# Effect of resistive exercise combined with vibration on body composition and energy balance – Results from the 21-day MNX bedrest study

Pierre Bourdier, Alexandre Zahariev, Floriane Rudwill, Helena Kenny, Donal O'gorman, Isabelle Chery, Marie-Pierre Bareille, Guillemette Gauquelin-Koch, Sylvie Normand, Stéphane Blanc, et al.

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## ► To cite this version:

Pierre Bourdier, Alexandre Zahariev, Floriane Rudwill, Helena Kenny, Donal O'gorman, et al. Effect of resistive exercise combined with vibration on body composition and energy balance – Results from the 21-day MNX bedrest study. Acta Astronautica, 2025, 232, pp.387-396. 10.1016/j.actaastro.2025.03.029. hal-05012310

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# Effect of resistive exercise combined with vibration on body composition and energy balance – Results from the 21-day MNX bedrest study

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Pierre Bourdier<sup>a,b,*</sup>, Alexandre Zahariev<sup>a</sup>, Floriane Rudwill<sup>a</sup>, Helena Kenny<sup>c,d</sup>, Donal J. O’Gorman<sup>c,d</sup>, Isabelle Chery<sup>a</sup>, Marie-Pierre Bareille<sup>e</sup>, Guillemette Gauquelin-Koch<sup>f</sup>, Sylvie Normand<sup>g</sup>, Stéphane Blanc<sup>a,1</sup>, Audrey Bergouignan<sup>a,h,1</sup>, Chantal Simon<sup>g,i,1</sup>

## ARTICLE INFO

### Keywords:

- Energy balance

- Bedrest

- Doubly labelled water

- Body composition

- Resistive-vibration exercise

## ABSTRACT

**Purpose:** High volume exercise is crucial in preventing spaceflight-induced muscle atrophy but can induce reductions in body mass and fat mass, due to a mismatch between energy intake and total energy expenditure (TEE). This randomized cross-over trial investigated whether short-duration resistive-vibration exercise (RVE) prevents muscle loss without inducing negative energy balance during a medium-duration bedrest (IDRCB n°2012-A00337-36).

**Methods:** Twelve healthy young males underwent a 21-day bedrest with RVE or without (CNT) in a randomized cross-over trial, each trial being separated by at least 120 days. Subjects could leave unfinished the meals designed to match their estimated TEE. TEE was assessed using doubly labelled water and energy balance from dual X-ray absorptiometry-generated body composition changes.

**Results:** Compared to strict bedrest, RVE limited by 25 % the bedrest-induced loss in body mass (P < 0.07) and lean body mass (P = 0.12), without reaching significance, and partially lessened the decline in overall muscle function and aerobic capacity. RVE reduced resting metabolic rate decrease (P = 0.03) without increasing activity energy expenditure and TEE. Both groups adjusted their energy intake to match the reductions in TEE, resulting in equilibrated energy balance.

**Conclusion:** RVE may offer some benefits in mitigating muscle mass and function decline under microgravity conditions, without affecting energy balance and fat mass.

## 1. Introduction

Body mass loss is a common threat observed during spaceflight [1]. It indicates a negative energy balance wherein energy intakes fall short of energy needs. Prolonged negative energy balance can exacerbate the adverse effects of microgravity, including muscle atrophy, loss of bone mass, cardiovascular deconditioning, and a decline in mood and well-being [2]. Given the foreseen long-term missions (e.g. towards the

Moon and Mars) [2,3], roadmaps for planetary exploration ranked energy balance management to a critical level. This requires understanding its regulation under microgravity conditions.

Negative energy balance may result from insufficient energy intake, elevated total daily energy expenditure, or a combination of both. By measuring free-living total energy expenditure with the gold standard doubly labelled water method in astronauts before and after at least three months spent onboard the International Space Station, it was

<footnote><sup>a</sup> Université de Strasbourg, CNRS, IPHC UMR 7178, 23 rue du Loess, CEDEX 2, 67037, Strasbourg, France</footnote>

<footnote><sup>b</sup> IRIMAS UR 7499, Université de Haute-Alsace, 12 rue des Frères Lumière, CEDEX, 68093, Mulhouse, France</footnote>

<footnote><sup>c</sup> 3U Diabetes Partnership, School of Health and Human Performance, Artane – Whitehall, Dublin City University, Dublin, Ireland</footnote>

<footnote><sup>d</sup> National Institute for Cellular and Biotechnology, 592 Collins Ave Ext, Whitehall, Dublin City University, Dublin, Ireland</footnote>

<footnote><sup>e</sup> Institut de Médecine et Physiologie Spatiales (MEDES), 2 Av. de l'Aérodrome de Montaudran, 41400, Toulouse, France</footnote>

<footnote><sup>f</sup> Centre National d'Études Spatiales, 2 Pl. Maurice Quentin, 75001, Paris France</footnote>

<footnote><sup>g</sup> Human Nutrition Research Centre of Rhône-Alpes, Hospices Civils de Lyon, 165 Chem. Du Grand Revoyet, Oullins-Pierre-Bénite, 69310, France</footnote>

<footnote><sup>h</sup> Anschutz Health and Wellness Center, Division of Endocrinology, University of Colorado, Anschutz Medical Campus, 12348 E Montview Blvd, Aurora, CO, USA</footnote>

<footnote><sup>i</sup> CarMEN INSERM U1060, Université de Lyon, INRA U1397, 59 Bd Pinel, 69500, Lyon, France</footnote>

<footnote>Abbreviations: BDC, baseline data collection; HDT, head down tilt position; CNT, Control; RVE, resistive-vibration exercise.</footnote>

<footnote>* Corresponding author. IRIMAS (UR 7499), Université de Haute-Alsace, 12 rue des Frères Lumière, CEDEX, 68093, Mulhouse, France. pierre.bourdier@uha.fr</footnote>

<footnote n="1">¹ Co last author.</footnote>
demonstrated that total energy expenditure was directly related to overall physical activity, i.e., the combined aerobic and resistive exercise training countermeasure coupled with daily activities [4]. Data further showed that the energy requirements of the most active astronauts were not met by adequate intakes. This uncoupling between energy intake and total energy expenditure induced a negative energy balance, hence a loss of fat mass. However, the important levels of physical activity (i.e., exercise and daily activities such as task maintenance) were associated with the maintenance of lean body mass. In contrast, the least active astronauts were in positive energy balance despite energy intake being below the prescriptions. While they achieved fat mass maintenance, they lost lean body mass [4]. These data obtained in space showed that performing exercise at significant volume may not be accompanied by compensatory changes in energy intake to match energy requirements, thus resulting in an energy imbalance.

