Muscle atrophy and bone demineralization can be induced by unloading, whether through disuse because of injury or illness (17), physical inactivity (27), ageing (2), or exposure to microgravity (34). These and associated cardiovascular deconditioning (28) are high priorities for both space agencies and public health organizations as amelioration would improve functionality and quality of life and reduce injury and mortality risk on Earth and in space. Although acute responses to microgravity (<2 weeks) seem to be moderate and reversible on return to Earth (28), longer term adaptations present serious risks, after prolonged space missions (34,35). Bone demineralization and loss of optimal structural architecture are particularly evident in locations that are typically weight-bearing such as the lumbar spine and trabecular head (27). Therefore, there is an increased fracture risk on re-exposure to gravity (34,35).
Furthermore, microgravity-induced muscle atrophy bears some similarities to age-related muscle loss or sarcopenia (2,29). Consequently, the deleterious effects of microgravity exposure have led to it being termed an “accelerated form of ageing” (34). Such muscle loss is predominantly observed in the postural muscles, with the gastrocnemius and hamstrings atrophying by approximately 20%, after more than 3 months in space or in terrestrial bed rest (19). The ability to generate power in the lower limbs (maximal explosive power, MEP) has also been documented to be effected by microgravity, with 1 astronaut documented to have reduced MEP during vertical jump testing by 67% after 21 days in space (4).
As a result, resistance in addition to functional (aerobic) exercises are major features of the astronaut health maintenance system (34) with emphasis upon “antigravity” muscles as little muscle atrophy and bone demineralization of the arms has been noted (23). Typically, NASA and European astronauts complete 4–6 treadmill, 2–3 cycles, and 6 resistance exercise sessions per week (18). However, this extensive countermeasure regimen has to date failed to completely protect against microgravity-induced physiological deconditioning (16). Furthermore, current countermeasures require significant power, volume, mass, and crew member time (15). Therefore, a new generation of more effective but low-resource intensive countermeasures are required.
Provision of gravitylike axial loading has obvious appeal with hypergravity, via centrifugation being proposed as a countermeasure during long-term space flight (35), to combat either disuse pathology or as a rehabilitation strategy on Earth (19). However, significant engineering and physiological issues such as motion sickness need to be overcome (15). A “simpler” approach is to provide static axial loading to the body. The Russian TNK V-1 Pingvin or “Penguin” suit, which uses bungee cords around the shoulders and feet tethered to a central waist belt, provides significant axial loading during walking (26) and around 70% body weight during treadmill running (13). Cosmonauts who adhered to treadmill exercise with the penguin suit experienced attenuated lumbar vertebrae bone mineral density loss (0–3%), compared with nonadherers (6–10%) (12). Furthermore, wearing the suit for 10 hours a day with 10-kg loading during bed rest preserved Soleus muscle size (20). However, anecdotal reports suggest the Pengvin suit imposes significant thermal and movement discomfort, rendering it inappropriate to be worn for prolonged periods or during exercise. In fact, the majority of cosmonauts refuse to don their suit (32) even though integration with resistance training could reduce both the required workload and length of sessions (13). Such discomfort may originate from the fact that the penguin suit's loading regimen creates pressure points, as it pulls from the central waist belt, to the shoulders and feet, which is not how the body is loaded on Earth, where when standing, segmental axial body weight loading occurs as a result of the pull of Earth's gravity (1 Gz) (18).
The gravity loading countermeasure skin suit (GLCS) has recently been developed using lightweight (<500 g) elastic, porous, bidirectional weaves, to better replicate the cumulative nature of axial loading as experienced on Earth (32, 31). Axial loading is progressively increased via material tension in the vertical axis fibers (with circumferential tension sufficient to prevent suit slippage). It uses each circumferential fiber of its elastic weave as a “belt” to produce numerous vertical stages, from the shoulders to the feet. Stirrups wrapped around shoes (or insoles) distribute the pressure across the sole.
Pilot studies with the first iteration of the GLCS (mark 1) were tested using the parabolic flight analog to simulate microgravity conditions, it was determined that there was a negligible impact on mobility when wearing the GLCS and skin pressure was similar to wearing tight socks (4–10 mm Hg) (18). However, whereas material stretch was assessed to calculate loading during the flight, actual axial loading experienced by the participant was not determined and it is unknown whether the GLCS is tolerable during ambulation, daily task performance, or resistance exercise. Therefore, the aim of this study was to assess axial loading provided by a newly designed SkinSuit, the Mk III GLCS, and thus determine whether the additional axial loading provided by the GLCS affects tolerability and joint range of motion (ROM), perceived exertion, ambulation tasks, and resistance exercise performance.
