Donald E. Watenpaugh

Lumbar spine disc height and curvature responses to an axial load generated by a compression device compatible with magnetic resonance imaging

Study Design. Axial load-dependent changes in the lumbar spine of supine healthy volunteers were examined using a compression device compatible with magnetic resonance imaging. Objective. To test two hypotheses: Axial loading of 50% body weight from shoulder to feet in supine posture 1) simulates the upright lumbar spine alignment and 2) decreases disc height significantly. Summary of Background Data. Axial compression on the lumbar spine has significantly narrowed the lumbar dural sac in patients with sciatica, neurogenic claudication or both. Methods. Using a device compatible with magnetic resonance imaging, the lumbar spine of eight young volunteers, ages 22 to 36 years, was axially compressed with a force equivalent to 50% of body weight, approximating the normal load on the lumbar spine in upright posture. Sagittal lumbar magnetic resonance imaging was performed to measure intervertebral angle and disc height before and during compression. Results. Each intervertebral angle before and during compression was as follows: T12–L1 (−0.8° ± 2.5° and −1.5° ± 2.6°), L1–L2 (0.7° ± 1.4° and 3.3° ± 2.9°), L2–L3 (4.7° ± 3.5° and 7.3° ± 6°), L3–L4 (7.9° ± 2.4° and 11.1° ± 4.6°), L4–L5 (14.3° ± 3.3° and 14.9° ± 1.7°), L5–S1 (25.8° ± 5.2° and 20.8° ± 6°), and L1–S1 (53.4° ± 11.9° and 57.3° ± 16.7°). Negative values reflect kyphosis, and positive values reflect lordosis. A significant difference between values before and during compression was obtained at L3–L4 and L5–S1. There was a significant decrease in disc height only at L4–L5 during compression. Conclusions. The axial force of 50% body weight in supine posture simulates the upright lumbar spine morphologically. No change in intervertebral angle occurred at L4–L5. However, disc height at L4–L5 decreased significantly during compression.

Cardiovascular adaptation to spaceflight

This article reviews recent flight and ground-based studies of cardiovascular adaptation to spaceflight. Prominent features of microgravity exposure include loss of gravitational pressures, relatively low venous pressures, headward fluid shifts, plasma volume loss, and postflight orthostatic intolerance and reduced exercise capacity. Many of these short-term responses to microgravity extend themselves during long-duration microgravity exposure and may be explained by altered pressures (blood and tissue) and fluid balance in local tissues nourished by the cardiovascular system. In this regard, it is particularly noteworthy that tissues of the lower body (e.g., foot) are well adapted to local hypertension on Earth, whereas tissues of the upper body (e.g., head) are not as well adapted to increase in local blood pressure. For these and other reasons, countermeasures for long-duration flight should include reestablishment of higher, Earth-like blood pressures in the lower body.

Near-Infrared Spectroscopy for Monitoring of Tissue Oxygenation of Exercising Skeletal Muscle in a Chronic Compartment Syndrome Model*

Variations in the levels of muscle hemoglobin and of myoglobin oxygen saturation can be detected non-invasively with near-infrared spectroscopy. This technique could be applied to the diagnosis of chronic compartment syndrome, in which invasive testing has shown increased intramuscular pressure associated with ischemia and pain during exercise. We simulated chronic compartment syndrome in ten healthy subjects (seven men and three women) by applying external compression, through a wide inflatable cuff, to increase the intramuscular pressure in the anterior compartment of the leg. The tissue oxygenation of the tibialis anterior muscle was measured with near-infrared spectroscopy during gradual inflation of the cuff to a pressure of forty millimeters of mercury (5.33 kilopascals) during fourteen minutes of cyclic isokinetic dorsiflexion and plantar flexion of the ankle. The subjects exercised with and without external compression. The data on tissue oxygenation for each subject then were normalized to a scale of 100 per cent (the baseline value, or the value at rest) to 0 per cent (the physiological minimum, or the level of oxygenation achieved by exercise to exhaustion during arterial occlusion of the lower extremity). With external compression, tissue oxygenation declined at a rate of 1.4 ± 0.3 per cent per minute (mean and standard error) during exercise. After an initial decrease at the onset, tissue oxygenation did not decline during exercise without compression. The recovery of tissue oxygenation after exercise was twice as slow with compression (2.5 ± 0.6 minutes) than it was without the use of compression (1.3 ± 0.2 minutes). CLINICAL RELEVANCE: The results of this study demonstrate that near-infrared spectroscopy can detect deoxygenation of skeletal muscle caused by elevated intramuscular pressure during exercise. Thus, this technique may prove to be a useful diagnostic tool for the non-invasive detection of chronic compartment syndrome.

