicina, Universidad de Magallanes, Punta Arenas, Chile
4Health Sciences Department, University of Aysén, Coyhaique, Chile
5Anatomy and Legal Medicine Department, Universidad de Chile, Santiago, Chile
6Interuniversity Center for Healthy Aging, Chile, Chile
ABSTRACT
Background. Age-related changes in body composition affect physical fitness in older
adults. However, whether the autonomic response is associated with body fat percentage
and its implication for physical fitness is not fully understood.
Aim. To understand the association between physical fitness, body composition, and
heart rate variability in older people and its mediating factors.
Methods. A cross-sectional study with 81 older adults was conducted, assessing
Short Physical Performance Battery (SPPB), Two-minute Step Test (TMST), body
composition, and cardiac autonomic response. Correlation and mediation analyses
were performed.
Results. Body fat percentage negatively correlated with physical fitness (SPPB: r=
−0.273, p=0.015; TMST: r= −0.279, p=0.013) and sympathetic activity (sympa-
thetic nervous system (SNS) index: r= −0.252, p=0.030), yet positively correlated
with parasympathetic tone (root mean square of successive differences (RMSSD):
r=0.253, p=0.029; standard deviation of NN intervals (SDNN): r=0.269, p=0.020).
Physical fitness associated with sympathetic nervous system index (SPPB: r=0.313,
p=0.006; TMST: r=0.265, p=0.022) and parasympathetic nervous system index
(TMST: r= −0.344, p=0.003). Muscle mass mediated body fat’s impact on physical
fitness, while physical fitness mediated body fat’s impact on autonomic response.
Conclusion. Body composition and cardiac autonomic response to exercise are
associated with physical fitness in older people, highlighting a possible protective effect
of muscle mass against the decline in physical fitness associated with increased body
fat.
Subjects Geriatrics, Global Health, Nutrition, Public Health, Sports Medicine
Keywords Exercise, Anthropometry, Exercise test, Aging, Heart rate, Performance
How to cite this article Mabe-Castro D, Castillo-Aguilar M, Mabe-Castro M, Méndez Muñoz R, Basualto-Alarcón C, Nuñez-Espinosa
CA. 2024. Associations between physical fitness, body composition, and heart rate variability during exercise in older people: exploring me-
diating factors. PeerJ 12:e18061 http://doi.org/10.7717/peerj.18061
INTRODUCTION
Aging is accompanied by a multitude of physiological changes that have the potential to
significantly impact people’s overall health and well-being (Partridge, Deelen & Slagboom,
2018;Feng, 2019). Body composition and cardiovascular function are particularly relevant,
as they are closely associated with aging (Malandrino et al., 2023;Liu et al., 2023;Ferrucci &
Fabbri, 2018;Xie et al., 2023;Gielen et al., 2021). Recently, there has been a growing interest
in comprehending the interplay between physical fitness, body composition, and cardiac
autonomic response to exercise in older individuals, as these factors play pivotal roles in
determining health outcomes within this age group.
Physical fitness is the ability to perform daily tasks with vigor, without undue
fatigue, and with ample energy to enjoy leisure-time pursuits and meet unforeseen
emergencies (Siscovick, LaPorte & Newman, 1985). Therefore, it is a crucial component of
healthy aging. The term encompasses many aspects, including cardiorespiratory endurance,
muscular endurance and strength, body composition, and flexibility (Caspersen, Powell &
Christenson, 1985). Numerous studies have demonstrated the positive impact of physical
fitness on overall health and longevity, emphasizing its role in reducing the risk of chronic
diseases and improving quality of life and functional independence in older adults (Lee,
Paffenbarger Jr & Hennekens, 1997;Strasser & Burtscher, 2018;Yang et al., 2019;López-
Bueno et al., 2022).
Moreover, the alteration of body composition, specifically the proportion of body fat, is
accompanied by significant changes with age, resulting in a tendency towards an increase
in adiposity and a decrease in lean muscle mass (Palmer & Jensen, 2022;Pataky, Young
& Nair, 2021). Excessive body fat accumulation, particularly visceral adiposity, has been
associated with a higher risk of cardiovascular disease, metabolic disorders, and functional
limitations in older individuals (Neeland et al., 2019;Powell-Wiley et al., 2021).
The autonomic nervous system (ANS) plays a critical role in regulating cardiovascular
function, with sympathetic and parasympathetic branches exerting opposing effects
on heart rate and vascular tone. Furthermore, during stressful situations, such as
physical exercise, the ANS ensures a sufficient cardiac response to higher metabolic
demands (Freeman et al., 2006). Heart rate variability (HRV) is a non-invasive indicator
of ANS activity and cardiovascular health (Zhao et al., 2024;Kubota et al., 2017), usually
measured by electrocardiogram or by a wireless heart rate sensor installed via a chest strap.