Experimental Approach to the Problem
Three sessions were conducted within a 7-day period with the first session comprising suit axial loading assessment, familiarization of joint motion and ambulation tests, and the determination of each subject's (safe) 12 repetition maximum (12RM) for 6 selected resistance exercises in loose-fitting gym clothing by completing several sets of each exercise with increasing weight when their technique was carefully monitored (experimenters were qualified fitness instructors). In the subsequent 2 sessions, the entire test battery was repeated, on one occasion when wearing the GLCS, and again at least 48 hours later when wearing gym attire (GYM). This order was randomized and balanced.
Because of the number of GLCS's available, 8 young healthy participants were recruited (5 male: 26 ± 3 years [22–24 years], 182 ± 0.1 cm, and 76.8 ± 6.7 kg; and 3 female: 32 ± 4 years [26–44 years], 170 ± 0.1 cm, and 71.3 ± 4.5 kg) and gave written informed consent to participate in the study, which received local ethics committee approval. All participants denied taking any medication or having a history of neurological, cardiorespiratory, and/or psychological disorders. None of the participants were in pain, and the female knew/suspected that they were not pregnant. Participants were instructed to abstain from vigorous exercise and alcohol for at least 24 hours and from caffeine for at least 2 hours before each session. Testing took place in a quiet, thermoneutral environment (∼23°C; ∼32% humidity).
All participants were provided with a custom-fabricated gravity loading countermeasure skin suit (Costume Works Inc, Boston, MA, USA), which necessitated circumferential measures every 2 cm vertically, from the top of the ankle to the yoke (roughly armpit level) for each subject. Sizing was performed twice to ensure accuracy with a linen tape measure. When participants had donned their suits, they were visually checked to ensure that the bottom of the suit was resting in line with the top of the ankle, as the material strain of the suit had been calculated from this point to the yoke line, based on the previous GLCS research (32, 31).
Axial loading was determined via TekScan (F-Scan; Boston, MA, USA) pressure sensor insoles inserted underneath the shoulder straps and between the sole of the foot and the shoes (flat, rigid-soled trainers to distribute the pressure) with GLCS foot straps fixed around the shoe. TekScan sensors were calibrated with known weights before testing. Two measures were taken, once with the subject wearing the GLCS and shoes but not strapped (i.e., not loaded), to get BASELINE loading, then again when the GLCS ankle straps were looped around the foot and clipped, thus stretching the material and inducing the loading. Bilateral pressure measurements were obtained for 6 seconds when standing upright, with arms relaxed by the sides (n = 8). Total pressure (Newton/m2) when wearing the GLCS was recorded at the foot and the shoulder. Loading recorded when wearing the GLCS was then expressed as an average difference (∆) from the BASELINE (without GLCS attached; 1 Gz)
Height was measured using a standiometer (Cambridge Measuring Systems, Cambridge, United Kingdom) when participants had donned the suit, and at the end of the experiment, subjects were asked to stand shoulder width apart during measurement and to fix their gaze forward. Joint flexibility (maximal ROM) was determined bilaterally from 3 attempts (with measures taken from the best stable attempt) via a bubble Inclinometer (Medical Research Ltd, Leeds, United Kingdom) during knee flexion/extension, hip abduction/adduction, shoulder flexion/extension, and spinal flexion/extension (at both the yoke line and T12, when standing). Back flexibility was assessed via Sit and Reach (33) testing, where participants sat upright on a level surface, with straight legs and bare feet flat against the vertical surface of a Sit and Reach Board. Subjects reached forward as far as possible on 3 occasions with the furthest attempt recorded. Participants were timed performing the Get Up and Go test (21), which required rising from a seated position, walking around a stationary cone (3 m away), and returning safely to the seated position, as quickly as possible.
Participants performed 3 sets of 12 repetitions of each exercise: Dumbbell Shoulder Press and Squat (Free weights; Reebok, China), Machine Chest Press and Seated Row (GLX Strength training system; Bodycraft, Lewis Centre, Ohio, USA), Horizontal Leg Press (Laying leg press; Technogym, Cesena, Italy), and Seated Calf Raise (Ultimate Workout, Nottingham, United Kingdom) at their predetermined 12RM with breaks of 1 minute between sets and 3 minutes between exercises. The technique was observed with improper or incomplete movement leading to exercise termination and the number of completed repetitions, per set, recorded. Core body temperature was monitored throughout with wireless pill telemetry (CorTemp sensor; HQinc, Palmetto, FL, USA). Upon completion of each set, participants rating of perceived exertion (RPE; 6 = rest to 20 = maximum effort), thermal comfort (ASHRAE 7-point; 0 = neutral to 3 = hot) (5), movement discomfort (1 = nude comfort to 10 = too uncomfortable for 10 minutes) (8), and body control (1 = unrestricted to 10 = no control) (7) on scales used to assess space suits (30).