These inflight data confirmed results previously obtained on Earth. In 2005, the Women International Space Simulation for Exploration (WISE) study tested the preventive effects of a countermeasure program combining aerobic and resistive exercises, similar to space training protocols, during a 60-day head-down tilt (HDT) bedrest study in female adults [5]. Participants in strict bedrest without an exercise countermeasure approached energy balance. This resulted in stable fat mass but diminished lean body mass due to muscle disuse [5]. As expected, participants who performed the exercise countermeasure had higher doubly labelled water-derived total energy expenditure, compared to those in strict bedrest. Simultaneously, an excessive and non-adapted reduction in energy intake was observed, leading to a negative energy balance and loss of fat mass. Of note, combining the two training modalities at high volumes only partially protected lean body mass. On the other hand, a training protocol based on resistive exercise alone (flywheel ergometer, 35 min/d, 3-4 days/week) allowed lean body mass maintenance following a 2-month bedrest in male participants [6], with no concomitant decrease in fat mass. Based on previous bedrest studies, incorporating vibration during resistive training appears to have beneficial effects on the musculoskeletal system by enhancing exercise efficacy through increased muscle activation [7]. At rest, muscle vibrations trigger the tonic vibration reflex, stimulating muscle spindles and transmitting neural signal transmission via Ia afferents to activate motor neurons. During voluntary contraction, vibration amplifies motor neuron activation [8], enabling comparable adaptations to conventional resistance training but with fewer sessions. Taken together these data suggest that, in contrast to combined aerobic and resistive exercise protocols, resistive exercise alone or combined with whole-body vibration may effectively prevent lean body mass loss without adversely affecting energy balance regulation.

European and French Space Agencies (ESA and CNES, respectively) designed the Medium Duration Nutrition and Resistance-Vibration Exercise (MNX) bedrest study to assess the effects of a resistive-vibration exercise (RVE) efficacy of countermeasure in mitigating the detrimental effects of 21 days of bedrest on muscle and bone mass. The present study was a protocol conducted during the MNX bedrest study aiming to assess the effects of a RVE training on energy balance regulation. The hypothesis is that the limited RVE impact on activity-related energy expenditure and total energy expenditure, as measured by doubly labelled water coupled with indirect calorimetry, will not significantly alter energy balance and fat mass regulation.

# 2. Material & methods

## 2.1. Participants

An initial call for participants was issued online through the French space clinic MEDES and ESA websites and media channels. From approximately 400 applicants, 63 volunteers were selected for medical screening. Following a rigorous selection assessing compliance with inclusion criteria, as well as comprehensive physical and psychological

evaluations, twelve healthy male volunteers were ultimately enrolled in the Medium Duration Nutrition and Resistance-Vibration Exercise (MNX) bedrest study. Inclusion criteria were age between 25 and 45, body mass index (BMI) within the range of 20-26 kg/m², moderate physical activity levels and VO₂ peak of 35-60 ml/min/kg if younger than 35 years old, or 30-60 ml/min/kg if older than 35 years old. Additional details about participants' inclusion and exclusion criteria have been previously published [9,10].

The MNX bedrest study is 21-day three-arm randomized crossover study funded by the ESA and CNES. The study was conducted between 2012 and 2013 at the MEDES located in Rangueil Hospital, Toulouse, France. The study tested the effects of RVE with or without nutritional supplementation compared to strict bedrest (control or CNT), where each participant was planned to complete each trial as previously described [10]. Given the primary focus on examining the isolated effects of RVE on energetic and body composition outcomes, only data from the RVE and CNT trials were analyzed, excluding the third cross-over arm that combined RVE with nutritional supplementation [9, 10]. Notably, one participant withdrew from the study after the first trial (RVE) and did not participate in the CNT trial. Consequently, the present analyses were based on data collected from twelve participants in the RVE and eleven in the CNT trials.

This study was performed in line with the principles of the Declaration of Helsinki. Ethical approval was obtained from the Comité de Protection des Personnes/CPP Sud-Ouest Outre-Mer I) and French Health Authorities (Agence Française de Sécurité Sanitaire des Produits de Santé) under the IDRCB n°2012-A00337-36. All participants underwent and provided written informed consent.

## 2.2. Overall study design

Study protocol was a randomly assigned crossover design, wherein all participants undertook CNT and RVE trials, each lasting 21 days with a minimum 4-month washout-period between trials (Fig. 1 During each campaign, four participants were assigned to the CNT trial, four participants to the RVE trial and four to the RVE + nutritional supplementation (NeX) trial. The trial assignments shifted for each campaign, following one of three sequences: CNT/RVE/NeX or RVE/NeX/CNT or RVE/CNT/NeX [9,10]. Participants spent 7 days prior to each trial at the MEDES testing facility to enable baseline data collection (BDC) and ensure standardized initial conditions. During this period, they were instructed to maintain moderate activity levels by performing at least one daily session of aerobic or resistive exercise and moving within the MEDES building. Following the 7-day BDC period, participants maintained a supine position with at least one shoulder in contact with the bed for 21 days in the CNT trial. They were also in -6° head down tilt (HDT) position to mimic the vascular changes associated with microgravity. The RVE trial mirrored this setup, but participants were additionally asked to engage in six sessions of RVE throughout the bedrest period.

## 2.3. Resistive and vibration exercise countermeasure

Exercise training sessions were performed twice weekly, specifically on HDT days 2, 5, 9, 12, 16, and 20 (Fig. 1), employing a revised prototype of the Galileo Space exercise device (Novotec Medical, Pforzheim, Germany) previously used in the 2nd Berlin bedrest study [11]. This device, which permitted to exercise in HDT position, combines a standard leg press machine with a vibration platform located on the foot plate that was set to 8 mm peak-to-peak and 25 Hz vibration. Details of the exercise protocol have been previously published [10,12]. Briefly, each 30-min exercise session consisted of the following sequence: a warm-up comprising 8 bilateral squats, followed by 10 bilateral squats, single heel raises on each side until reaching exhaustion, and bilateral heel raises until reaching exhaustion. The load, initially determined by pre-bedrest performance (i.e., measured during the BDC period), was
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Schematic representation of the overall study design. AC = aerobic capacity; MF = muscle function; PSY = psychometric tests; RMR = resting metabolic rate; DXA = dual X-ray absorptiometry; DLW = period of measurement of total daily energy expenditure with the doubly labelled water (DLW) method; BDC = period from BDC-7 to HDT1; 1st half-HDT from HDT1 to HD10; 2nd half-HDT = period from HDT10 to HDT21; R = recovery period; CNT = Control; RVE = resistive-vibration exercise; NeX = RVE + nutritional supplementation (training days are shaded in light grey).</figcaption></figure>

adjusted by ±5% of the prescribed load according to participants' capacity to complete the exercise set. Overall, this amounted to about 2-to-6 min/session under tension [12]. The rationale behind the training modalities was based on the data obtained during the 2nd Berlin Bedrest study [8,11].