It encompasses a wide range of variables derived from mathematical calculations based on
heart rate records. Therefore, the variables used in this study are defined in the Instruments
section. A reduction in resting HRV has been linked to various adverse outcomes, including
cardiovascular events and morbidity, while insufficient HRV reduction during exercise
may lead to impaired physical capacity (Kubota et al., 2017;Tiwari et al., 2021;Mongin et
al., 2022).
However, HRV is not only influenced by physical and environmental stressors.
Psychological factors such as anxiety and depression have also been linked to changes
in ANS activity (Brown et al., 2018;Cheng et al., 2022). Older individuals may be more
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 2/21
susceptible to these psychological factors, making it crucial to measure and control for
their effects (Schlechter, Ford & Neufeld, 2023;Andreescu & Varon, 2015).
Despite extensive research into physical fitness, body composition, and cardiovascular
health in older adults, gaps persist in understanding their interrelationships and the factors
mediating them. One area of interest is the relationship between physical fitness, body fat
percentage, and cardiac autonomic response to exercise in older individuals. This study
explores their collective influence on physical fitness in the aging population.
The conceptualization of this investigation is based on recognizing the interplay between
physical fitness, body composition, and autonomic cardiovascular regulation in aging. We
aim to answer the following research question: How does physical fitness relate to body
fat percentage and cardiac autonomic responses to exercise in older people? By elucidating
these correlations, we hope to gain valuable insights into the relationships that underlie
age-related modifications in physical fitness and identify potential interventions to enhance
health outcomes in older adults. Even more, we seek to understand the mediating effect of
the variables, including physical aspects and frequent psychological traits, such as geriatric
depression and anxiety.
We hypothesized that physical fitness would be inversely related to body fat percentage
and associated with cardiac autonomic response during exercise in older individuals.
We believe an optimal cardiac autonomic response, characterized by a shift towards
sympathetic activity predominance, will be essential for this population to achieve greater
performance during physical fitness testing.
MATERIALS & METHODS
Aims and study design
A prospective observational, cross-sectional study was conducted in one data collection
session to determine the associations between physical fitness, body composition, and
cardiac autonomic response to moderate-intensity exercise.
Setting
This study was conducted at the Centro Asistencial Docente e Investigación (Assistance,
Teaching, and Research Center), which belongs to the University of Magallanes (CADI-
UMAG) in Punta Arenas, Chile. All the assessments were made between 09:00 and 11:00
a.m. The privacy and comfort of the subjects were ensured; the room temperature was set
at 20◦C, and white artificial lighting was used.
Participants
A total of 81 community-dwelling older adults were recruited and selected by non-
probabilistic sampling from the Region of Magallanes and Chilean Antarctica, Chile. If
they were 60 years or older, residents of the Region of Magallanes and Chilean Antarctica,
Chile, they were included and understood the study aims and instructions. However,
they were excluded in the case of diagnosis of congenital heart disease, consumption of
beta-blocker drugs, taking stimulant substances within 24 h before assessment session,
motor or cognitive disability, inability to understand instructions or written content, or
presence of pain during cardiac or physical assessments.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 3/21
Figure 1 The flow diagram of the included participants.
Full-size DOI: 10.7717/peerj.18061/fig-1
All participants gave their permission and provided informed written consent before
participation. The Ethics Committee of the University of Magallanes (N◦10/CEC-
UMAG/2023) approved this study, following the regulations established by the Declaration
of Helsinki on ethical principles in human beings.
The flow diagram of the included participants can be seen in Fig. 1.
Procedures
During recruitment, the participants were instructed to avoid the use of psychoactive
substances for 24 hours before the assessment and to sleep for at least 7 hours the previous
night. Upon arrival, participants were informed about the study aims and risks associated
with their participation, and sociodemographic and medical information was collected
during the initial interview. Then, body composition parameters were measured using
bioimpedance analysis, and psychological questionnaires for anxiety and depression
screening were applied by a supervised psychology undergraduate student.
Cardiac autonomic response to physical exercise was measured as previously validated
for this age group (Castillo-Aguilar et al., 2023). The protocol is briefly described below:
HRV was recorded through a non-invasive chest band immediately before, during, and
after executing the two-minute step test (TMST). Vital signs, including blood pressure,
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 4/21
were monitored throughout the test, and participants’ well-being was visually checked to
ensure they were comfortable and prepared. For resting HRV measurements (before and
after the application of the TMST), the volunteers remained seated in a chair, with feet
and back supported, ensuring avoidance of talking during the recordings. R-R intervals
were recorded continuously during the last 10 min of rest and were analyzed for 5 min on
each occasion. The breathing rate was spontaneous. As part of the protocol, it was ensured
that the participant had a blood pressure of less than 140/90 mmHg to start the HRV
measurements.
