Feb. 08, 2024
Orthostatic intolerance is often observed in individuals after prolonged inactivity that occurs during bed rest or spaceflight. This condition can increase the risk of falls, the leading cause of injury-related death among older adults. Researchers suspect that the deconditioning that accompanies prolonged bed rest can affect cardiac and muscle-pump baroreflex activation, processes that help the body respond to sudden changes in blood pressure that can occur after sudden movements or postural changes. But the extent to which this deconditioning impacts the neural integrity of cardio-postural reflexes is still unknown.
To further examine how bed rest impacts cardiac and muscle-pump baroreflexes, and whether exercise can mitigate these effects, a team of researchers from Mayo Clinic and Simon Fraser University conducted a study examining these processes in older individuals. The study results were published in Frontiers in Human Neuroscience in 2023. This study was a collaborative effort between Isobel A. Scarisbrick, Ph.D., director of the Neural Repair Laboratory within Mayo Clinic's Department of Physical Medicine and Rehabilitation, and Andrew Blaber, Ph.D., at Simon Fraser University, with experiments performed at McGill University Health Centre Research Institute.
"In this pilot study, we examined levels of several inflammatory and neurodegenerative markers to assess their relationship with neural deficits in cardiac and muscle-pump baroreflexes, and with physical and physiological declines commonly associated with bed rest," explains Dr. Scarisbrick. "We also explored whether 60 to 75 minutes of daily exercise can reduce bed rest-induced cardiovascular and skeletal muscle deconditioning."
Methods
The researchers recruited 22 healthy study participants ages 55 to 65 years and randomly assigned them to either the exercise group or the control group. Both participant groups spent 14 days in continuous head-down bed rest with a 6-degree tilt. During the bed rest phase, the exercise group performed 60 to 75 minutes of daily exercise, while the control group remained inactive. The daily exercises included upper- and lower-body strength training, and aerobic and high-intensity interval training. Immediately after day 14 of bed rest, the participants began ambulatory recovery.
Data collection occurred before, during and after the bed rest phase and included tests to determine levels of the following: hemoglobin, neurofilament light chain (NfL), glial fibrillary acidic protein (GFAP), tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6), total tau protein (t-Tau), insulin-like growth factor 1 (IGF-1), brain-derived neurotrophic factor (BDNF), and ubiquitin carboxy-terminal hydrolase L1 (UCH-L1).
The researchers also analyzed several physical variables, including the participants' cardio-postural control system, using a supine-to-stand baroreflex and postural sway test performed once before bed rest and twice after bed rest. Participants also performed physical jump, muscle activity and strength tests before and after the bed rest phase. The researchers used false discovery rate with Huber M-estimation to correlate the changes in biomarkers with the changes in cardiovascular and muscular function that occurred during bed rest.
Results
Overall, Dr. Scarisbrick and colleagues observed several correlations of changes in biomarker levels with physical and physiological changes occurring during bed rest:
- Levels of NfL, GFAP, TNF-α and IL-6 were elevated in all participants after bed rest. Levels of IGF-1 decreased in females only.
- Changes in heart rate, blood pressure and lower limb muscle motoneuron activity during standing correlated with changes in TNF-α and BDNF.
- Baroreflex control, leg muscle maximal voluntary contraction and postural sway test results correlated with GFAP and NfL levels.
- The exercise and control groups exhibited similar reductions in orthostatic tolerance related to bed rest. Although daily exercise reduced the number of physiological interactions of biomarkers, significant cardio-postural correlations were observed in the exercise group with no protection against post-bed rest orthostatic intolerance.
"Our research findings indicate that a substantial part of the cardiovascular and neuromuscular deconditioning linked to bed rest is correlated with elevated levels of serum markers indicating neural injury and inflammation. These markers are associated with declines in neural reflex control," summarizes Dr. Scarisbrick. "These observations may help us better understand the role of bed rest in cardiovascular and postural deconditioning, and in neurodegeneration in older adults. This new perspective could help us develop improved exercise and biotherapeutic interventions."
For more information
Blaber AP, et al. Elevated biomarkers of neural injury in older adults following head-down bed rest: links to cardio-postural deconditioning with spaceflight and aging. Frontiers in Human Neuroscience. In press.
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