Central venous pressure in space.

Gravity affects cardiac filling pressure and intravascular fluid distribution significantly. A major central fluid shift occurs when all hydrostatic gradients are abolished on entry into microgravity (microG). Understanding the dynamics of this shift requires continuous monitoring of cardiac filling pressure; central venous pressure (CVP) measurement is the only feasible means of accomplishing this. We directly measured CVP in three subjects: one aboard the Spacelab Life Sciences-1 space shuttle flight and two aboard the Spacelab Life Sciences-2 space shuttle flight. Continuous CVP measurements, with a 4-Fr catheter, began 4 h before launch and continued into microG. Mean CVP was 8.4 cmH2O seated before flight, 15.0 cmH2O in the supine legs-elevated posture in the shuttle, and 2.5 cmH2O after 10 min in microG. Although CVP decreased, the left ventricular end-diastolic dimension measured by echocardiography increased from a mean of 4.60 cm supine preflight to 4.97 cm within 48 h in microG. These data are consistent with increased cardiac filling early in microG despite a fall in CVP, suggesting that the relationship between CVP and actual transmural left ventricular filling pressure is altered in microG.

Upright exercise or supine lower body negative pressure exercise maintains exercise responses after bed rest

Adaptation to bed rest or space flight is accompanied by an impaired ability to exercise in an upright position. We hypothesized that a daily, 30-min bout of intense, interval exercise in upright posture or supine against lower body negative pressure (LBNP) would maintain upright exercise heart rate and respiratory responses after bed rest. Twenty-four men (31 +/- 3 yr) underwent 5 d of 6 degree head-down tilt: eight performed no exercise (CON), eight performed upright treadmill exercise (UPex), and eight performed supine treadmill exercise against LBNP at -51.3 +/- 0.4 mm Hg (LBNPex). Submaximal treadmill exercise responses (56, 74, and 85% of VO2peak) were measured pre- and post-bed rest. In CON, submaximal heart rate, respiratory exchange ratio, and ventilation were significantly greater (P < or = 0.05) after bed rest. In UPex and LBNPex, submaximal exercise responses were similar pre- and post-bed rest. Our results indicate that a daily 30-min bout of intense, interval upright exercise training or supine exercise training against LBNP is sufficient to maintain upright exercise responses after 5 d of bed rest. These results may have important implications for the development of exercise countermeasures during space flight.

Wise-2005: Exercise and Nutrition Countermeasures for Upright V˙o2pk during Bed Rest

PURPOSEnExercise prescriptions for spaceflight include aerobic and resistive countermeasures, yet few studies have evaluated their combined effects on exercise responses after real or simulated microgravity. We hypothesized that upright aerobic capacity (VO2pk) is protected during a 60-d bed rest (BR) in which intermittent (40%-80% pre-BR VO2pk) aerobic exercise (supine treadmill exercise against lower body negative pressure) was performed 2-4 d x wk(-1) and resistive exercise (inertial flywheel exercises) was performed 2-3 d x wk(-1). Further, we hypothesized that ingestion of an amino acid supplement that was shown previously to counteract muscle atrophy, would reduce the decline in VO2pk in nonexercising subjects during BR.nnnMETHODSnTwenty-four healthy women (8 nonexercise controls (CON), 8 exercisers (EX), and 8 nonexercisers with nutritional supplementation (NUT)) underwent a 20-d ambulatory baseline period, 60 d of 6 degrees head-down tilt BR, and 21 d of ambulatory recovery. VO2pk was measured pre-BR and on the third day of recovery from BR (R3).nnnRESULTSnIn the EX group, VO2pk (mean +/- SE) was not different from pre-BR (-3.3 +/- 1.2%) on R3, although it decreased significantly in the CON (-21.2 +/- 2.1%) and NUT (-25.6 +/- 1.6%) groups.nnnCONCLUSIONSnThese results indicate that alternating aerobic and resistive exercise on most days during prolonged microgravity simulated by BR is sufficient to preserve or allow quick recovery of upright aerobic capacity in women but that a nutritional supplementation alone is not effective.