## 2.4. Body mass and composition

BM and body composition were measured throughout the bedrest period in fasting participants in underwear. Daily body mass measurements were taken using a calibrated scale designed to weigh individuals in a supine position. Fat mass and lean body mass, defined as fat-free mass minus bone mass, were assessed by dual-energy x-ray absorptiometry (DXA, Hologic QDR-4500C) and its companion software version 11.2 (Hologic, Inc., Marlborough, MA, USA) 7 days before the start of the bedrest (BDC-7), on the first (HDT1, i.e., before bedrest period), 10th (HDT10) and 20th (HDT 20) days of bedrest.

## 2.5. Energy components and balance

### 2.5.1. Total energy expenditure

Total energy expenditure was determined using the doubly labelled water method over three consecutive periods: at baseline (from BDC-7 to HDT1), during the first half (from HDT1 to HDT10) and the second half (HDT10 to HDT21) of the bedrest (Fig. 1).

Participants received three doubly labelled water doses during each trial: a 2.00 g/kg of estimated total body water premixed dose composed of 0.20 g/kg of total body water $H_2^{18}$O enriched at 10% and of 0.15 g/kg of total body water $^{2}$H$_2$O enriched at 99% (Eurisotop, Saint-Aubin, France) at BDC-7; a 1.60 g/kg of total body water premixed dose composed of 0.16 g/kg of total body water $H_2^{18}$O and of 0.12 g/kg of

total body water $^{2}$H$_2$O at HDT 1; and a 1.30 g/kg of total body water premixed dose composed of 0.13 g/kg of total body water $H_2^{18}$O enriched at 10% and 0.10 g/kg of total body water of $^{2}$H$_2$O enriched at 99% at HDT10. Doses were calculated to obtain a maximal enrichment of 150 ppm for $^{18}$O and 130 ppm for $^{2}$H after each of the three successive doubly labelled water ingestions. For each of the three total energy expenditure measurement periods, urines were collected before and 1, 4, and 5 h after the dose ingestion. The 1h samples were discarded; the 4 and 5 h samples were stored to measure isotope enrichments at equilibration [13]. Urines of the second void were then collected daily until the end of the measurement period to calculate total energy expenditure using the multi-point doubly labelled water approach [14]. As previously described, the urines were cleaned with black carbon and filtered to extract water [15]. Hydrogen and oxygen isotope ratios were measured using a high-temperature conversion elemental analyzer (TC/EA) interfaced with a Delta V Plus Isotope-Ratio Mass spectrometer and a Conflo III interface (Thermo Fisher Scientific, Bremen, Germany). Analyses were performed in quadruplicate and repeated if the SD exceeded 2% for deuterium and 0.2% for 18-oxygen.

The dilution spaces for $^{2}$H and $^{18}$O were calculated from the baseline and equilibration samples, according to Coward et al. [16]. $^{2}$H and $^{18}$O constant elimination rates were calculated by least-squares linear regression on the natural logarithm of isotope enrichments from all urine samples as a function of elapsed time from dose administration. CO$_2$ production was then calculated according to Schoeller et al. [17] modified by Racette et al. [18], and total energy expenditure was derived using Weir's formula [19] based on food quotient from actual food intake [20].

### 2.5.2. Resting metabolic rate and activity-related energy expenditure

Resting metabolic rate was measured by indirect calorimetry (Del-
tatrac II, Datex-Ohmeda, Madison, WI, USA) following an overnight fast at the beginning of each of the three total energy expenditure measurement periods and at the end of the bedrest period, with each measurement lasting 45 min (Fig. 1). Since RVE volume was low, the impact of exercise post-oxygen consumption (EPOC) on resting metabolic rate measurement was considered negligible. Oxygen consumption and carbon dioxide production were continuously recorded using a ventilated hood (canopy) and resting metabolic rate was calculated using Weir's formula [19]. For each total energy expenditure measurement, resting metabolic rate recorded at the beginning and end of the doubly labelled water corresponding period were averaged and activity-related energy expenditure was calculated as Total energy expenditure × 0.9 – resting metabolic rate, assuming a diet-induced thermogenesis of 10 % total energy expenditure. Physical activity level was calculated as the ratio between total energy expenditure and the corresponding mean resting metabolic rate.

Daily time spent sedentary and physically active was determined using a tri-axial accelerometer (ActiGraph GT3X+; Actigraph, Pensacola, USA). During the experimental trials, participants were instructed to wear the accelerometer at their right hip 24 h a day. Vector magnitude in counts per minute, steps, and raw acceleration were downloaded using Actilife software (version 6.4). Vector magnitude served as an indicator of overall physical activity throughout the experimental trials. For the ambulatory period, which serves as baseline reference, after identification of sleep and non-wear time, an automatic activity-recognition algorithm [21] was further applied to the raw signal to determine time spent in different postures and activities, and in activities of different intensities [22]. Valid days were defined as at least 10 h of wear-time during waking hours. Sedentary behavior was determined as the time spent awake in lying, reclining, or sitting postures. A cut point of 3 METs was also applied to estimate time spent in moderate-to-very vigorous activity.

## 2.6. Energy intake

Daily prescribed energy intake was adjusted to match estimated total daily energy expenditure throughout the trials to maintain energy balance and fat mass (from DXA), using a macronutrient composition of 30 % fat, 1.2 g/kg protein and approximately 55 % carbohydrates. Prescribed energy intake was 150 % and 125 % of resting metabolic rate (including 10 % of total energy expenditure for diet-induced energy expenditure) for the BDC and bedrest periods, respectively. An additional energy intake prescription of 280 kJ/d was added in the RVE trial during bedrest to account for the energy cost of RVE. Meals were provided quick-frozen by an industrial manufacturer (Davigel, France) and prepared in the metabolic kitchen of the MEDES. Nutritional values were calculated by the Nutrilog® software. Food quotient was derived using Black's equation [20]. Participants were encouraged to consume the provided daily meals fully but were free to eat less. Actual energy intake was quantified from prescribed energy intake and leftovers.

## 2.7. Energy balance

The equations formulated by Ravelli & Schoeller [23] were employed to estimate energy balance from changes in energy stores (fat mass and lean body mass) as assessed by DXA, over the entire bedrest period.

