Igor B. Mekjavic

Acute heat stress reduces cerebral haemodynamics and orthostatic tolerance. However, the haemodynamic response to orthostasis during prolonged heat stress typical of heatwaves is unknown. We hypothesised that thermoregulatory‐induced elevations in peripheral haemodynamics during 3 days of simulated heatwave would exacerbate orthostatic‐induced reductions in cerebral haemodynamics observed in temperate conditions. Seven males (22 ± 1 years) maintained a standard work/rest ratio during temperate (∼25.4°C) and heatwave (∼35.4°C) conditions. Gastrointestinal core, mean skin and body () temperatures, haemoglobin concentration, mean arterial pressure (MAP), cerebral and peripheral haemodynamics (middle cerebral artery blood velocity (MCA _v ), internal carotid, vertebral, and superficial femoral artery blood flow (sFA _bf )) were assessed at rest. MAP, middle cerebral artery vascular conductance index (MCA _VCi ), and superficial femoral artery vascular conductance (sFA _VC ) were measured during orthostasis caused by 60° head‐up tilt. Heatwave increased resting (+0.76 ± 0.24°C, P  < 0.001, d =  3.02 vs. temperate), haemoglobin concentration ( P  = 0.004), sFA _bf ( P  = 0.001) and sFA (+34.0 ± 14.0 mL O ₂ min ⁻¹ , P  < 0.001, d  = 2.61) and global cerebral (+21.3 ± 16.8 mL O ₂ min ⁻¹ , P  = 0.009, d =  0.71) oxygen delivery, whereas MAP ( P  = 0.729) and MCA _v ( P  = 0.687) were unchanged. During tilt, MCA _VCi reductions were comparable between conditions (day × posture interaction, P  = 0.959; posture main effect, −0.19 ± 0.06 cm s ⁻¹ mmHg ⁻¹ , P  < 0.001, d  = 2.05) whilst the heatwave‐induced elevation in sFA _VC was reduced (interaction, P  = 0.008). In conclusion, orthostatic‐induced reductions in cerebral haemodynamics were not exacerbated across heatwave days, whereas the heatwave‐induced elevation in resting supine peripheral haemodynamics was attenuated during orthostasis. Additionally, resting global cerebral oxygen delivery was increased during the latter heatwave days. These divergent cerebral and peripheral haemodynamic responses in males suggest cerebral perfusion and oxygen delivery are prioritised during prolonged heat stress.

Head‐up tilt (HUT) and lower‐body negative pressure (LBNP) both reduce central blood volume but differ in heart orientation and regional fluid shifts. Whether they evoke comparable cardiovascular regulation when matched for preload remains uncertain. We characterized differences in fluid redistribution and haemodynamic responses during graded HUT and LBNP at intensities eliciting equivalent reductions in thoracic blood volume (TBV). In a randomized crossover design, 30 young adults (16 healthy control subjects and 14 with type 1 diabetes) underwent both HUT (22°, 42°, 58° and 80°) and LBNP (−10, −20, −35 and −50 mmHg). Segmental bioimpedance, haemodynamic and ECG responses were recorded continuously. Equivalent reductions in TBV were obtained for 22° vs. −10 mmHg, 42° vs. −20 mmHg, and 80° vs. −35 mmHg (all P > 0.9). However, regional fluid shifts differed, in that HUT induced splanchnic pooling, whereas LBNP promoted pelvic pooling. At matched reductions in TBV, stroke volume decreased more during HUT than during LBNP ( P = 0.016). Cardiac output was maintained during HUT owing to more pronounced tachycardia, but it declined during LBNP at −35 mmHg. Type 1 diabetics exhibited consistently higher heart rate across all conditions, but preserved stroke volume, cardiac output and mean arterial pressure relative to control subjects. Although HUT and LBNP generate comparable reductions in TBV, they elicit distinct cardiovascular adjustments. HUT engages gravitational influences that help to maintain cardiac output, whereas LBNP isolates baroreflex‐mediated responses, underscoring the unique contribution of gravity to orthostatic regulation. Collectively, these findings refine interpretation of orthostatic models and highlight their utility for detecting early reductions in autonomic reserve in type 1 diabetes.

