I recently read a review article published by The Journal of the American Academy of Orthopaedic Surgeons which discussed the effects of spaceflight and microgravity on the musculoskeletal system. As we plan for longer distance and duration space flights and the private expansion into space travel, understanding the impacts of microgravity on the musculoskeletal system is increasingly important. The review article discussed the effects of space travel on bone mineral density, fractures, muscles, tendons, cartilage, arthritis, and the spine.
Bottom line up front:
-Astronauts experience a loss of 1-1.5% bone density per month while in microgravity
-Muscles atrophy at a rapid rate in microgravity, with studies noting a 30% decrease over 3-6 months
-NASA has developed an Advanced Resistive Exercise Device (ARED) to counter these losses
-Fractures heal differently in microgravity and will require new management techniques
-Intervertebral discs hyperhydrate and increase risk for herniation upon return to gravity
Decreased bone mineral density is a known risk factor for fractures. Astronauts that have completed 4-6 months on space missions experienced a 1-1.5% decline in bone density per month for normal weight-bearing skeletal sites. Most theories for the decline explain this as a result of the alteration in bone metabolism that is created by microgravity. Newer theories say that bone loss could be a result of effect of microgravity on the vestibular system instead. Comparative studies on earthbound patients that were confined to prolonged bedrest found that bone mineral density in those patients typically plateaued at 69% of its original value after an initial period of decline. However, the studies on astronauts returning from microgravity noted an increase in bone formation, but bone strength does not always return to pre-flight levels.
Current measures to minimize bone mineral density reductions include the creation of an artificial, weight-bearing environment in space, such as NASA’s Advanced Resistive Exercise Device (ARED). The ARED, which looks like a hybrid between a squat rack and a bench press rack, uses vacuum cylinders with flywheels to generate weight-bearing force. While there is not yet a formal protocol or strength training program to help to prevent bone density loss, the use of ARED in combination with antiresorptive osteoporosis medications such as diphosphonate has shown an effect at reducing bone density decline.
Fractures
Decreased bone density is a known risk factor for fractures. However, after 50 years of space flight research, the risk of fracture from reduced bone mineral density during and after space flight is minimal. When fractures do occur, models predict that astronauts will experience fracture patterns similar to osteoporotic Earthmongers, including fractures of the femoral neck, lumbar spine, and distal radius. Due to the lack of gravity, fracture healing in space may not follow the same healing pattern necessitating different management techniques. Some technologies that are emerging to enhance fracture healing in space and improve bone recovery includes intermittent parathyroid hormone therapy with and low-intensity pulsed ultrasonography.
Muscles and tendons
Humans move around by means of what's called bipedal ambulation, or two legged walking, on Earth due to gravity. In space, astronauts use their upper extremities more often to get around. Spaceflight with microgravity exposure can cause muscle atrophy, or muscle wasting, at a rapid rate as high as a 30% loss of muscle mass for those astronauts who participate in missions between 3 to 6 months. In similar studies of bedbound patients, decrease of muscle mass will plateaus after four months.
Spaceflight has been noted to lead to a decrease in muscular endurance with a decrease in the body’s slow twitch muscle fibers occurring more rapidly than the decrease of the fast twitch muscle fibers, which are responsible for the muscles explosive ability. As far as tendons, spaceflight decreases stiffness of tendons and their attachment to bones. Loss of strength is typically more than the loss of mass. For example, one space-related study demonstrated that astronauts who experienced a 20% decrease in calf muscle volume also experienced a 50% decrease in calf muscle explosive force. This loss is likely due to muscle atrophy and increased tendon laxity.
However, an astronaut's strength and volume do not disappear forever. In one study, astronaut’s muscle mass returned to pre-fight levels between one and four years after their return from space, or as soon as two months upon return with participation in aggressive rehabilitation and conditioning.
In spaceflight exercise, including resistance exercise, only reduces the quantity of muscle volume and strength loss without completely preventing it. This means it slows the loss of muscle but it continues even with spaceflight resistance exercise. However, in-flight exercise remains the main means of muscle atrophy prevention.
Cartilage and arthritis
Cartilage is responsive to biomechanical signals based on loading. Loss of loadbearing stimuli leads to atrophy and thinner cartilage, similar to osteoarthritic changes. Studies are currently in progress to further assess cartilage health in microgravity.
Spine
Astronauts experience prolonged periods of spinal decompression during spaceflight due to microgravity, mixed with intermittent spinal compression from spacecraft acceleration. In space, the spine elongates 4-6 centimeters, the lumbar curve flattens, the vertebral discs swell, and the paraspinal muscles, or the muscles adjacent to the spine, lose mass and tone. Because in space the spine is not constantly pulled down by gravity, the vertebral discs swell and can increases the risk of herniation upon return to Earth. Studies have shown 4.3 times greater risk of cervical or lumbar disc herniation after spaceflight compared with controls. The risk of herniation is the greatest in the first year after returning to Earth, with a 35.9 times increase risk of cervical disc herniation. Current strategies astronauts use to prevent these changes include wearing a penguin suit and sleeping in the fetal position. The penguin suit is the jumpsuit that you typically see astronauts wearing in pictures of them floating inside space stations. Inside this suit, there are a series of bungee cords from their shoulders to their feet meant to compress the spine to mimic the effects of gravity. Additional preventive measures are in-flight passive spinal loading, post-flight limited spinal flexion, and post-flight exercises to reduce disc hyperhydration. Currently, ARED does not allow for cervical spine weight-bearing. More research is needed to identify programs to protect the regions of the spine closer to the head.
The biggest risk of injury is when returning to a gravitational environment after spaceflight. Injury prevention strategies need to be developed to address this period of increased risk, as astronauts will likely need to engage in physical labor upon return to a gravitational environment.
In conclusion, Microgravity spaceflight influences every component of the musculoskeletal system. Existing research provides a solid knowledge base, but future research is needed to adequately mitigate the effects of spaceflight and microgravity to ensure the health of astronauts as well as extended spaceflight mission success.
It seems Mr. Gravity isn’t so mean after all.
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