Data were plotted to assess normality in SPSS (histogram, boxplots) with tests of normality (Shapiro-Wilk test [SW test]). Data are reported as mean ± SEM except for changes in height (mean ± SD) and subjective ratings expressed as median (interquartile range). A paired-samples t-test was used to compare the average difference (∆) of loading produced at the foot and shoulder, and the total height and specific height difference between the Calcaneus and Iliac crest were also compared between GYM and GLCS.
As no differences >5° were observed in joint ROM, the averaged best attempt for each side was compared between GYM and GLCS with Student's paired t-tests, except for hip abduction (p = 0.02) and spinal flexion at T12 (p = 0.01), which were nonnormally distributed (SW test) and thus Wilcoxon tests were used. Student's paired t-tests were also used to compare GLCS and GYM for Sit and Reach (centimeter), Get Up and Go (seconds), number of repetitions completed in the final set (third), and average time taken for completion of exercise sets (seconds). Subjective RPE, discomfort, control, thermal comfort, and core temperature change (SW test p ≤ 0.05) after exercise performance were compared with Wilcoxon nonparametric test. Statistical analyses were performed using Statistical Package for Social Sciences 19.0 (SPSS Inc., Chicago, IL, USA) with statistical significance assumed when p ≤ 0.05.
Gravity Loading Countermeasure Skinsuit Loading
The Mk III GLCS provided significant axial loading (∆Gz) in all subjects imparting 0.7 ± 0.3 Gz at the feet, significantly (p < 0.005) greater than that recorded at the shoulders (0.1 ± 0.1 Gz; Figure 1).
The GLCS (178.7 ± 9.6) when standing induced a small nonsignificant reduction of height in 5 of the 8 participants vs. when wearing GYM clothing (179.7 ± 9.9 cm) garments. No difference in height between the Calcaneus and Iliac crest was observed (GYM [66.9 ± 5.1 cm] and GLCS [66.9 ± 3.6 cm]).
Joint Motion and Ambulation
Gravity loading countermeasure skinsuit significantly (p ≤ 0.05) attenuated the ROM of all movements except shoulder extension and hip adduction (Table 1). Sit and Reach was also significantly impaired, whereas Get Up and Go time was prolonged with the GLCS.
Participants were able to complete the 3 sets of 12 repetitions for nearly all the selected strength exercises in both attires. The exception was shoulder press, where a mean of 9 repetitions was completed in the last set (p ≤ 0.05) when wearing the GLCS (Table 2). This in turn significantly reduced the average time to complete the set of shoulder exercises. Core temperature remained unchanged apart from shoulder press where a greater increase was reported after exercise in the GYM condition.
Significant movement discomfort (p < 0.01) and body control impairment (p < 0.01) were induced by the GLCS during all resistance exercises (Table 3).
Rating of perceived exertion was significantly (p ≤ 0.05) higher during shoulder press (vs. GYM) only (Table 4). No significant differences in thermal comfort were reported.
The main findings of the study were that wearing the GLCS (Mk III) provides approximately 0.7-Gz axial loading at the feet, whereas there was a significant reduction in the maximal joint ROM. This only had a minor encumbrance in the ability to perform resistance exercise. Core temperature and thermal comfort during strength exercise did not differ between attire, though there was a significant increase in movement discomfort and control required to perform the exercises. During the initial trials, subjects found donning and doffing the GLCS challenging, especially with getting their shoulders into the garment and tightening the stirrups; however, participants noted this became easier with more practice.
The GLCS provided an additional axial load of approximately 0.7 Gz, albeit with a broad range (0.53–1.12 Gz). Variation in axial loading between subjects seemed not to be dependent upon gender or stature but may relate to inaccuracies in fitting measurements and/or wear of the suit. This range in loading may also be because of differences in stirrup tightening/loosening as once the study commenced no adjustments were made. Having live feedback on what axial loading is being provided might facilitate greater consistency. This is especially important across multiple donnings and where microgravity/immobilization-induced fluid redistribution (10), anthropometric changes (9,6), or exercise could conceivably alter the loading. Furthermore, although not explicitly tested, the axial loading seems to be dependent upon posture. Thus, the ability to adjust axial load with real-time feedback would be advantageous for application in both user groups on Earth and space where astronauts adopt a “neutral” floating (15).
Though significant loading was recorded, it is presently unknown as to what an appropriate static axial load stimulus might be to attenuate musculoskeletal deconditioning experienced in disuse and microgravity, either alone or in combination with exercise (29). Although no direct comparison with the Pingvin suit was conducted, the Mk III GLCS seems to induce axial loading not dissimilar to the 70% of the subject's bodyweight reported during running. However, unlike the Pingvin suit, no thermal tolerance issues arose when wearing the GLCS during exercise, presumably because of fabrication with porous material (26,32). The material tension created by the elastic weaves in the GLCS also creates vertical tension, which is more analogous to the way the body is loaded on Earth, than the bungee cords in the Pingvin suit. This can be observed in the low pressure recorded at the shoulder and increased pressure at the feet, which likely contributes to its improved tolerability during exercise (31).