## 2.8. Muscular strength and aerobic capacity assessments

Muscular strength and aerobic capacity were assessed before (BDC-6 for muscular function and BDC-5 for aerobic capacity) and the day following the bedrest period.

The muscular function was evaluated using an isokinetic dynamometer (CON-TREX, Medimex, France). Following a familiarization session, participants were securely seated and strapped onto the

dynamometer in a neutral position according to the manufacturer's instructions. They were asked to perform maximal voluntary isometric contraction (MVIC) of the knee and ankle flexors and extensors of their dominant leg. Each participants completed 3 sets of MVICs lasting 5–7sec with a 30-sec rest between contractions, and a 2-min rest between sets. The maximal torque (in N.m) generated during the MVICs was recorded as the MVIC force. Following a minimum 30-min rest period after the MVIC test, a fatigue test was conducted on the dominant leg with the knee at an 80° angle. Participants were required to sustain a 90-sec contraction at 50 % of their MVIC force prior to bedrest. The fatigue index was calculated as the ratio of the final torque (between 84 and 86 s) to the initial torque (between 4 and 6 s).

Aerobic capacity assessments were conducted using an Ergometrics 800S cycle ergometer (Ergoline, Bitz, Germany), with breath-by-breath oxygen consumption ($VO_2$) monitored by an Oxycon Pro metabolic cart (Jaeger, Hochberg, Germany). During the selection phase, participants were asked to cycle 3 min at 30, 60, 90, 120, and 150 W, followed by a 20 W increase every minute until they could no longer maintain the target cycling cadence of 75 rotations per minute (rpm) and/or chose to stop, reaching $VO_2$ max. Before and after the HDT period, participants were asked to cycle at intensities corresponding to 25 %, 50 %, and 75 % of their $VO_2$ max at selection. At completion of the third stage, the workload will increase 25 W every minute until exhaustion, when participants could no longer sustain the required 75 rpm cadence, as previously detailed [10].

## 2.9. Mood, well-being and sleep

Participants completed questionnaires to evaluate their psychological state and well-being before and at the end of the bedrest period (HDT18). These assessments included the General Health Questionnaire (GHQ-28) [24], which gauges overall health perception over the preceding weeks, with higher scores indicating poorer well-being. The Profile of Mood State (POMS) in its 65-item version [25,26] measured transient mood states across six dimensions (anxiety, anger, activity, fatigue, depression, and confusion), providing a global score of emotional distress. Additionally, the Pittsburgh Sleep Quality Index questionnaire (PSQI) [27] assessed sleep quantity and quality based on measures of sleep latency, duration, and efficiency, with higher total scores indicating poorer sleep quality.

# 3. Statistical analysis

Baseline characteristics are presented as means (standard deviation [SD]). Unless otherwise indicated, data are reported as least squared means (LS means) with their standard error (SE) or 95 % confidence interval (CI).

Linear mixed-effects models, accounting for repeated measures across and within trial periods were employed to evaluate the impact of RVE on anthropometric and energetic outcomes during bedrest. Fixed effects included trial (CNT vs RVE), time (bedrest), and their interactions, along with campaign and sequence (order of the trials) to address the crossover design. A subject-specific random intercept was incorporated to account for between-subject heterogeneity, and an autoregressive or a compound-symmetric covariance structure (selected based on the smallest information criteria) was applied to consider dependencies among measurements within trial periods. Statistical inference centered on estimating between-trial differences (ETD) in net changes at the end of or after the bedrest compared to BDC. ETD (RVE vs CNT) were determined with their corresponding 95 % confidence interval (CI). Additional models, adjusting for lean body mass and fat mass, were estimated for total energy expenditure, activity-related energy expenditure, and resting metabolic rate by adding fat mass and lean body mass as fixed effects. Model residuals were checked for normality using residuals and QQ plots. Effect size were calculated using Cohen's d as previously described [28].
A-posteriori analyses revealed that a statistical power of 0.80 would allow the detection of a minimal between-trial difference in total energy expenditure changes from BDC to HDT20 (ETD) of 1.60 MJ/d, as well as a minimal ETD of 0.85 kg in lean body mass.

Analyses were conducted using SAS version 9.4 (SAS Institute, Cary, North Carolina, USA) with a two-sided 5 % significance level. Given the exploratory nature of this study, no statistical adjustment was made for multiple comparisons. Figures were generated with Prism 10.1 (GraphPad, San Diego, California).

# 4. Results

## 4.1. Characteristics of the participants

Table 1 presents the participants' baseline characteristics measured during the week prior to the HDT period. The participants had an average age of 34.3 years (SD 8.4; range 29-44), with a BMI of 22.3 kg/m² (2.0) and a lean body mass index of 18.2 kg/m² (1.4). Specifically selected for moderate activity levels and fitness, they had a mean VO₂max of 37.4 ml/min/kg (4.1). During the baseline ambulatory period in the MEDES testing facility, accelerometry data revealed an allocation of 790 min/d (123) to sedentary activities and 46 min/d (8) to ambulatory activities, including 36 min/d (11) spent in MVPA.

## 4.2. Effects of RVE on anthropometric outcomes during bedrest

As illustrated in Fig. 2 and Table 2, the 21-day bedrest period resulted in a progressive decline in both body mass (bedrest P < 0.001) and lean body mass (bedrest P < 0.001), while fat mass exhibited a slight increase (bedrest P < 0.01) occurring during the second half period of the bedrest.

In the CNT group, body mass and lean body mass presented declines of 4.0 % and 5.3 %, respectively, from baseline values. The RVE intervention limited the decrease in body mass by almost one quarter (estimated trial difference RVE vs CNT [ETD] +0.79 kg, 95 %CI -0.07 to 1.65, P = 0.07) and that of lean body mass by one fifth (ETD +0.64 kg; 95 %CI -0.16 to 1.43, P = 0.12; Cohen's d = 0.23) without reaching significance. RVE had no impact on fat mass variations (P = 0.56).

<caption>**Table 1**</caption>

Inclusion characteristics of the participants (n = 12) during the ambulatory period (BDC).

