NM Balance Regulation With ULLS and Loss of Sleep

Effects of Unloading and Sleep Restriction on Balance Regulation and Quadriceps Structure and Contractile Function

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.

Subjects will be randomized to different arms via a matched-pairs design using the following criteria: age, sex, and race. Within each arm, the loaded limb will serve as an internal control to assess effects of unloading.

The research team will provide participants with a pair of shoes to wear throughout the study. The right shoe will be modified to include a 5cm (approximately 2") rocker-style platform which will keep the left leg suspended during walking. Participants will also be provided with a pair of short crutches to assist with walking. Participants in the SR group will sleep only 5 hours for three consecutive nights.

The research team will provide participants with a pair of shoes to wear throughout the study. The right shoe will be modified to include a 5cm (approximately 2") rocker-style platform which will keep the left leg suspended during walking. Participants will also be provided with a pair of short crutches to assist with walking. Participants in the SA group will sleep for 9 hours for three consecutive nights.

Subjects will sit upright in an isokinetic dynamometer with the knee at 60° flexion. Gel electrode pads will be placed over the femoral nerve and just proximal to the patellar tendon. Square wave, constant current, doublet pulses from an electrical stimulator will be used to evoke twitches prior to, during, and following a maximal voluntary isometric contraction (MVIC).

Balance will be assessed using a dual force plate system. Participants will be asked to stand on one leg and maintain their balance for 15 seconds

Diffusion Tensor Imaging (DTI) will be used to quantify muscle volume. Subjects will lay supine, and slightly flex the knee (~10°) to elongate and straighten the quadriceps. A spine array coil will be placed on the MR-scanner table and a flexible body coil wrapped around the upper thigh and centered over mid-thigh of the dominant leg. Multiple images will be taken. DSI Studio program will be used to analyze raw images.

Balance will be assessed using a dual force plate system. Participants will step off a wooden platform and land with one leg only in the middle of the respective force plate. As soon as the participant lands, they will be asked to maintain their position for 10 seconds.

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)