Elsevier

Acta Astronautica

Volume 210, September 2023, Pages 29-35
Acta Astronautica

Axial reloading during body weight unloading: Relationship between g-level and cardiorespiratory responses to running – A case study

https://doi.org/10.1016/j.actaastro.2023.05.008Get rights and content

Highlights

  • Changes in gravitational conditions leads to physiological adaptation.

  • Human locomotion is affected by gravity, such that locomotion changes have been observed between Lunar and Earth.

  • Lunar and Martian conditions can be simulated to study locomotion effects on Earth.

  • Countermeasures are required to support the optimal function of the human body.

  • These must be studied and evaluated in simulated conditions to understand compatibility.

Abstract

Objective

Prolonged microgravity exposure induces physiological de-conditioning that is partially mitigated by aerobic exercise. De-conditioning is also anticipated during partial gravity habitation (e.g., Moon or Mars). However, the relationship between gravity, resultant running speed, associated biomechanics, and the cardiorespiratory response is unknown. Thus, this case study evaluated responses to self-selected running across a continuum of simulated gravity levels generated by bodyweight unloading and axial loading via the Mk III Gravity Loading Countermeasure SkinSuit (GLCS).

Methods

A healthy male ran at a self-selected speed for 5 min at six simulated gravity (Gz) levels. Bodyweight suspension unloaded the subject's mass to simulate Martian (0.38Gz) and Lunar (0.16Gz) gravity, whilst the GLCS reloaded the subject by 0.8Gz, thereby simulating 1.8Gz and 0.96Gz – comparable to 1Gz. Gait kinematics, heart rate (HR), respiratory variables, subjective ratings were evaluated in the final minute of each run and evaluated across conditions.

Results

Cardiorespiratory, biomechanical and subjective responses were broadly similar in 1Gz and 0.96Gz (GLCS + Lunar). Positive linear associations were observed between Gz and minute ventilation (VE) oxygen consumption (VO2) and cost of transport responses to self-selected running, where HR responses correlated with self-selected running speed only.

Conclusion

Respiratory and biomechanical responses (but not HR) to self-selected running appear related to simulated Gz. Significance: Mk III GLCS is able to induce broadly G-equivalent biomechanical and cardiorespiratory responses, and thus may be a tool to facilitate g-dependence research and hypogravity exercise.

Introduction

Humans have evolved in the presence of Earth's gravity: shaping anatomy and physiology at rest, and during motion [1,2]. As a result, prolonged exposure to microgravity (μG) on the International Space Station (ISS) is associated with skeletal [3], muscular [4], and cardiorespiratory de-conditioning [5]. This de-conditioning is observed, albeit to variable extents [6] despite daily performance of exercise countermeasures, which includes treadmill-based running [7]. Extended habitation in low (hypo)gravity is also anticipated to induce de-conditioning, although whether it will be as profound as in μG is currently unknown, although even confinement (and thus reduced activity in 1G) is associated with physiological de-conditioning [8]. Thus, whilst de-conditioning in μG has limited consequences in-flight, de-conditioning in hypogravity may present significant operational challenges [9].

Furthermore, despite Apollo astronauts having walked/hopped/skipped significant distances on the Lunar surface [10], the inter-relationship between gravity, the biomechanics of running, and the cardiorespiratory responses required to support such locomotion are unknown. On Earth, partial unloading (i.e., hypogravity) via body suspension systems has been shown to result in lower speeds [11] and reduced ground reaction forces [12], in addition to prolonged ground contact and flight times broadly consistent with the loping gait observed on the Lunar missions. Such gait is also associated with reduced rates of oxygen consumption during both walking and running [13,14].

On Earth, backpack (mass) loading is known to significantly affect walking [15] and running [16] biomechanics, elevating its metabolic cost. Whilst on the Lunar surface the Apollo astronauts wore Extra-Vehicular Activity (EVA) suits with a Portable Life Support System (PLSS) backpack. Despite being in 1/6th of Earth's Gz, the ∼90 kg PLSS displaced crewmembers centre of mass (CoM) backwards, requiring compensatory forward leaning and negatively affecting stability [10], thereby potentially contributing to falls [17].

Interestingly, skipping was observed on the Lunar surface, and has been proposed as biomechanically efficient by virtue of incorporating both walking (pendulum-like) and running (elastic bouncing) mechanical energy savings [18]. However, more recent findings during simulated hypogravity, have shown skipping's metabolic economy to be similar to running (2 J·kg1·m1) [19]. As a result, the skipping seen on the lunar surface was presumably also driven by suit-related factors, such as reducing the mechanical work to move the limbs – in particular, hip flexion-extension as the suits were pressurized and created strain energy when compressed. Thus, whilst the effect of Lunar hypogravity per se, and gravity generally, upon locomotion are unknown.

