By Avery Hinks Fifty-four years ago, humans went to the Moon. While at that time traveling even further seemed like science fiction dreamed up by Philip K. Dick, we are now apparently a decade away from the first manned trips to Mars. It won’t be long before space travel becomes more available to the general public. Researchers are of course already considering the impact long-term spaceflight would have on the human body, in particular its muscles. What has perhaps been given less consideration is the different populations that will travel to space as it becomes more available. In our lab, we especially think about older individuals. What happens to muscles during spaceflight?You probably know that physical inactivity is not good for your muscles. Whether you’re on bed rest at the hospital, your arm is in a sling after an injury, or you’re too immersed in Game of Thrones to leave your couch, your muscles will atrophy if you continue to not use them. But did you know that even when you’re not actively using your muscles, they are still bearing some weight that helps sustain them? It’s the same reason an apple once fell on Isaac Newton’s head. Gravity. In space, there is no gravity acting on your limbs for your muscles to resist, so they begin losing mass rapidly. With as few as 8 days in space, astronauts can lose 6% of the muscle volume in their calf muscles. When approaching 6 months in space, this loss is closer to 25% (see the dark blue line in the figure below). These numbers are also regardless of any in-flight countermeasures such as exercise. Currently, traveling to Mars is projected to take about 9 months. That gives a lot of time for zero-gravity to wear away at one’s muscles! This loss of muscle mass has severe consequences on physical function, most notably a reduction in strength. The % loss of strength actually seems to exceed the loss of muscle mass. Looking again at the calf muscles, strength can decline around 40% for a 6-month mission. The reason for the decline in strength exceeding that of muscle mass is likely due to the neural component of muscle contraction. That means the connection between the brain and muscle declines during spaceflight as well! We can also assess muscle strength using electrically stimulated contractions to eliminate the neural component, isolating performance of the muscle itself. A study by Koryak in 2001 on Russian astronauts tested both voluntary and electrically stimulated strength of the calf muscles after 115-380 days of spaceflight. They indeed saw that the average decline in voluntary strength was 42% (the top graph below) while the decline in electrically stimulated strength was only 26% (the bottom graph below). A study by Narici and colleagues in 2003 compared this to only 17 days of spaceflight. In the graph below, focus on the points at 50 Hz, as that represents the maximum electrical stimulation. Interestingly, when assessing electrically stimulated calf muscle strength, there was no decline in strength at the end of the mission (FD16 in the graph below). Instead, researchers observed the biggest loss of strength (around 22%) 8 days after returning to Earth (R + 8 in the graph below). They concluded this delayed strength loss was due to the muscles undergoing damage while being re-introduced to gravity. The loss of muscle mass with aging = the loss of muscle mass during spaceflight?A recent article in Aging Research Reviews by Capri and colleagues argued the physiological stresses that occur during spaceflight can be seen as an “acceleration” of what is seen in natural aging. To that end, the loss of muscle mass with age is comparable to the (much faster) loss of muscle mass during spaceflight. Comparing thigh muscle size between men aged ~70 and ~30, Lexell and colleagues found the 70-year old men’s muscles were 18% smaller. A study by Frontera and colleagues later tracked men for 12 years starting at about age 65, and observed a 14-16% loss of thigh muscle size over time. That’s up to a 1.3% loss of muscle size per year after age 65. Like during spaceflight, the loss of muscle strength with age may be even more drastic. In the study mentioned above, they also assessed strength (torque in the graph below) of those muscle groups in shortening contractions. Across the 12-year period, strength declined by 24-29%. These effects of aging on muscle are not only a concern above the age of 60. A review by Vandervoort from the University of Western Ontario in 2001 compiled data across several