NM Balance Regulation With ULLS and Loss of Sleep

The goal of this trial is to learn about how restricted sleep and not bearing weight on the leg affects muscle strength and posture control. Participants will walk exclusively on one leg, sleep at differing intervals, and complete posture tests, muscle strength tests, and muscle imaging. Researchers will compare adequate sleep and restricted sleep to see if muscle strength and posture are affected.

During space flight, astronauts experience multiple physiological stimuli (such as microgravity and altered sleep cycles) which independently impair balance and strength. The goal of this project is to determine how these stimuli interact to affect static single-leg balance, balance during dynamic tasks, and muscle recruitment, strength, and remodeling.

Subjects (healthy adults age 18-50) will complete 13 days of unilateral lower limb suspension (ULLS, the gold standard analog for measuring space flight in ambulatory subjects). For the final three days of the study, subjects will be randomly assigned (via a matched pairs approach) to either normal sleep (7-9 hours per night) or restricted sleep (5 hours per night). Before and after the study, subjects will complete balance assessments and measures of muscle strength and recruitment. At the end of the study, subjects will also undergo diffusion tensor imaging for the thighs.

Balance testing will be conducted by examining sway in center of gravity during single leg stance and when stepping down from a box. Muscle strength and recruitment will be assessed via submaximal and maximal voluntary isometric contractions. Additionally, an interpolated twitch experiment will be performed to assess muscle recruitment and twitch properties. Diffusion Tensor imaging will be used to quantify muscle size and structure.

As prophylactic treatment against deep vein thrombosis (the major risk of ULLS), subjects will take an aspirin (81mg daily), wear compression stockings, complete passive range of motion exercises, and use intermittent pneumatic compression devices during sedentary times.

Eligibility Criteria:
Inclusion Criteria:

* Age between 18 and 50 years
* Regularly engaging in aerobic exercise (\> 150min/wk) and resistance exercise (\>1 time per week) for the past 12 months
* Regularly sleep at least 7 hours per night

Exclusion Criteria:

* Age \< 18 or \> 50 years
* Height between \<150 (F) or \<170 (M) and \> 190 cm (both sexes)
* Waist circumference \< 55cm or \> 90cm (F) and \< 75cm or \> 100cm (M)
* Body mass index \< 18.5 or \> 27.5
* Do not regularly sleep between 7 and 9 hours each night
* Have a known sleep disorder
* Not regularly engaging in exercise for previous 12 months
* Blood clotting disorder
* Heart arrhythmia
* Implanted device which could negatively be affected by electrical impulse or strong magnetic field such as pacemaker, internal defibrillator, or cochlear implant
* Diagnosed cardiovascular, pulmonary, renal, or metabolic disease
* Pregnancy (within previous 6 months)
* Oral contraceptive use (within previous three months)
* High resting blood pressure (\>140 systolic and/or \> 90 diastolic)
* Currently or previously undergone gender-affirming therapy (hormone therapy or sexual reassignment surgery)
* Low back or leg injury in previous 6 months
* Currently taking medication to assist with sleep
* Muscle, bone, or joint injury that limits physical activity within previous 6 months
* Neurological disorder which affects balance (such as multiple sclerosis or Parkinson's disease)