Finally, after 15 minutes of resting from the TMST, the Short Physical Performance
Battery (SPPB) was administered to the participants. Before the session ended, the subjects’
blood pressure, heart rate, and general appearance were verified.
Physical and physiological assessments, as well as the initial interview, were made by
professional physiotherapists.
Instruments
Short physical performance battery
SPPB consists of a physical test used to measure three components of physical fitness,
described below (Guralnik et al., 1994):
Balance: to achieve the maximal score (4 points), the subject should be able to stay
balanced for at least 10 seconds in a side-by-side stand (feet together), semi-tandem stand,
and tandem stand.
Usual gait speed: The subjects are asked to walk normally at a 4-meter distance. They
are given two chances, and the best of both is registered. To achieve the maximal score (4
points), they should obtain 4.82 sec. or less.
Lower body fitness: The subjects are asked to perform five chair stands without using
their arms, and time is registered upon completion. A time lower or equal to 11.19 sec.
gives the maximal score (4 points).
To obtain the final score, the sum of the three components is calculated, with a maximal
total score of 12 points.
Body composition
Body mass (kg) and total body fat (%) were assessed by bioimpedance using the Tanita
BC-558 Ironman Segmental Body Composition Monitor (Tanita Ironman, Arlington
Heights, IL, USA), with a concordance of 89.3% compared to the Dual X-ray Absorption
test using standard measurement protocols (Mialich, Martinez & Júnior, 2011).
Cardiac autonomic activity
The cardiac autonomic activity was assessed using R-R interval recordings (HRV) obtained
through the Polar Team2 system (Polar®) application. Artifacts and ectopic heartbeats were
excluded, not exceeding 3% of the recorded data (Malik, 1996). Time-domain parameters
considered for analysis included the square root of the mean squared differences of
successive R-R intervals (RMSSD, expressed in ms) as an index of parasympathetic
activity (Buchheit et al., 2010) and the standard deviation of RR intervals (SDNN), reflecting
total variability encompassing both sympathetic and parasympathetic contributions to
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 5/21
cardiac autonomic function (Berntson et al., 1997;Buchheit & Gindre, 2006). The Stress
Index (SI) and Parasympathetic and Sympathetic Nervous System Index (PNS and
SNS) were computed. The PNS Index, indicative of total vagal stimulation, was derived
from mean R-R intervals, RMSSD, and Poincaré Plot Index SD1 in normalized units
(linked to RMSSD), representing deviations from normal population averages (Berntson
et al., 1997;Rajendra Acharya et al., 2006). The SNS Index, reflecting total sympathetic
stimulation, was derived from mean R-R intervals, Baevsky’s Stress Index (positively
related to cardiovascular system stress and cardiac sympathetic activity), and the Poincaré
Plot Index SD2 in normalized units (related to SDNN) with interpretation similar to the
PNS Index (Berntson et al., 1997;Rajendra Acharya et al., 2006). The SI indicates the ANS
control system’s workload (Yoo et al., 2020), normalized by the square root of Baevsky’s
SI (Baevsky, 2008). All analyses were conducted to compute HRV-related indices using
Kubios HRV®software (Kuopio, Finland).
Two-minutes step test
The TMST is a sub-test from the Senior Fitness Test, demanding a low to moderate
intensity (Castillo-Aguilar et al., 2023;Rikli & Jones, 1999). It consists of a two-minute
assessment designed to evaluate cardiorespiratory fitness. Participants are instructed to
raise their right knee to a marked point on a wall as often as possible within the given time
frame, ensuring that each raise reaches at least a 70◦angle at the thigh-femoral joint. The
number of valid steps was recorded for each subject.
Geriatric depression scale
The 30-question Geriatric Depression Scale (GDS-30) was employed to assess the subject’s
depressive symptoms (Gana et al., 2017). It consists of a dichotomous questionnaire, where
participants are asked about their past-week feelings about depressive symptoms (for
instance, ‘‘Do you feel that your life is empty?’’), with higher scores (i.e., ‘‘yes’’ responses)
representing more depressive symptoms. It was first developed by Yesavage et al. (1982) and
is currently widely used (Zenebe et al., 2021). The Spanish version employed in this study
has been previously validated, with a Cronbach alpha coefficient of 0.82 (Fernández-San
Martín et al., 2002). The authors have permission to use this instrument from the copyright
holders.