Understanding the relationship between gravity and running responses is important as running is a candidate exercise modality on the Lunar surface – either in lunar hypogravity, or with some form of axial reloading. Whilst treadmill running on the ISS with the subject loading system is uncomfortable [7], reloading in some form may be a pragmatic approach given the operational constraints compared to those currently on the ISS [20]. However, even low mass loading has been shown to generate sub-optimal gait biomechanics including prolongation of ground contact times [21] and thus poor metabolic economy, and exaggerated knee flexion potentially reflecting attempts to maintain the CoM within the base of support [22] and may even increase injury risk [23].

A novel alternative approach may be the Mk III Gravity Loading Countermeasure Skinsuit (GLCS), a lightweight (<500 g) garment based on graduated elastic principles [24], which provides an axial compressive load of ∼0.8Gz via accumulation of tension (resistance to longitudinal stretch) from the shoulders to the feet proportional to the estimated body mass at that segment. Whilst high-resistance lycra-based garments have been employed to restrict, constrain, or negate inappropriate movement such as with developmental dyspraxia [25], donning the Mk III GLCS has been shown to be tolerable during moderate cycle ergometer exercise [26] and locomotion (including 3 m get up and go) [27] in 1Gz as part of compatibility evaluation for spaceflight. As a result, the Mk III GLCS may afford the opportunity to provide a ‘gravity-like’ stimulus - potentially compatible with actual, and simulated Lunar and Martian hypogravity.

However, whether such an approach is feasible, tolerable and able to induce G-equivalent biomechanical and cardiorespiratory responses is unknown. Indeed, studies evaluating walking and running in simulated hypogravity suggest that whilst metabolic costs are broadly reduced, the specific biomechanics and thus resultant cost of oxygen transport (COT) are highly sensitive to several factors, including gravity level [28], unloading method [2], and resultant gait speed [29].

Thus, this case study evaluated the relationship between biomechanical, cardiorespiratory and subjective responses to self-selected running across a continuum of simulated Gz levels from 0.16 to 1.8Gz, including comparison of actual, and simulated 1Gz, by employing a unique combination of body suspension and axial loading via the Mk III GLCS.

Section snippets

Methods

The volunteer (male; 72 kg; 1.70 m; 26 yr; healthy, moderately physically active (a gymgoer 2–3 times a week) with no musculoskeletal or cardiorespiratory conditions) gave written informed consent to participate in the study, that received approval by the Ethics Committee of Ponticia Univ Catolica do RGS (273.668) and was conducted in accordance with the Helsinki Declaration of 1975, as revised in 2000.

The volunteer attended the laboratory on three occasions, more than 2 days apart. The first

Results

The volunteer successfully completed all runs, although in both attires discomfort was reported at the harness pressure points.

Self-selected running speed was 63% lower, whereas stride length was essentially maintained (9% lower) in the Lunar GLCS (1Gz) vs. Earth GYM condition (Table 1). Knee and ankle angle were similar (4% and 5% higher values respectively obtained in the Lunar GLCS (1Gz) condition).

Running speed was reduced with greater bodyweight suspension (weight unloading) in both

Discussion

This case study is the first to utilize the combination of body suspension and an axial-loading (Mk III GLCS) textile-based suit to create a continuum of simulated Gz levels below, around, and above 1Gz, including those pertinent to human exploration i.e., Lunar and Martian gravities. The main findings were that self-selected running speed was lower, whereas stride length, knee angle, ankle angle, RPE and thermal comfort were similar in the Lunar Unloading + GLCS (yielding 1Gz) vs. Earth GYM

Conclusion

This case study utilized a unique combination of body suspension and the GLCS axial loading suit to evaluate a continuum of simulated Gz levels from 0.16Gz to 1.8Gz. It is the first to establish broad cardiorespiratory, biomechanical and subjective equivalence between 1Gz and the GLCS + Lunar unloaded 1Gz simulation. Furthermore, positive linear associations between Gz loading and VE, VO2 and COT responses to self-selected running, in addition to RPE were observed, whereas HR responses

Funding

The Mk III Gravity Loading Countermeasure SkinSuit (GLCS) was loaned having been fabricated for another study funded by the Space Medicine Team of the European Space Agency. K.L and V.J were supported as part of the Space Physiology & Health MSc at King's College London, UK.).

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.

Acknowledgment

The study received no direct funding although the authors would like to thank the European Space Agency Space Medicine Team for the loan of the Mk III Gravity Loading Countermeasure SkinSuit (GLCS). K.L's travel to Brazil was supported by King's College London, UK as part of their Space Physiology & Health MSc course. V.J was supported by a NIHR Clinical Research Fellowship. We also thank Ms Thilini Subasinghe for her support in data collection.

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