<table><thead><tr><th rowspan="2"></th><th colspan="2">Baseline</th></tr><tr><th>Mean (SD)</th><th>Minimum - Maximum</th></tr></thead><tbody><tr><td><strong>Anthropometric variables</strong></td><td></td><td></td></tr><tr><td>Height (m)</td><td>1.76<br>(0.06)</td><td>1.72-1.80</td></tr><tr><td>Body mass (kg)</td><td>69.5 (9.0)</td><td>64.6-75.9</td></tr><tr><td>Fat mass (kg)</td><td>10.3 (4.2)</td><td>7.7-13.0</td></tr><tr><td>Lean body mass (kg)</td><td>56.6 (7.1)</td><td>52.6-61.9</td></tr><tr><td>Body mass index (kg/m²)</td><td>22.3 (2.0)</td><td>21.3-23.8</td></tr><tr><td>Fat mass index (kg/m²)</td><td>3.3 (1.3)</td><td>3.5-4.2</td></tr><tr><td>Lean body mass index (kg/m²)</td><td>18.2 (1.4)</td><td>17.5-19.3</td></tr><tr><td><strong>Cardio-respiratory variables</strong></td><td></td><td></td></tr><tr><td>VO₂max (l/min)</td><td>2.6 (0.4)</td><td>2.4-2.9</td></tr><tr><td>VO₂max (ml/min/kg)</td><td>37.4 (4.1)</td><td>34.8-40.0</td></tr><tr><td><strong>Accelerometer-derived variables<sup>a</sup></strong></td><td></td><td></td></tr><tr><td>Steps (n/d)</td><td>3992<br>(642)</td><td>3532-4451</td></tr><tr><td>Moderate-to-vigorous physical activity (min/d)</td><td>36 (11)</td><td>28-44</td></tr><tr><td>Sedentary time (min/d)</td><td>790 (123)</td><td>702-878</td></tr><tr><td>Ambulatory activities (min/d)</td><td>46 (8)</td><td>41-52</td></tr></tbody></table>

Values are presented as mean (SD).

## 4.3. Effects of the RVE on energetic and physical activity outcomes during bedrest

Energetic outcomes are presented in Fig. 3 and Table 2. A significant reduction in total energy expenditure occurred in the initial half of the bedrest period and persisted over the second half in both groups (CNT: 1.67 MJ/d [SE 0.40]; RVE: 1.41 MJ/d [0.38]; bedrest P < 0.001), with no between-treatment difference (ETD = 0.26 MJ/d; 95 %CI -0.85 to 1.36; P = 0.64; Cohen's d = 0.07). The decrease in absolute total energy expenditure during bedrest was related to decreases in both absolute resting metabolic rate (bedrest P < 0.001) and absolute activity-related energy expenditure (bedrest P < 0.001), all remaining significant after adjustment for fat mass and lean body mass (Table 2).

The RVE intervention limited the bedrest-induced drop in absolute resting metabolic rate by about 40 % compared to the CNT condition (ETD = 0.19 MJ/d; 95 %CI -0.02 to 0.36; P = 0.03) but not in absolute activity-related energy expenditure (ETD = 0.04 MJ/d; 95 %CI -1.04 to 1.12; P = 0.94). After adjustment for lean body mass and fat mass, the preventing effect of RVE on resting metabolic rate changes was only slightly reduced and remained nearly significant (ETD = 0.16; 95 %CI -0.02 to 0.34; P = 0.08).

Accelerometry data provided additional evidence of the decline in physical activity induced by bedrest (Table 2) with a 75 % reduction in vector magnitude (bedrest P < 0.001), a measure of overall physical activity. The RVE intervention did not mitigate this decrease (ETD P = 0.87).

## 4.4. Effects of the RVE on energy intakes and energy balance

Although participants were encouraged to fully consume the provided meals, whose energy intake was calculated to match the estimated decreased total energy expenditure during the bedrest and account for RVE cost, they were free to eat less. Actual energy intake (Fig. 3 and Table 2), quantified from provided energy intake and leftovers followed a similar pattern as total energy expenditure, with a decrease during the initial half of the bedrest that was maintained over the second half in both groups (CNT: -2.21 MJ/d [0.15]; RVE: -2.02 MJ/d [0.15]; bedrest P < 0.001) The decrease in energy intake during bedrest remained significant after adjustment for fat mass and lean body mass (bedrest P < 0.001) and was not modified by the RVE intervention (ETD = +0.19 MJ/d; 95 %CI -0.24 to 0.61; P = 0.38).

The reduction in energy intake to match the decrease in total energy expenditure was further illustrated by the equilibrated energy balance, as determined from DXA body composition changes over the bedrest period (Fig. 4). No significant between-group difference was noted (CNT = -0.29 MJ/d [0.38]; RVE = 0.18 MJ/d [SD 0.36]; between-groups P = 0.39).

## 4.5. Effects of the RVE on muscular strength and aerobic capacity

Results of muscular strength and aerobic capacity are presented in **supplemental table A (Online Resource)**.

Bedrest induced in a significant decrease in VO₂max (bedrest P < 0.001) and maximal aerobic power (bedrest P < 0.001). The RVE intervention partially attenuated these declines, with significant mitigation observed in both VO₂max (ETD = 4.6 ml/min/kg; 95 %CI 0.3 to 9.0; P = 0.04) and maximal aerobic power (ETD = 33 W; 95 %CI 3 to 62; P = 0.03).

Strength in both knee extensors and flexors significantly decreased during bedrest (bedrest P < 0.001 and P = 0.02, respectively). Similarly, strength in ankle extensors and flexors was reduced (bedrest P < 0.01 and P < 0.001, respectively). The RVE intervention did not significantly counteract these alterations. Notably, the fatigue index remained unchanged throughout the bedrest period (bedrest P = 0.25).

<footnote>a. Accelerometer-derived variables were measured during the 7-day ambulatory period in the MEDES testing facility prior to the HDT period.</footnote>
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 2.** Changes in body mass and composition during the 21-day bedrest with or without an RVE countermeasure. Values are LS means (SE) (a, b, and c), and estimated trial-differences (ETD) (d, e and f) presented with their 95 % confidence interval (95 %CI), both being from linear mixed-effects model accounting for repeated measurements. BM= Body mass; LBM = lean body mass; FM= Fat mass.</figcaption></figure>

## 4.6. Effects of the RVE trial on well-being, mood, and sleep quality

Results of psychometric tests during bedrest are detailed in **supplemental Table B (Online Resource)**. Bedrest was associated with a deterioration of well-being, emotional distress, and sleep quality, as indicated by the increase in GHQ28 (bedrest P < 0.001), POMS (bedrest P < 0.01), and PSQI (bedrest P = 0.02) global scores with no significant impact of the RVE intervention.