Beck anxiety inventory
The Beck Anxiety Inventory was employed to assess anxiety symptoms. It consists of
a 21-item questionnaire based on usual anxiety symptoms and a 4-option Likert scale
from 0 (‘‘Not at all’’) to 3 (‘‘Severely’’), meaning the severity reported by the subject in
each one. It was originally developed by Beck et al. (1988). This study uses the Spanish
version, demonstrating a high internal consistency in older people (α=0.94) (Rodríguez
Reynaldo, Martínez Lugo & Rodríguez Gómez, 2001). The authors have permission to use
this instrument from the copyright holders.
Statistical analysis
We used mean and standard deviation (SD) to describe continuous variables and absolute
and relative frequencies to describe discrete variables. We used Pearson’s product-moment
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 6/21
correlation (r) to assess the relationship between continuous variables and Spearman’s
rank correlation (rho) to evaluate the relationship between body composition variables
and the ordinal constituents of the SPPB. To assess differences between groups, we used
standardized mean difference (SMD) and 95% confidence intervals (CI95% ).
As a way of controlling for the influence of psycho-physiological variables on the cardiac
autonomic response of the subjects to exercise, we assess the average causal mediation
effect (ACME), the average direct effect (ADE) of the main effects after taking into account
the effect of moderator variables into the observed relationships and the proportion of
the effect that its mediated by these variables (Imai, Keele & Yamamoto, 2010). To assess
the significance of moderating variables, we used nonoverlapping CI95% , estimated based
on nonparametric bootstrapping using Monte Carlo resampling, and bias-corrected. We
accelerated CI95% , using the mediation R package to this end (Tingley et al., 2014).
We defined a type I error rate of 5% (α=0.05) as our threshold for null hypothesis
significance testing and nonoverlapping CI95% over the null effect for sex differences and
mediation analyses.
To estimate the confidence in our conclusions and, consequently, the statistical power of
our analyses, we calculated the probability of correctly rejecting the null hypothesis for the
correlation tests post hoc, considering a low (i.e., r =0.3) to moderate effect size (r=0.5).
All analyses were computed using the Rprogramming language for statistical
computing (R Core Team, 2021).
RESULTS
Sample characteristics
The sample consisted of 81 adults, averaging 71.1 years (±6.2), with a BMI of 31
(±6.2 kg/m2), and predominantly female (82.7%, n=67). Table 1 presents the detailed
characteristics of the sample.
Body composition and autonomic response
We observed a negative correlation between body fat percentage and the sympathetic
nervous system (SNS) response (r = −0.252, p =0.030) and the Stress Index (r = −0.258,
p=0.027) during the TMST. Conversely, body fat percentage positively correlated with
heart rate variability (HRV) metrics during exercise, specifically RMSSD (r =0.253, p =
0.029) and SDNN (r =0.269, p =0.020) (See Fig. 2).
When evaluating the sex-specific effects on autonomic response, most correlations held
true for females. Body fat percentage correlated negatively with the SNS Index (females:
r= −0.356, p =0.005; males: r =0.174, p =0.589) and the Stress Index (females: r
= −0.316, p =0.012; males: r =0.204, p =0.525). However, there was no sufficient
statistical evidence to suggest a sex-specific response for RMSSD (females: r =0.246, p =
0.054; males: r = −0.259, p =0.416) and SDNN (females: r =0.247, p =0.053; males: r =
−0.058, p =0.859). No other body composition variables showed a significant correlation
with cardiac autonomic response to exercise.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 7/21
Table 1 Main sample characteristics. Body composition and characteristics are displayed for the overall sample and separated by sex. Differences
between males and females are displayed as SMD and CI95% for continuous variables.
Sex
Characteristic Overall, N = 811Female, N = 671Male, N = 141Difference295% CI23
Age (years) 71.5 ±5.6 71.4 ±5.7 71.9 ±5.5 −0.08 −0.66, 0.50
Hypertension 33 (41%) 26 (39%) 7 (50%)
Diabetes 12 (15%) 9 (13%) 3 (21%)
Body weight (kg) 74.6 ±13.2 72.8 ±12.6 85.0 ±12.7 −0.98 −1.6, −0.33*
Height (cm) 155.8 ±9.2 153.3 ±7.6 167.8 ±6.3 −2.1 −2.8, −1.4*
BMI (kg/m2) 31.0 ±6.2 31.2 ±6.5 29.7 ±3.2 0.28 −0.36, 0.92
BMI category
Normal 7 (9%) 6 (9%) 1 (9%)
Overweight 35 (45%) 29 (43%) 6 (55%)
Obese 36 (46%) 32 (48%) 4 (36%)
Muscle mass (kg) 43.9 ±8.2 41.0 ±4.3 59.9 ±6.5 −3.5 −4.4, −2.7*
Fat mass (%) 37.5 ±9.3 39.5 ±8.3 26.2 ±6.4 1.8 1.2, 2.5*
Bone mass (%) 2.3 ±0.4 2.2 ±0.2 3.1 ±0.3 −3.6 −4.5, −2.8*
Water (%) 47.0 ±6.7 45.4 ±5.6 55.8 ±5.0 −2.0 −2.7, −1.3*
Notes.