Artificial gravity (AG) combined with resistive vibration exercise has been proposed as a multi‐system countermeasure for long‐duration spaceflight; however, its operational feasibility during prolonged unloading remains insufficiently characterised. This study primarily evaluated the feasibility, tolerability, and adherence to a structured resistive vibration exercise protocol delivered either under artificial gravity (AGRVE) or in horizontal loading (HRVE) during 60 days of hypoxic ( = 0.14) 6° head‐down tilt bed rest. Secondary observations included delayed‐onset muscle soreness (DOMS) and changes in eight‐repetition maximum strength (8RM) to inform load progression. Sixteen healthy male participants were allocated to HRVE ( n  = 9) or AGRVE ( n  = 8) and completed near‐daily 30‐min training sessions at light (30% 1RM) and moderate (50% 1RM) intensities. Adherence was high in both groups (HRVE: 98.5%; AGRVE: 96.3%). In AGRVE, early‐session interruptions were primarily due to motion sickness or presyncopal symptoms; motion sickness severity declined significantly across repeated centrifugation exposures ( P < 0.001), consistent with vestibular habituation. HRVE interruptions were predominantly fatigue‐related. DOMS remained low throughout the intervention (typically 0–3/10). 8RM increased over time in both groups ( P < 0.001). Progressive resistive vibration exercise can be implemented with high adherence during prolonged hypoxic bed rest, including when combined with AG. Motion sickness represents the principal early operational constraint specific to squat‐based AG exposure but diminishes with repeated sessions.

We evaluated the feasibility and efficacy of a 2‐week training programme comprising resistance vibration exercise (RVE) without and with artificial gravity (AG). Participants ( n = 24) were divided into three groups: (i) URVE: upright loaded squat exercise; (ii) HRVE: horizontal loaded squat exercise; and (iii) AGRVE: loaded squat exercise conducted on a short‐arm human centrifuge. All participants followed the same protocol and were exposed to the same ground reaction force, whilst performing exercise comprising bilateral squats, triple extension squats and single/bilateral calf raises. Before and after the 2‐week training period, we measured thigh and calf muscle strength with isokinetic dynamometry, muscular power with a jump test, volume with functional muscle magnetic resonance imaging, and body composition with dual‐energy X‐ray absorptiometry. All groups showed significant improvements in eight‐repetition maximum squat strength ( P < 0.0001, G > 0.80), whilst only the AGRVE group demonstrated a small effect in jump height ( G = 0.26). The AGRVE group significantly increased knee extension and flexion maximum voluntary contraction (MVC), with no comparable changes in the HRVE or URVE groups. The AGRVE group increased total thigh muscle volume ( P = 0.03), with notable hypertrophy in the vastus medialis, semitendinosus, and vastus intermedius muscles. These findings demonstrate that AGRVE is significantly superior to HRVE and URVE in enhancing knee MVC and thigh muscle volume, thus indicating that artificial gravity improves the outcome of resistance vibration exercise in ambulatory participants.

Prolonged exposure to microgravity, simulated via 6° head‐down tilt bed rest (HDT), induces musculoskeletal deconditioning and negatively impacts body composition. This study evaluated whether a combination of aerobic exercise with artificial gravity (AG) offers superior protection in comparison to exercise alone. Twenty‐four healthy male participants completed 60 days of HDT, randomized into control (C), exercise‐only (EX) and exercise with AG (EX‐AG) groups. Muscle volume, intramuscular fat, body composition and isokinetic strength were assessed via whole‐body MRI and isokinetic dynamometry. All groups experienced thigh fat‐free muscle volume loss: C (10.5% ± 2.6%), EX (6.9% ± 2.4%) and EX‐AG (4.3% ± 2.4%), with EX‐AG showing significantly less atrophy than C ( p  < 0.001). Compared with C, EX‐AG preserved more muscle in both anterior ( p < 0.001) and posterior ( p  < 0.05) compartments, whilst EX preserved more muscle only anteriorly ( p  < 0.05). The fat ratio increased more in C (8.9% ± 6.0%) compared with EX‐AG (−0.8% ± 3.8%; p  < 0.05) but not EX (6.5% ± 9.8%). Muscle fat infiltration increased across all groups (C, 7.0% ± 3.7%; EX, 6.2% ± 4.3%; EX‐AG, 3.1% ± 4.7%) but was not different between groups ( p  > 0.05). Maximal isokinetic torque decreased in all groups over all measured angular velocities but was not different between groups ( p  > 0.05). This is the first study to investigate the combination of AG and exercise as a countermeasure to body composition changes induced by long‐term bed rest. We showed that EX‐AG provided partial protection against muscle atrophy and fat accumulation but did not outperform exercise alone in preserving muscle quality, strength or overall body composition.