## 5. Discussion

Exercise is the cornerstone of the countermeasure programs aiming to preserve astronauts' health and performance during space missions. However, high volumes of combined aerobic and resistive exercise training do not entirely prevent lean body mass loss in microgravity conditions but also induce an energy deficit, contributing to potential health risks during long-term planetary missions [4]. This study was a

part of the MNX bedrest protocol aiming to assess the effects of an RVE countermeasure in mitigating the detrimental effects of 21 days of bedrest on muscle and bone mass. It specifically explored the hypothesis that RVE could contribute to maintaining lean body mass without altering energy balance regulation during 21 days of bedrest. Current findings indicate that the RVE intervention did not alter the coupling between energy intake and total energy expenditure, allowing a stable energy balance to be maintained throughout the bedrest period. However, while the intervention effectively limited the decline in resting metabolic rate during bedrest, it only partially prevented lean body mass loss by around 25 %, without statistical significance. Notably, although it did not preserve lower limb muscle strength, it partially lessened the decrease in aerobic capacity and overall muscle function, as measured by maximal aerobic power. Reductions in self-perceived fatigue and mood were not mitigated by the RVE intervention.

From an energetic standpoint, RVE is an interesting strategy with limited and non-significant impact on total energy expenditure. Unlike
<caption>**Table 2**</caption>

Changes in anthropometric, energetic, and accelerometer-derived physical activity outcomes during the 21-day bedrest with or without an RVE countermeasure.

<table><thead><tr><th rowspan="2"></th><th colspan="2">Baseline</th><th colspan="2">Mid bedrest (changes)</th><th colspan="2">End of bedrest (changes)</th><th rowspan="2">Bedrest P</th><th colspan="4">ETD in end-changes from baseline</th></tr><tr><th>CNT</th><th>RVE</th><th>CNT</th><th>RVE</th><th>CNT</th><th>RVE</th><th>LS means (SE)</th><th>95%CI</th><th>P</th><th>Cohen's d</th></tr></thead><tbody><tr><td colspan="12"><strong>Anthropometric variables</strong></td></tr><tr><td>Body mass (kg)</td><td>70.7 (2.1)</td><td>70.7 (2.1)</td><td>-1.52<br>(0.22)</td><td>-1.18<br>(0.21)</td><td>-2.85<br>(0.31)</td><td>-2.06<br>(0.29)</td><td>&lt;0.001</td><td>0.79<br>(0.43)</td><td>-0.07 to<br>1.65</td><td>0.069</td><td>0.28</td></tr><tr><td>Fat mass (kg)</td><td>11.1 (1.1)</td><td>10.7 (1.1)</td><td>-0.13<br>(0.15)</td><td>0.06 (0.14)</td><td>0.20 (0.20)</td><td>0.37 (0.20)</td><td>0.009</td><td>0.17<br>(0.28)</td><td>-0.40 to<br>0.74</td><td>0.558</td><td>0.09</td></tr><tr><td>Lean body mass<br>(kg)</td><td>57.0 (1.8)</td><td>57.3 (1.8)</td><td>-1.40<br>(0.24)</td><td>-1.23<br>(0.23)</td><td>-3.04<br>(0.29)</td><td>-2.41<br>(0.27)</td><td>&lt;0.001</td><td>0.64<br>(0.40)</td><td>-0.16 to<br>1.43</td><td>0.116</td><td>0.23</td></tr><tr><td>Body mass index<br>(kg/m<sup>2</sup>)</td><td>22.7 (0.5)</td><td>22.7 (0.5)</td><td>-0.49<br>(0.07)</td><td>-0.38<br>(0.07)</td><td>-0.91<br>(0.10)</td><td>-0.66<br>(0.09)</td><td>&lt;0.001</td><td>0.25<br>(0.14)</td><td>-0.02 to<br>0.53</td><td>0.072</td><td>0.28</td></tr><tr><td>Fat mass index (kg/<br>m<sup>2</sup>)</td><td>3.6 (0.4)</td><td>3.5 (0.4)</td><td>-0.04<br>(0.05)</td><td>0.01 (0.04)</td><td>0.07 (0.07)</td><td>0.11 (0.06)</td><td>0.009</td><td>0.04<br>(0.09)</td><td>-0.14 to<br>0.22</td><td>0.647</td><td>0.07</td></tr><tr><td>Lean body mass<br>index (kg/m<sup>2</sup>)</td><td>18.3 (0.4)</td><td>18.4 (0.4)</td><td>-0.45<br>(0.08)</td><td>-0.39<br>(0.07)</td><td>-0.98<br>(0.09)</td><td>-0.76<br>(0.09)</td><td>&lt;0.001</td><td>0.21<br>(0.12)</td><td>-0.04 to<br>0.46</td><td>0.092</td><td>0.25</td></tr><tr><td colspan="12"><strong>Energetic variables<sup>a</sup></strong></td></tr><tr><td>Non-adjusted</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total energy<br>expenditure<br>(MJ/d)</td><td>10.7 (0.4)</td><td>10.7 (0.4)</td><td>-1.49<br>(0.40)</td><td>-1.75<br>(0.38)</td><td>-1.67<br>(0.40)</td><td>-1.41<br>(0.38)</td><td>&lt;0.001</td><td>0.26<br>(0.55)</td><td>-0.85 to<br>1.36</td><td>0.641</td><td>0.07</td></tr><tr><td>Activity-related<br>energy<br>expenditure<br>(MJ/d)</td><td>3.1 (0.3)</td><td>3.2 (0.3)</td><td>-1.09<br>(0.39)</td><td>-1.38<br>(0.37)</td><td>-1.06<br>(0.39)</td><td>-1.02<br>(0.37)</td><td>&lt;0.001</td><td>0.04<br>(0.54)</td><td>-1.04 to<br>1.12</td><td>0.943</td><td>0.01</td></tr><tr><td>Actual energy<br>intake (MJ/d)</td><td>10.1 (0.3)</td><td>10.2 (0.3)</td><td>-1.98<br>(0.15)</td><td>-1.76<br>(0.15)</td><td>-2.21<br>(0.15)</td><td>-2.02<br>(0.15)</td><td>&lt;0.001</td><td>0.19<br>(0.21)</td><td>-0.24 to<br>0.61</td><td>0.378</td><td>0.13</td></tr><tr><td>Adjusted</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Total energy<br>expenditure<br>(MJ/d)</td><td>10.5 (0.4)</td><td>10.6 (0.4)</td><td>-1.35<br>(0.40)</td><td>-1.61<br>(0.38)</td><td>-1.33<br>(0.41)</td><td>-1.13<br>(0.39)</td><td>&lt;0.001</td><td>0.20<br>(0.54)</td><td>-0.90 to<br>1.29</td><td>0.718</td><td>0.06</td></tr><tr><td>Activity-related<br>energy<br>expenditure<br>(MJ/d)</td><td>3.1 (0.3)</td><td>3.2 (0.3)</td><td>-1.09<br>(0.35)</td><td>-1.37<br>(0.34)</td><td>-1.02<br>(0.36)</td><td>-0.98<br>(0.34)</td><td>&lt;0.001</td><td>0.04<br>(0.49)</td><td>-0.94 to<br>1.02</td><td>0.934</td><td>0.01</td></tr><tr><td>Actual energy<br>intake (MJ/d)</td><td>10.1 (0.2)</td><td>10.0 (0.2)</td><td>-1.85<br>(0.16)</td><td>-1.65<br>(0.15)</td><td>-1.93<br>(0.18)</td><td>-1.80<br>(0.16)</td><td>&lt;0.001</td><td>0.13<br>(0.22)</td><td>-0.30 to<br>0.56</td><td>0.552</td><td>0.09</td></tr><tr><td colspan="12"><strong>Resting metabolic rate<sup>b</sup></strong></td></tr><tr><td>Non-adjusted</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Resting metabolic<br>rate (MJ/d)</td><td>6.5 (0.2)</td><td>6.5 (0.2)</td><td>-0.25<br>(0.05)</td><td>-0.19<br>(0.04)</td><td>-0.44<br>(0.06)</td><td>-0.25<br>(0.06)</td><td>&lt;0.001</td><td>0.19<br>(0.08)</td><td>0.02 to<br>0.36</td><td>0.026</td><td>0.33</td></tr><tr><td>Adjusted</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Resting metabolic<br>rate (MJ/d)<sup>b</sup></td><td>6.4 (0.2)</td><td>6.4 (0.2)</td><td>-0.18<br>(0.06)</td><td>-0.13<br>(0.05)</td><td>-0.30<br>(0.09)</td><td>-0.14<br>(0.08)</td><td>0.005</td><td>0.16<br>(0.09)</td><td>-0.02 to<br>0.34</td><td>0.076</td><td>0.26</td></tr><tr><td colspan="12"><strong>Accelerometer-derived variables</strong></td></tr><tr><td>Vector magnitude<br>(cpm/d)</td><td>385,960<br>(12,554)</td><td>387,831<br>(11,648)</td><td>-264,885<br>(14,073)</td><td>-27,5327<br>(13,062)</td><td>-28,0591<br>(14,794)</td><td>-28,3807<br>(13,062)</td><td>&lt;0.001</td><td>-3215<br>(19,735)</td><td>-42,906 to<br>36,475</td><td>0.871</td><td>-0.02</td></tr></tbody></table>