When wearing the GLCS for an acute period of time (∼2 hours), total standing height was reduced by ∼1 cm, presumably because of the compression on the intravertebral disks, as leg length measured from the Calcaneus to the Iliac crest remained unchanged. If confirmed, this may be advantageous in mitigating spinal elongation during immobilization on Earth (29,11) and when in space, which has been reported to be as much as 7 cm (24). Such elongation can be painful and dehabilitating and lead to increased risk of disc herniation (24). However, in potential future studies focusing on elongation, standardization of height assessment to improve reliability should be implemented. Because gaze stabilization was only subjectively controlled, this could be improved by placing fixed markers and pointers to reduce error.
Although all maximal joint ranges of motion tested were attenuated by GLCS wear, it is rare to require the full ROM during normal daily activity, and as subjects reported few difficulties, functional significance seems minor. Timed Get Up and Go was slower, but from anecdotal reports, it may have at least in part been because of a reluctance to tear the seams of the suit, rather than locomotion impedance per se. This might be a potential limitation and could indicate greater familiarization with the GLCS is required before testing.
The Pingvin suit has been reported to elevate core temperature and induce thermal discomfort during exercise (14). In contrast, the GLCS had no effect upon strength exercise-induced core temperature or thermal comfort in normal ambient conditions (analogous to the international space station) (18). Movement discomfort and body control were significantly increased when wearing the GLCS compared with GYM clothes, suggesting the GLCS could be optimized to improve comfort, especially near the shoulder. However, it is important to note that comparison with loose-fitting clothing is potentially misleading and a limit of this study; thus, direct comparison with the Pingvin and/or another compression garment affecting performance (22,25) would provide a more appropriate comparative model.
The ability to perform resistance exercise was not significantly impeded by wearing the GLCS in the majority of the exercises performed. However, difficulties were encountered when performing the shoulder press, with 3 individuals unable to complete the prescribed 3 sets of 12 repetitions when maintaining adequate control. This could be attributed to increased effort required by the participant to overcome the loading provided by the GLCS during the standing shoulder press. Whether this additional effort provides a useful adjunct to resistance exercise would require further study, including an assessment of muscle activity. However, all subjects did report that for the same exercise load (GLCS vs. GYM), wearing the GLCS increased the perceived effort. Thus, the axial loading provided by the GLCS, if adjusted to the appropriate level, might provide a training stimulus across a range of joints and, in postures appropriate to the individuals' requirements and capabilities, offer a potential physiological/training augmentation strategy for use in microgravity and terrestrial settings, as reported with the use of whole-body compression garments in male athletes (25).
A main limitation of this study is the small, gender-unbalanced sample size, and therefore, more data from additional gender-matched groups should be investigated further. Loading also needs to be reassessed with integrated force sensors in the shoes, during different body positions both when in contact with the ground and when floating in microgravity, to more accurately capture the axial load produced by the GLCS. The characterization of loading should also be performed during the exercise, along with measurements of muscle activity and exercise response, as predominantly only subjective measures of performance during exercise were assessed in this first trial. This could then determine if additional axial load during exercise is effecting muscle recruitment, as this could have intriguing applications for modifying training response. However, further refinements to the GLCS are suggested to improve comfort, tolerability, and the ability to don and doff the garment with ease, as this was an issue for several participants.
This approach of combined wear with exercises such as running and task-specific body weight exercises could be investigated to determine if additional axial loading augments athletic training in healthy populations. Additional axial loading in the future may also aid to provide a stimulus to support bone fracture healing (3) and rehabilitation from musculoskeletal degradation induced by disuse, disease, or injury (29,1) after further investigation.
The GLCS demonstrates potential as a lightweight, low-volume/cost countermeasure against the loss of axial loading in microgravity, by providing static axial loading broadly analogous to Earth. Such axial loading has minor effects on ambulation and ROM and renders strength exercise 12RM completions more challenging, without apparent thermal issues. With the increase of smart clothing in athletic disciplines, loading suits primarily designed for use in space may have potential terrestrial benefit as either training augmentation or rehabilitation tools; however, more research is required in this area. Thus, the GLCS, with further design improvements and future investigations, may provide a useful adjunct to exercise, potentially either by providing a complimentary training modality or through virtue of its static loading, and assist in ameliorating musculoskeletal deconditioning associated with space, disuse, or injury.
The authors thank Costumeworks, Lindsey Marjoram and Tony Christopher for technical support, Leandro Disiuta and Ingrid Lamadrid for their testing support, and the participants for their cooperation. The authors have no competing interests to declare. The study was conducted as part of a Space Physiology & Health MSc project, where financial support was provided by Wyle GmbH on behalf of the Space Medical Office of the European Space Agency. The results of the present study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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