* shows significant sex differences.
Figure 2 Bivariate dispersion plots between body fat and HRV-related measures. Significance values
for Pearson’s product-moment correlation test are shown.
Full-size DOI: 10.7717/peerj.18061/fig-2
Body composition and physical fitness
Lower body fat percentage was associated with higher SPPB scores in the sit-to-stand (rho
= −0.279, p =0.013) and gait speed (rho = −0.261, p =0.021) subtests, but not in the
balance score (rho =0.052, p =0.656). Similarly, lower BMI values correlated with higher
SPPB scores in sit-to-stand (rho = −0.325, p =0.004) and gait speed (rho = −0.305, p =
0.007), but not in balance (rho = −0.083, p =0.474).
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 8/21
Figure 3 Bivariate dispersion plots between physical fitness and body composition-related measures.
Significance values for Pearson’s product-moment correlation test are shown.
Full-size DOI: 10.7717/peerj.18061/fig-3
Furthermore, total steps from the TMST were inversely correlated with body fat
percentage (r = −0.279, p =0.013) and body weight (r = −0.232, p =0.041). No
significant correlations were found between total muscle mass and SPPB scores (r =
−0.063, p =0.584) or TMST steps (r = −0.070, p =0.541) (See Fig. 3).
Sex-specific analyses revealed relationships between body composition variables and
physical fitness measures. For females, body fat percentage and BMI were linked to
sit-to-stand (body fat: rho = −0.347, p =0.004; BMI: rho = −0.324, p =0.008) and gait
speed (body fat: rho = −0.309, p =0.012; BMI: rho = −0.318, p =0.009). However, these
correlations were not significant in males (sit-to-stand, body fat: rho = −0.304, p =0.337;
BMI: rho = −0.294, p =0.380; gait speed, body fat: rho =0.172, p =0.593; BMI: rho =
−0.108, p =0.752).
Identical findings were observed for TMST total steps, with significant correlations for
females between steps and body fat percentage (r = −0.311, p =0.01) and body weight
(r = −0.275, p =0.024). No such correlations were found in males (steps, body fat: r =
0.085, p =0.792; body weight: r = −0.198, p =0.559).
Physical fitness and cardiac autonomic response to exercise
Physical fitness, reflected through total steps in TMST and SPPB sit-to-stand score, was
positively correlated with greater sympathetic activity during exercise. This was indicated
by the SNS Index (SPPB sit-to-stand: rho =0.345, p =0.003; TMST steps: r =0.265, p
=0.022), mean HR (SPPB sit-to-stand: rho =0.387, p =0.001; TMST steps: r =0.338,
p=0.003), and its counterpart mean R-R interval (SPPB sit-to-stand: rho = −0.394, p =
0.001; TMST steps: r = −0.311, p =0.007). Other SPPB sub-domains did not exhibit this
behavior (i.e., p >0.05 for balance and gait speed scores).
Sex-specific analyses showed that the relationship between the SNS Index during exercise
and the SPPB sit-to-stand score was stronger in males (rho =0.607, p =0.028) compared
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 9/21
to females (rho =0.294, p =0.021). However, the correlation between total TMST steps
and the SNS Index was significant only in females (r =0.326, p =0.01) and not in males (r
= −0.211, p =0.51). A similar sex-specific effect was observed between TMST steps and
mean HR during exercise (females: r =0.414, p =0.001; males: r = −0.17, p =0.598) and
mean R-R interval (females: r = −0.398, p =0.001; males: r =0.16, p =0.62).
The total number of TMST steps was inversely correlated with parasympathetic indices
during exercise, such as RMSSD (r = −0.285, p =0.014) and PNS Index (r = −0.344, p =
0.003). Sex-specific analyses indicated similar patterns for RMSSD (females: r = −0.288, p
=0.023; males: r = −0.08, p =0.805) and PNS Index during exercise (females: r = −0.41,
p=0.001; males: r =0.115, p =0.722).