Unless otherwise indicated, values are LS means (SE) from linear mixed-effects model accounting for repeated measurements. Total energy expenditure, activity-related energy expenditure, and resting metabolic rate are presented in absolute values (i.e., non-adjusted) and adjusted for lean body mass and fat mass.

CNT = control; RVE = resistive-vibration exercise; ETD = estimated trial difference.

a Total energy expenditure, activity-related energy expenditure and actual energy intake are calculated over entire bedrest half-periods (from HDT1 to HDT10 and HDT10 to HDT21) respectively.

b Resting metabolic rates are average of measures performed at the beginning and at the end of each bedrest half-periods.

aerobic exercise with or without resistive exercise training [5,29], current investigation found that the RVE intervention does not disrupt energy balance regulation. The rapid decline in total energy expenditure, primarily attributed to reduced physical activity energy expenditure, occurred in both the RVE and CNT trials during the initial bedrest phase, stabilizing subsequently. Accordingly, accelerometry data revealed a substantial reduction in vector magnitude, a proxy of overall physical activity, at the onset of the bedrest period, with no discernible difference between the RVE and control groups. Resting metabolic rate also decreased in these subjects during the bedrest period. The nearly 7 % reduction in the control group was of the same order as the commonly reported 10 % decrease in bedrest studies with no exercise countermeasure [6,30]. In contrast, while aerobic exercise training was shown to maintain a stable resting metabolic rate [30], the present RVE intervention only mitigated the drop in resting metabolic rate by 40 %. This is in line with the incomplete prevention of lean body mass loss. Remarkably, the decline in resting metabolic rate and the protecting

effect of RVE remained significant after adjusting for both fat mass and lean body mass as previously observed [6], thus suggesting that mechanisms other than the changes in body mass and composition contribute to the observed resting metabolic rate variations in the CNT and RVE groups. These mechanisms can include changes in sympathetic nervous activity and norepinephrine levels that were shown to increase in response to strength training in healthy older men [31] while decreasing during prolonged bedrest [32]. Additionally, myokines released from the skeletal muscle during contractions [33] and thyroid hormones [34] may also stimulate thermogenesis and contribute to adaptations in resting metabolic rate not directly linked to changes in lean body mass. Notably, the RVE intervention was associated with a slight but statistically significant higher body core temperature during the bedrest, as compared to the control group (data not shown).

Energy intake prescriptions were adjusted during the bedrest to maintain fat mass balance. Even if participants were not provided an *ad libitum* diet, they were allowed to leave their meals unfinished. Notably,
<figure><img src="image_12.png" /><figcaption>**Fig. 3.** Changes in energetic outcomes during the 21-day bedrest with or without an RVE countermeasure. Values are LS means (SE) from linear mixed-effects model accounting for repeated measurements. BDC = period from BDC-7 to HDT1; 1st half-HDT from HDT1 to HDT10; 2nd half-HDT = period from HDT10 to HDT21. TEE = total daily energy expenditure; AEE = activity-related energy; RMR = resting metabolic rate; EI = energy intake; ETD = estimated trial-difference.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 4.** Energy balance (EB) calculated from changes in body composition during the 21-day bedrest for the CNT and RVE trials. Values are mean (SE) during the full HDT period.</figcaption></figure>

minimal leftovers were reported. Although appetite sensations were not directly measured, the limited leftovers suggest that appetite might not have been significantly altered during the 21-day bedrest, regardless of the RVE countermeasure. This coupling between total energy expenditure and energy intake during bedrest aligns with findings from the control group (no exercise) in the WISE bedrest study [5]. However, in contrast to the current results in the RVE group, a greater reduction in energy intake compared to total energy expenditure was reported in the combined aerobic and resistive exercise group of the WISE study, thus leading to fat mass loss [5]. This suggested a potential anorexigenic effect of the combined aerobic and resistive exercise training as previously observed in space [4]. Taken together, these observations support existing models that describe a linear relationship between energy expenditure and intake, which helps to maintain energy balance, and consequently, body mass and composition [35-37]. However, this

relationship might be impaired when physical activity levels become extreme or deviate significantly from habitual patterns, potentially leading to changes in body mass and composition [38]. This underscores the complex challenge of optimizing energy requirements for long-term spaceflights.