studies. As seen in their graph below, muscle strength declines steadily throughout adulthood, with on average about a 10% decline from age 20 to 50 for isometric (static muscle length) and concentric (shortening) contractions. In 2021, at 90 years old, William Shatner became the oldest person to travel to space. The previous record, set earlier that year, was Wally Funk at age 82. As they only remained in space for about 10 minutes, the risks on muscle function were minimal. However, options for longer flights will inevitably arise as space tourism continues to advance. When that happens, should we be prepared for the effects of aging and spaceflight compounding? Maybe we should already be concerned—in the spaceflight studies discussed earlier, some of the astronauts were over 50 years old! How can we assess this on Earth before going to space?We of course can’t send people to space every time we want to gain insight on how spaceflight impacts muscle mass and strength. Thankfully, there are a few options we can test down here on Earth that yield similar outcomes. Namely, there are three methods: bed rest, lower-limb suspension, and immobilization. Bed rest is exactly as it sounds—a person is confined to lying in bed. With lower-limb suspension, the person is free to move, but one leg is kept suspended off the ground. Immobilization is similar, but the leg is instead held in a cast. Bed rest is the most popular because it can be combined with a head tilt, which mimics the fluid shift that occurs in space to reproduce some of spaceflight’s systemic effects. Regarding muscle, however, all three options produce reductions in mass and strength that are comparable to spaceflight. A review by Narici and de Boer in 2011 demonstrated similarities among all models. The graph below is from their review, showing decreases in size of the calf (PF) and quadriceps (KE) muscle groups for spaceflight (SF), bed rest (BR), and lower-limb suspension (ULLS). While data for immobilization aren’t shown on the graph, the authors noted the loss of muscle size is 10-12% for the quadriceps after 10-14 days of immobilization, similar to spaceflight. These Earth-based models have provided insight both on the decline of aged muscle during spaceflight, and the recovery of aged muscle following spaceflight compared to younger counterparts. A study by Suetta and colleagues immobilized the quadriceps of young (age 21-27) and older (age 61-74) men for 2 weeks, then investigated their recovery following 4 weeks of strength training of the once-immobilized leg. The graph below shows that young men experienced a greater percent reduction in muscle volume following the cast than older men. However, young men recovered closer to their initial muscle volume after 4 weeks of recovery. The literature outside of the above study are split, however. Some have reported a greater magnitude of muscle atrophy in older adults with immobilization, while others have reported greater atrophy in younger adults. These disparities are best seen in the figure below from a systematic review by Hodgson and colleagues. Performing studies on old rodents also provides an opportunity to assess age-related differences in disuse atrophy and recovery more closely. For example, a study by Fisher and Brown in 1998 tested 31 and 37-month-old rats after their calf muscles were immobilized for 4 weeks. These age groups represent about 75 and 90 years old in comparison to humans. Using rats allowed the researchers to directly assess the muscle mass and the strength of individual muscles in a controlled environment. They found that both the losses of muscle mass and strength (“Po” in the graphs below) with immobilization were less in the 37-month-old rats compared to the 31-month old rats. These trends are clearly seen in the figures below when comparing the control (white) and immobilized (grey) bars. From these results, it appears that the muscle atrophy associated with disuse may be diminished when superimposed on the atrophy that already comes with aging. But how do muscles of old rats recover from immobilization compared to young rats? A study by Zarzhevsky and colleagues in 2001 provided insight, comparing rats aged 6 months to rats aged 24 months. Like the study on humans by Suetta and colleagues, they found that the muscles of old rats had a slower return to their original mass than young rats. This trend is shown in the graph below, with the bars representing the percent increase in muscle mass following 