Development of countermeasures to minimise spaceflight deconditioning is of paramount importance, such as the short-arm human centrifuge (SAHC); however, sex differences in cardiovascular responses should be considered. 20 participants (female = 10, male = 10) conducted an identical centrifugation protocol of 10-min phases of standing (NG), 1Gz centrifugation (1GRF), and 2Gz centrifugation (2GRF). Separated by 10-min each in hot (29.1 ± 0.8 °C) conditions, and either normoxia (PO₂ = 133 mmHg) or hypoxia (PO₂ = 92 mmHg). Haemodynamics, microvascular blood flow (arm and leg) and temperatures (skin and gastrointestinal) were continuously recorded. At 2GRF, females lasted 2.7 ± 1.6 min less than males. Arm blood flow significantly decreased, and leg blood flow increased, in 2GRF (p > 0.001); with higher female leg blood flow in 2GRF (p = 0.002). 2GRF altered all haemodynamic variables and females exhibited significantly higher heart rate, and lower stroke volume index (p < 0.001). Female participants exhibited greater cardiovascular strain, and encountered pre-syncopal symptoms earlier, during 2GRF. Future research should consider individualised and tolerable gravitational loads.

We evaluated whether vibration augmented muscle use during squat exercise with magnetic resonance imaging, with two methods: spin–spin relaxation time (T2 mapping), and volume analysis. Male participants ( n = 13) visited the facility on two occasions. During the first visit, each participant completed a resistance exercise (RE) comprising four sets of 12 repetitions of triple extension squats. Before and immediately after completion of the exercise, magnetic resonance imaging of their thigh and calf muscles was performed. During the second visit, participants performed resistance vibration exercise (RVE) using the same protocol, but standing on a vibration plate that provided a whole-body vibration stimulus (20 Hz, 3.5 mm amplitude) during the exercise. RE promoted increased T2 in vastus lateralis , vastus medialis and vastus intermedius muscles ( p < 0.0001). The same activation was observed for RVE, with no differences compared to RE. The volumetric method was less effective in assessing muscle activation. Only the vastus medialis and vastus intermedius muscle volumes were significantly greater post-exercise compared to pre-exercise for both RE (both muscles: p = 0.0009) and RVE (respectively: p = 0.005 and p = 0.009). No muscular activation was detected for calf either with T2 or volumetric method. Resistance exercise increased T2 and volume of some muscles of the thigh. The addition of whole-body vibration to resistance exercise does not enhance T2 relaxation time or volume accumulation.

Introduction: Resistive exercise combined with whole-body vibration (WBV) and short-arm human centrifugation is being considered as a countermeasure to mitigate muscle and bone loss in astronauts during prolonged space missions. WBV may provide exercise benefits or adverse effects on organs and the lower back. These effects may result from vibration transmitted throughout the body. The objective of this study was to compare vibration transmission (VT) from the ground to the body during resistance vibration exercise (RVE) comprising squat and calf raise performed on a rotational vibration plate. Specifically, we compared VT during RVE in the upright position (URVE) and on a short-arm human centrifuge (artificial gravity, AG) establishing a similar ground reaction force. The latter (AGRVE) is considered a potential countermeasure for microgravity-induced musculoskeletal deconditioning.Methods: Fifteen healthy males participated. They were assigned to two groups: one (n = 8) performed URVE at 20% of 1 repetition maximum (RM) squat; the other (n = 7) performed horizontal RVE during AG exposure (AGRVE), with a matching ground reaction force. Both groups were exposed to vibration at 20 Hz and 3–4 mm displacement (RMS value: 4 ± 0.14 g). VT was recorded during two sets of squats and calf raises. Three accelerometers recorded VT at: (i) the platform surface at the feet, (ii) lower back (L5), and (iii) forehead.Results: In both conditions, the lower limbs attenuated vibration transmission to the lower back (p < 0.0001). During AGRVE, both VT and pelvis octave-band RMS values were lower compared to URVE in both squat (p = 0.008 and p = 0.01) and calf raise (p = 0.007 and p = 0.01), suggesting potentially greater safety for the lower back.Conclusion: During RVE, whether in URVE or AGRVE, lower limbs effectively attenuated vibrations, resulting in negligible pelvic exposure. AGRVE may represent a safer alternative to URVE due to reduced transmission to the lower back and adjacent sensitive regions.