Contrary to the initial hypothesis, as already reported by Kenny et al. [39], the RVE program used in this study did not fully prevent the loss of total and lean body mass. Furthermore, it did not attenuate the decline in bedrest-induced muscle strength. Magnetic resonance imaging also indicated that it did not specifically prevent muscle atrophy in the spinal musculature of subjects from the MNX study [12]. This contrasts with the protecting effect of RVE on lumbar, abdominal, hip, and thigh muscle mass, and to a lesser degree on leg muscle strength, observed in the 2nd Berlin 60-day bedrest study [8,40-42]. Despite a similar RVE regimen sequence during sessions between the latter study and the present one, the reduction in frequency from 3 to 2 sessions/week in the current MNX study may explain the lack of impact on muscle mass and strength. This aligns with observations from prolonged spaceflights where the least active astronauts lost lean body mass while maintaining fat mass [4], thus supporting that a physical activity workload needs to reach a minimal level to be effective. The protective effects of RVE on specific muscle mass and strength were even more pronounced in the first Berlin bedrest study, during which participants engaged in two RVE sessions per day, amounting to 11 sessions per week, underlining its significant impact [8,43,44].

Training load and modalities can also be examined in light of the 90-day LTBR study, where flywheel-based resistive training enabled male adults to achieve lean body mass and fat mass maintenance [6]. As participants engaged in two sessions lasting 35 min per week, similar to the present MNX study, it is plausible that continuous cycling exercise may induce a higher muscular stimulus than the short-duration total tension time (2-to-6 min per session) in the current study. Importantly, because this study did not include a group with a resistive exercise countermeasure alone, results could not delineate the independent and combined effects of resistance exercise training versus vibration.
Taken altogether, findings from prior bedrest studies underscore the importance of providing a substantial muscle stimulus to counteract bedrest-induced muscle atrophy [12]. Recently, high-intensity interval training (HIIT) combining resistive and aerobic exercise has emerged as a potential alternative that could address the limitations of the current exercise countermeasures. Indeed, Matsuo et al. [45,46] showed that a HIIT protocol performed on ergocyte was associated with lower exercise energy expenditure than continuous ergocyte exercise, as currently prescribed onboard the International Space Station. However, this finding should be considered cautiously, given how challenging it is to match exercise modalities precisely for session duration or total workload [47,48]. Moreover, intermittent training is suspected to reduce 30- to 90-min post-exercise hunger perception more than usual continuous training [49]. Nevertheless, appetite control is a complex process [50], and different time/intensity ratios and individuals' characteristics (i.e., habitual physical activity level or body composition) may elicit various responses in appetite sensations, thus affecting food intake. In any case, these limitations require careful consideration in exercise prescription for long-term spaceflight, especially considering that space anorexia is commonly reported during these missions [51].

The current findings indicate that the RVE does not impair the regulation of energy balance. Even if the RVE effect on lean body mass was limited, this absence of detrimental effect should be considered in conjunction with its known protective role against bedrest-induced insulin resistance, impaired mitochondrial metabolism, alterations in protein synthesis and degradation pathways [39,52], and reduced aerobic capacity [10]. Altogether these data further highlight the potential benefits of RVE as a countermeasure during extended periods of immobility. However, further studies are needed to evaluate the effect of i) an increased total workload with adequately stimulated muscles to fully prevent muscle mass loss and function and ii) RVE protocols, including whole-body exercises as currently prescribed for aerobic and resistive training onboard the International Space Station to optimize the benefits of training.

Finally, strengths and limitations should be acknowledged. While prior research indicated that acute exercise combined with vibrations increases energy expenditure above resting levels [53,54], there was a notable dearth of information on the sustained impact of this exercise modality on total energy expenditure over more extended periods, such as days or weeks. This pioneering study filled this knowledge gap by objectively and accurately measuring total energy expenditure and its components using the gold standard methods of doubly labelled water and indirect calorimetry. This study also highlighted that RVE exerted a partial preventive effect on aerobic capacity. However, the study has limitations. The relatively small number of participants may have constrained the statistical power to detect significant between-trials differences in total energy expenditure. Nevertheless, the crucial observation is that the impact of RVE on total energy expenditure was modest and did not alter energy intake, energy balance, and fat mass maintenance. Another limitation was related to the food prescription methodology. Participants were allowed to consume less food than the provided meals, but an *ad libitum* approach was not adopted, and participants were restricted from requesting additional food. Also, the study did not measure appetite sensation or gut hormones, limiting the comprehensive interpretation of the RVE intervention's impact on food intake regulation. Lastly, the exclusive inclusion of male participants implies that these results may not apply to females.

## 6. Conclusion

Resistive exercise, coupled with whole-body vibration, was investigated as a potential strategy to mitigate microgravity-induced losses in body mass and lean body mass loss, thereby preserving functional capabilities. The present findings indicate that the RVE intervention used in this study, while not impacting energy balance, only partially preserved lean body mass. Although it did not protect lower limb muscle

strength, it partially attenuated the decline in aerobic capacity and overall muscle function induced by bedrest. Therefore, ongoing optimization of exercise countermeasure studies remains vital to ensure the health and well-being of astronauts, especially by integrating both aerobic and resistive exercises at optimal intensity and duration ratios.

## CRediT authorship contribution statement

**Pierre Bourdier:** Writing – review & editing, Writing – original draft, Formal analysis. **Alexandre Zahariev:** Writing – review & editing, Data curation. **Floriane Rudwill:** Writing – review & editing, Data curation. **Helena Kenny:** Data curation. **Donal J. O’Gorman:** Funding acquisition, Data curation. **Isabelle Chery:** Writing – review & editing, Formal analysis, Data curation. **Marie-Pierre Bareille:** Writing – review & editing, Conceptualization. **Guillemette Gauquelin-Koch:** Writing – review & editing, Conceptualization. **Sylvie Normand:** Writing – review & editing, Data curation. **Stéphane Blanc:** Writing – review & editing, Funding acquisition, Data curation, Conceptualization. **Audrey Bergouignan:** Writing – review & editing, Funding acquisition, Data curation, Conceptualization. **Chantal Simon:** Writing – review & editing, Writing – original draft, Formal analysis, Data curation, Conceptualization.

## Data statement

Data are accessible upon reasonable request to the authors.

## Fundings

This work was supported by was funded by the French Space Agency (CNES), the European Space Agency (ESA), the French National Center of Scientific Research (CNRS), and funding to Pierre Bourdier from the Graduate School of Life Science (ED 414) of the University of Strasbourg.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors thank all the participants and the staff at MEDES who contributed to the success of this bedrest study.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.actaastro.2025.03.029.

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