Additionally, there was a proportional decrease in the PNS Index during exercise with
increasing levels of physical fitness, as shown by the SPPB sit-to-stand score (rho = −0.407,
p <0.001). A similar effect was observed for the PNS Index post-exercise, particularly in the
SPPB balance score for females (females: rho = −0.259, p =0.046; males: rho = −0.077, p
=0.802), unlike the sit-to-stand score, which showed a negative correlation in males only
(females: rho = −0.087, p =0.506; males: rho = −0.570, p =0.042).
Likewise, RMSSD during exercise decreased with increasing physical fitness, as indicated
by the sit-to-stand score (rho = −0.327, p =0.005). This relationship was significant only
for females (females: rho = −0.333, p =0.009; males: rho = −0.419, p =0.154) (See Fig.
4).
Mediation analysis
Mediation analyses suggest many potential influential effects on the relationships between
body composition and parasympathetic indices. In this context, the average direct effect
(ADE) of body fat percentage on RMSSD during exercise (ADE =0.088, CI95% [−0.005,
0.200], p=0.077) was accentuated when considering the influence of SPPB score (ACME
=0.0289, CI95% [0.003, 0.110], p=0.057; Total effect =0.117, CI95% [0.028, 0.240],
p=0.012). Similar findings were observed when considering the mediation effect of TMST
steps into account (ACME =0.0283, CI95% [0, 0.07], p=0.079; Total effect =0.117,
CI95%[0.028, 0.240], p=0.012). No other parasympathetic indicators were influenced or
mediated by fitness or psychological-related measures.
Additionally, and in the case of sympathetic indicators, the effect of body fat on SNS
index during exercise (ADE = −0.050, CI95% [−0.107, 0.01], p=0.077) was influenced
by SPPB score in similar way as with RMSSD, enhancing the original main observed effect
(ACME = −0.016, CI95% [−0.049, 0], p=0.041; Total effect = −0.066, CI95% [−0.125,
−0.010], p=0.016). Similar effects were observed with TMST steps in this regard (ACME
= −0.015, CI95% [−0.042, 0], p=0.050; Total effect = −0.066, CI95% [−0.125, −0.010],
p=0.016). No other sympathetic indices were notoriously modified in the presence of
either SPPB score, TMST steps, or psychological variables.
When assessing potential mediators for the effect of body composition on fitness-related
measures, we found that the impact of body fat percentage on SPPB score (ADE = −0.075,
CI 95% [−0.123, −0.020], p=0.007) is partially diminished when considering the influence
of total muscle mass (ACME =0.011, CI95%[0, 0.040], p =0.107; Total effect = −0.064,
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 10/21
Figure 4 Bivariate dispersion plots between physical fitness and HRV-related measures. Significance
values for Pearson’s product-moment correlation test are shown.
Full-size DOI: 10.7717/peerj.18061/fig-4
CI95% [−0.111, −0.010], p =0.014). Similar mediation effects of total muscle mass were
observed (ACME =0.118, CI95%[0.005, 0.350], p =0.075) when assessing the effect of
body fat percentage on TMST steps (ADE = −0.773, CI95%[−1.193, −0.300], p=0.001;
Total effect = −0.655, CI95% [−1.084, −0.210], p=0.004). No other mediating effects
were observed for any of the psychological measures or the relationships between body
composition and physical fitness measures.
A visual summary of the results is presented in Fig. 5.
Post-hoc power analysis
Finally, when evaluating the statistical power for the observed effects, with correlations of
0.3, 0.4, and 0.5, considering our previously established confidence level and the current
sample size, we estimated a statistical power of 76.9%, 95.8%, and 99.8%, respectively.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 11/21
Figure 5 Diagram of main results observed from the data. Blue bidirectional arrows and boxes repre-
sent the correlation between variables, while orange unidirectional arrows and boxes represent the factors
mediating the above correlations. ‘‘+’’ means that the mediating factor enhances the main observed effect,
contrary to ‘‘-’’, reflecting a diminished main effect when considering the mediating factor.
Full-size DOI: 10.7717/peerj.18061/fig-5
DISCUSSION
This study investigated the relationship between physical fitness, body fat percentage, and
cardiac autonomic response to exercise in community-dwelling older people (aged: 71.5 ±
5.6). We elucidated the interplay between these factors and their implications for cardiac
autonomic function during physical efforts through correlational and mediating analyses.
Sex-separated analyses were carried out to avoid incorrect interpretation of the results and
to clarify the effect of the difference in sample size between men and women on our main
results.