4 weeks of recovery after cast removal. Conclusion Humankind is quickly advancing accessibility to space travel. As space travel becomes more common, we should consider the effects it may have on muscles of non-“young, healthy” populations, including the elderly. We may assess this without having to go to space by investigating models of bed rest, lower-limb suspension, and immobilization. Studies so far in both humans and animals suggest older adults do not experience a heightened magnitude of disuse atrophy, however, they require a longer recovery period. In order to further apply these models to spaceflight, we should aim to better understand the effects of longer periods of muscle disuse, and optimal in-flight countermeasures and pre-training. We should also investigate the effects on measures of muscle performance other than strength, such as muscle power and velocity, which provide indications of performance during movement. References
“China plans for first manned mission to Mars in 2033” https://www.aljazeera.com/news/2021/6/24/china-plans-for-first-manned-mission-to-mars-in-2033 “William Shatner oldest astronaut at 90 – Here's how space tourism could affect older people” https://www.space.com/how-space-tourism-could-affect-older-people “Aviation pioneer Wally Funk, the oldest person to fly in space, can't wait to go back after Blue Origin launch” https://www.space.com/wally-funk-blue-origin-new-shepard-launch-reaction LeBlanc A, Lin C, Shackelford L, Sinitsyn V, Evans H, Belichenko O, Schenkman B, Kozlovskaya I, Oganov V, Bakulin A, Hedrick T, Feeback D. Muscle volume, MRI relaxation times (T2), and body composition after spaceflight. J Appl Physiol (1985). 2000 Dec;89(6):2158-64. doi: 10.1152/jappl.2000.89.6.2158. PMID: 11090562. Koryak YU. Electrically evoked and voluntary properties of the human triceps surae muscle: effects of long-term spaceflights. Acta Physiol Pharmacol Bulg. 2001;26(1-2):21-7. PMID: 11693395. Narici M, Kayser B, Barattini P, Cerretelli P. Effects of 17-day spaceflight on electrically evoked torque and cross-sectional area of the human triceps surae. Eur J Appl Physiol. 2003 Oct;90(3-4):275-82. doi: 10.1007/s00421-003-0955-7. Epub 2003 Sep 17. PMID: 13680242. Capri M, Conte M, Ciurca E, Pirazzini C, Garagnani P, Santoro A, Longo F, Salvioli S, Lau P, Moeller R, Jordan J, Illig T, Villanueva MM, Gruber M, Bürkle A, Franceschi C, Rittweger J. Long-term human spaceflight and inflammaging: Does it promote aging? Ageing Res Rev. 2023 Jun;87:101909. doi: 10.1016/j.arr.2023.101909. Epub 2023 Mar 12. PMID: 36918115. Lexell J, Henriksson-Larsén K, Winblad B, Sjöström M. Distribution of different fiber types in human skeletal muscles: effects of aging studied in whole muscle cross sections. Muscle Nerve. 1983 Oct;6(8):588-95. doi: 10.1002/mus.880060809. PMID: 6646161. Frontera WR, Hughes VA, Fielding RA, Fiatarone MA, Evans WJ, Roubenoff R. Aging of skeletal muscle: a 12-yr longitudinal study. J Appl Physiol (1985). 2000 Apr;88(4):1321-6. doi: 10.1152/jappl.2000.88.4.1321. PMID: 10749826. Vandervoort AA. Aging of the human neuromuscular system. Muscle Nerve. 2002 Jan;25(1):17-25. doi: 10.1002/mus.1215. PMID: 11754180. Narici MV, de Boer MD. Disuse of the musculo-skeletal system in space and on earth. Eur J Appl Physiol. 2011 Mar;111(3):403-20. doi: 10.1007/s00421-010-1556-x. Epub 2010 Jul 9. PMID: 20617334. Suetta C, Hvid LG, Justesen L, Christensen U, Neergaard K, Simonsen L, Ortenblad N, Magnusson SP, Kjaer M, Aagaard P. Effects of aging on human skeletal muscle after immobilization and retraining. J Appl Physiol (1985). 2009 Oct;107(4):1172-80. doi: 10.1152/japplphysiol.00290.2009. Epub 2009 Aug 6. PMID: 19661454. Hodgson H, Wilkinson M, Bowen S, Giannoudis P, Howard A. Older adults are not more susceptible to acute muscle atrophy after immobilisation compared to younger adults: a systematic review. Eur J Trauma Emerg Surg. 2022 Apr;48(2):1167-1176. doi: 10.1007/s00068-021-01694-0. Epub 2021 Jun 3. PMID: 34081160; PMCID: PMC9001571. Fisher JS, Brown M. Immobilization effects on contractile properties of aging rat skeletal muscle. Aging (Milano). 1998 Feb;10(1):59-66. doi: 10.1007/BF03339635. PMID: 9589753. Zarzhevsky N, Menashe O, Carmeli E, Stein H, Reznick AZ. Capacity for recovery and possible mechanisms in immobilization atrophy of young and old animals. Ann N Y Acad Sci. 2001 Apr;928:212-25. doi: 10.1111/j.1749-6632.2001.tb05651.x. PMID: 11795512.
0 Comments
|
AuthorAvery Hinks Archives
September 2023
Categories |