LBNP exercise protects aerobic capacity and sprint speed of female twins during 30 days of bed rest

We have shown previously that treadmill exercise within lower body negative pressure (LBNPex) maintains upright exercise capacity (peak oxygen consumption, Vo(2peak)) in men after 5, 15, and 30 days of bed rest (BR). We hypothesized that LBNPex protects treadmill Vo(2peak) and sprint speed in women during a 30-day BR. Seven sets of female monozygous twins volunteered to participate. Within each twin set, one was randomly assigned to a control group (Con) and performed no countermeasures, and the other was assigned to an exercise group (Ex) and performed a 40-min interval (40-80% pre-BR Vo(2peak)) LBNPex (51 +/- 5 mmHg) protocol, plus 5 min of static LBNP, 6 days per week. Before and immediately after BR, subjects completed a 30.5-m sprint test and an upright graded treadmill test to volitional fatigue. These results in women were compared with previously reported reductions in Vo(2peak) and sprint speed in male twins after BR. In women, sprint speed (-8 +/- 2%) and Vo(2peak) (-6 +/- 2%) were not different after BR in the Ex group. In contrast, both sprint speed (-24 +/- 5%) and Vo(2peak) (-16 +/- 3%) were significantly less after BR in the Con group. The effect of BR on sprint speed and Vo(2peak) after BR was not different between women and men. We conclude that treadmill exercise within LBNP protects against BR-induced reductions in Vo(2peak) and sprint speed in women and should prove effective during long-duration spaceflight.

Intramuscular pressure and torque during isometric, concentric and eccentric muscular activity

Intramuscular pressures, electromyography (EMG) and torque generation during isometric, concentric and eccentric maximal isokinetic muscle activity were recorded in 10 healthy volunteers. Pressure and EMG activity were continuously and simultaneously measured side by side in the tibialis anterior and soleus muscles. Ankle joint torque and position were monitored continuously by an isokinetic dynamometer during plantar flexion and dorsiflexion of the foot. The increased force generation during eccentric muscular activity, compared with other muscular activity, was not accompanied by higher intramuscular pressure. Thus, this study demonstrated that eccentric muscular activity generated higher torque values for each increment of intramuscular pressure. Intramuscular pressures during antagonistic co‐activation were significantly higher in the tibialis anterior muscle (42–46% of maximal agonistic activity) compared with the soleus muscle (12–29% of maximal agonistic activity) and was largely due to active recruitment of muscle fibers. In summary, eccentric muscular activity creates higher torque values with no additional increase of the intramuscular pressure compared with concentric and isometric muscular activity.

Intramuscular Pressures Beneath Elastic and Inelastic Leggings

Leg compression devices have been used extensively by patients to combat chronic venous insufficiency and by astronauts to counteract orthostatic intolerance following spaceflight. However, the effects of elastic and inelastic leggings on the calf muscle pump have not been compared. The purpose of this study was to compare in normal subjects the effects of elastic and inelastic compression on leg intramuscular pressure (IMP), an objective index of calf muscle pump function. IMP in soleus and tibialis anterior muscles was measured with transducer-tipped catheters. Surface compression between each legging and the skin was recorded with an air bladder. Subjects were studied under three conditions: (1) control (no legging), (2) elastic legging, and (3) inelastic legging. Pressure data were recorded for each condition during recumbency, sitting, standing, walking, and running. Elastic leggings applied significantly greater surface compression during recumbency (20±1 mm Hg, mean±SE) than inelastic leggings (13±2 mm Hg). During recumbency, elastic leggings produced significantly higher soleus IMP of 25±1 mm Hg and tibialis anterior IMP of 28±1 mm Hg compared to 17±1 mm Hg and 20±2 mm Hg, respectively, generated by inelastic leggings and 8±1 mm Hg and 11±1 mm Hg, respectively, without leggings. During sitting, walking, and running, however, peak IMPs generated in the muscular compartments by elastic and inelastic leggings were similar. Our results suggest that elastic leg compression applied over a long period in the recumbent posture may impede microcirculation and jeopardize tissue viability. On the other hand, inelastic leggings do not compress leg tissues at levels above 20 mm Hg during recumbency. Therefore inelastic leggings may be more effective in improving venous circulation in the legs of patients with chronic venous insufficiency.

Exercise against lower body negative pressure as a countermeasure for cardiovascular and musculoskeletal deconditioning

Exposure to lower body negative pressure (LBNP) with oral salt and water ingestion has been tested by astronauts as a countermeasure to prevent postflight orthostatic intolerance. Exercise is another countermeasure that astronauts commonly use during spaceflight to maintain musculoskeletal strength. We hypothesize that a novel combination of exercise and simultaneous exposure to lower body negative pressure during spaceflight will produce Earth-like musculoskeletal loads as well as cardiovascular stimuli to maintain adaptation to Earths gravity. Results from recent studies indicate that leg exercise within a LBNP chamber against the suction force of 100 mmHg LBNP in horizontal-supine posture produces an equivalent, if not greater exercise stress compared to similar leg exercise in upright posture (without LBNP) against Earths gravity. Therefore, the concept of LBNP combined with exercise may prove to be a low cost and low mass technique to stress the cardiovascular and the musculoskeletal systems simultaneously.