A higher body fat percentage was hypothesized to be inversely correlated to
physical fitness. Our results align with this hypothesis and previous research regarding
body composition and physical performance, with a proportional decrease in older
people (Paranhos Amorim et al., 2022;Fatyga-Kotula et al., 2022). Using the SPPB, we
examined three components of physical fitness: gait speed, lower limb relative strength, and
balance. Additionally, using the number of steps in the TMST, we assessed aerobic capacity.
To better understand the interaction between body composition and physical performance,
we analyzed each component separately. We found that the negative correlation with fat
percentage and BMI was consistent for gait speed, lower limb relative strength, and aerobic
capacity, but not for balance. This analysis contributes to a more specific understanding of
how these two widely studied variables are related.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 12/21
When making this analysis separated by sex, it was found that the main observed
correlations maintained significant only for women, which shows that these results were
not increased by the presence of a limited number of men in the sample.
However, the mediation analysis showed that total muscle mass diminished the main
observed effect of body fat on physical fitness, with more muscled individuals displaying
better physical performance despite the body fat percentage. This finding suggests that
older adults could benefit from activities promoting muscle mass conservation, even when
they have high body fat percentages (Ramírez-Vélez et al., 2016), offering insights into the
protective role of muscle mass during aging. Nevertheless, caution is advised in interpreting
this result, as the close physiological association between increasing body fat and decreasing
muscle mass during aging has been well-studied (Morgan, Smeuninx & Breen, 2020).
Regarding the link between physical fitness and cardiac autonomic response to exercise,
our results evidenced that individuals with a high sympathetic drive while exercising had
greater physical fitness and exhibited better performance during TMST. Previous research
had demonstrated the utility of HRV and exercise-induced responses in those metrics as
a potential marker of cardiorespiratory fitness in these individuals (Mongin et al., 2022),
supporting the current hypothesis that a greater sympathetic drive facilitates physical
performance during moderate-intensity activities. Even more, it has been previously
demonstrated that physically active individuals have an enhanced autonomic response to
exercise, playing a pivotal role and strengthening this hypothesis (Sarmento et al., 2017;
Soares-Miranda et al., 2014). Despite that, our results provide information about these
relationships in individuals aged 60 years or older.
When sex specific influence on main results was analyzed, it was observed that the
positive association between sympathetic drive and TMST performance only remained
true for women, however, the correlation between sympathetic activity and SPPB sit-to-
stand score was stronger in men, showing that men could have had a positive influence in
the correlations between physical fitness and autonomic response for the total sample, only
in certain variables. This highlights the importance of a more sex-equilibrated sample, as
discussed further in the study’s lmitations.
Furthermore, the increase in body fat percentage was related to reduced sympathetic
activity and a higher parasympathetic activity during TMST, as hypothesized (i.e.,
SNS index, SI, SDNN, and RMSSD). ANS is expected to shift into a sympathetic
predominance throughout physical efforts, ensuring a sufficient response to higher
metabolic demands (Marasingha-Arachchige et al., 2022). However, aged individuals may
present an impaired ANS response to physical exercise (Castillo-Aguilar et al., 2023). In
that sense, our results support body composition’s role in autonomic response. Moreover,
they suggest that the negative effects of body fat on physical fitness in older adults may be
partially attributable to less efficient autonomic inputs to the cardiovascular system (Sinha
et al., 2023), adding new insights into this issue. When analyzing this association by sex,
it was found that the results remained consistent in sympathetic activity markers form
women, but not for men; demonstrating that the observed main effect is not due to men
presence. However, the positive correlation between body fat and, RMSSD and SDNN
disappeared for both sexes when analyzed separately.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 13/21
Surprisingly, mediating analyses found that higher physical fitness (SPPB and TMST
scores) intensifies the negative impact of body fat on autonomic control. This unexpected
finding hints at a complex interplay between physical fitness, adiposity, and cardiac
autonomic regulation. Our results indicate that in individuals with greater functional
capacity, the influence of body fat on HRV is amplified rather than mitigated. This
observation challenges traditional conceptions about the relationship between physical
fitness and cardiac autonomic regulation and raises questions about the underlying
mechanisms involved (Tiwari et al., 2021).
As far as we know, no previous studies have addressed these findings. We posit that
further factors exist that influence the mediation effect. For instance, body distribution
of the adipose tissue was not explored, especially when it is known that visceral fat has
an important impact on cardiovascular health and autonomic regulation (Triggiani et
al., 2019;Chait & Hartigh, 2020). Participants with higher fitness may have a different
fat distribution than lower-performance individuals, which may affect the autonomic
drive to the heart (O’Donovan et al., 2009). Furthermore, other physical, physiological,
demographic, or psychological variables may be unexplored. However, these hypotheses
must be examined under scientific standards. Therefore, exhaustive research is needed to
address this intriguing interplay.
Our results support the interplay between physical fitness, body fat, and cardiac
autonomic response to exercise in older individuals. Higher body fat percentage negatively
affects physical fitness and is also associated with impaired cardiac autonomic response to
exercise, characterized by decreased sympathetic drive and increased parasympathetic tone.
Additionally, this altered autonomic response is related to lower physical performance
in older individuals, which are important predictors of health outcomes in this
population (Langhammer, Bergland & Rydwik, 2018;Tan et al., 2019). Overall, our findings
underscore the triangular and bidirectional relationship among these variables.
However, our results did not evidence a mediating effect of GDS-30 or BAI scores,
challenging our initial hypothesis. This finding suggests a nuanced interplay between
psychological variables and physical/physiological measures in older people, underscoring
the need for further research to elucidate underlying mechanisms and variables beyond
depression and anxiety. While the lack of evidence suggesting any mediating effects does
not diminish the significance of psychological factors in aging, it highlights the complexity
of their influence.
This study was strengthened by the fact that SPPB and TMST measures consistently
aligned in their interaction with other variables, which supports a global understanding
of physical fitness in older people rather than specific aspects, and by exploring mediating
factors in the primary results. Nonetheless, this cross-sectional study is not without
limitations. First, our study design prevents us from making causal inferences about
the direction of the relationships, highlighting the relevance of experimental and
longitudinal research exploring causal relations between our variables and potential
underlying mechanisms. Second, a relatively small sample was included and selected with
no randomization process, limiting the generalizability and statistical power of the analysis.
Furthermore, the sample consisted mainly of women (82.7%), limiting our ability to
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 14/21
conduct sex comparisons. Caution is advised in interpreting sex differences in sample
characteristics, presented in Table 1. Given the natural differences in body composition,
fitness, and HRV, our findings may be influenced by the presence of 14 males, therefore, we
recommend using an equilibrated male/female sample to be able to control sex influences
in the future. Further research could investigate a broader group of potentially mediating
factors to understand the impact of the personal context and the underlying mechanisms
of the associations while using longitudinal designs, which could be useful to improve
internal and external validity.
CONCLUSIONS
The results highlighted a structural bidirectional relationship among variables in our
sample. Body fat percentage was inversely correlated to physical fitness and cardiac
autonomic activity during exercise, with sympathetic drive associated with physical fitness
in older people. Furthermore, physical fitness may mediate the effect of body fat on cardiac
autonomic activity during exercise, and total muscle mass may mitigate the negative effect
of increased adiposity on physical fitness, highlighting its pivotal role in older people’s
health. Further research is needed to assess sex influences on this topic.
ACKNOWLEDGEMENTS
This project was part of Consolidating the binational (Chile-Argentina) research network
in Southern Patagonia (FOVI210061).
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
This study was supported with funds from C.N.-E. by ANID Proyecto Fondecyt Iniciación
N◦11220116. The funders had no role in study design, data collection and analysis, decision
to publish, or preparation of the manuscript.
Grant Disclosures
The following grant information was disclosed by the authors:
ANID Proyecto Fondecyt Iniciación N◦: 11220116.
Competing Interests
The authors declare there are no competing interests.
Author Contributions
•Diego Mabe-Castro conceived and designed the experiments, performed the
experiments, analyzed the data, prepared figures and/or tables, authored or reviewed
drafts of the article, and approved the final draft.
•Matías Castillo-Aguilar performed the experiments, analyzed the data, prepared figures
and/or tables, and approved the final draft.
Mabe-Castro et al. (2024), PeerJ, DOI 10.7717/peerj.18061 15/21
•Matías Mabe-Castro performed the experiments, authored or reviewed drafts of the
article, and approved the final draft.
•Ruby Méndez Muñoz conceived and designed the experiments, performed the
experiments, authored or reviewed drafts of the article, and approved the final draft.
•Carla Basualto-Alarcón performed the experiments, authored or reviewed drafts of the
article, and approved the final draft.
•Cristian Andrés Nuñez-Espinosa conceived and designed the experiments, performed
the experiments, analyzed the data, authored or reviewed drafts of the article, and
approved the final draft.
Human Ethics
The following information was supplied relating to ethical approvals (i.e., approving body
and any reference numbers):
The Ethics Committee of the University of Magallanes (N◦10/CEC-UMAG/2023)
approved this study following the regulations established by the Declaration of Helsinki on
ethical principles in human beings.
Data Availability
The following information was supplied regarding data availability:
The raw data are available in the Supplemental File.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.18061#supplemental-information.
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