Self-generated lower body negative pressure exercise.

BACKGROUND: Exercise during spaceflight helps prevent musculoskeletal and cardiovascular deconditioning to Earth gravity. This report evaluates the aerobic and anaerobic exercise stimulus provided by self-generated lower body negative pressure. METHODS: A lower body negative pressure cylinder expands and collapses longitudinally, but not radially. As the legs push footward to expand the cylinder, the air pressure in the cylinder decreases, increasing the force required to continue expanding the cylinder. In addition, valves control air flow into and out of the cylinder, and thus workload. In seven supine subjects, knee bend exercise was performed at 19 cycles per minute for 6 min. Footward force was measured with load cells, cylinder pressure with a transducer, heart rate from ECG, and oxygen consumption with turbine volumetry and gas analysis. RESULTS: Maximum footward force at the peak of the exercise cycle averaged 1120+/-88 N (114+/-9 kg), and pressure within the cylinder concomitantly decreased 26+/-3 mmHg below ambient. Heart rate and oxygen consumption increased 75+/-4 bpm and 26.3+/-1.4 ml O2/kg x min(-1) from supine resting values, respectively. CONCLUSIONS: With the air inlet valve nearly closed, exercise with this device approximates a resistance-type leg press. With more inflow of air, more rapid, aerobic knee bends can be performed. This exercise device/concept provides simultaneous dynamic musculoskeletal and cardiovascular stresses without an external power source.

Leg intramuscular pressures during locomotion in humans.

To assess the usefulness of intramuscular pressure (IMP) measurement for studying muscle function during gait, IMP was recorded in the soleus and tibialis anterior muscles of 10 volunteers during treadmill walking and running by using transducer-tipped catheters. Soleus IMP exhibited single peaks during late-stance phase of walking [181 +/- 69 (SE) mmHg] and running (269 +/- 95 mmHg). Tibialis anterior IMP showed a biphasic response, with the largest peak (90 +/- 15 mmHg during walking and 151 +/- 25 mmHg during running) occurring shortly after heel strike. IMP magnitude increased with gait speed in both muscles. Linear regression of soleus IMP against ankle joint torque obtained by a dynamometer produced linear relationships (n = 2, r = 0.97 for both). Application of these relationships to IMP data yielded estimated peak soleus moment contributions of 0.95-1.65 N . m/kg during walking, and 1.43-2.70 N . m/kg during running. Phasic elevations of IMP during exercise are probably generated by local muscle tissue deformations due to muscle force development. Thus profiles of IMP provide a direct, reproducible index of muscle function during locomotion in humans.

Monitoring acute whole-body fluid redistribution by changes in leg and neck volumes.

BACKGROUND: Acute fluid shifts initiate chronic cardiovascular acclimation to altered posture or gravity. HYPOTHESIS: We hypothesized that neck volume increases with acute tilt from vertical to horizontal and head-down positions, and that neck volume correlates negatively with leg volume during tilting. METHODS: Strain gauges measured changes in calf and neck volumes in 9 subjects during the following tilt table protocol: 90 degrees (upright control), 54 degrees, 30 degrees, 12 degrees, 0 degree (horizontal supine), -6 degrees (head-down tilt), -12 degrees, -6 degrees, 0 degree, 12 degrees, 30 degrees, 54 degrees, and 90 degrees. Each position was held for 30 s. RESULTS: Tilting from 90 degrees upright to 0 degree supine increased neck volume 3.09 +/- 0.37% (mean +/- SE); neck volume increased further above upright control to 4.26 +/- 0.39% at -12 degrees head-down tilt. In the calf, tilting produced significant volume decrements of 1.66 +/- 0.36% below 90 degrees control at 0 degree supine, and 2.03 +/- 0.50% below control at -12 degrees tilt. Neck volume elevation consistently exceeded the absolute magnitude of calf volume reduction at a given tilt angle by a factor of about 1.5, and the two were linearly correlated (r2 = 0.60). CONCLUSIONS: Responses of body segment volumes to tilt were practically instantaneous, indicating that venous blood volume translocation accounted for the changes. We conclude that leg and neck volume changes provide a convenient, non-invasive, and sensitive means of assessing acute regional fluid shifts in humans.

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).

Prediction of human gait parameters from temporal measures of foot-ground contact.

Investigation of the influence of human physical activity on bone functional adaptation requires long-term histories of gait-related ground reaction force (GRF). Towards a simpler portable GRF measurement, we hypothesized that: 1) the reciprocal of foot-ground contact time (1/tc); or 2) the reciprocal of stride-period-normalized contact time (T/tc) predict peak vertical and horizontal GRF, loading rates, and horizontal speed during gait. GRF data were collected from 24 subjects while they walked and ran at a variety of speeds. Linear regression and ANCOVA determined the dependence of gait parameters on 1/tc and T/tc, and prediction SE. All parameters were significantly correlated to 1/tc and T/tc. The closest pooled relationship existed between peak running vertical GRF and T/tc (r2 = 0.896; SE = 3.6%) and improved with subject-specific regression (r2 = 0.970; SE = 2.2%). We conclude that temporal measures can predict force parameters of gait and may represent an alternative to direct GRF measurements for determining daily histories of habitual lower limb loading quantities necessary to quantify a bone remodeling stimulus.

Cutaneous microvascular flow in the foot during simulated variable gravities.

Our objective was to understand how weight bearing with varying gravitational fields affects blood perfusion in the sole of the foot. Human subjects underwent whole body tilting at four angles: upright [1 gravitational vector from head to foot (Gz)], 22 degrees (0.38 Gz), 10 degrees (0.17 Gz), and supine (0 Gz), simulating the gravitational fields of Earth, Mars, Moon, and microgravity, respectively. Cutaneous capillary blood flow was monitored on the plantar surface of the heel by laser Doppler flowmetry while weight-bearing load was measured. At each tilt angle, subjects increased weight bearing on one foot in graded load increments of 1 kg beginning with zero. The weight bearing at which null flow first occurred was determined as the closing load. Subsequently, the weight bearing was reduced in reverse steps until blood flow returned (opening load). Mean closing loads for simulated Earth gravity, Mars gravity, Moon gravity, and microgravity were 9.1, 4.6, 4.4, and 3.6 kg, respectively. Mean opening loads were 7.9, 4.1, 3.5, and 3.1 kg, respectively. Mean arterial pressures in the foot (MAP(foot)) calculated for each simulated gravitational field were 192, 127, 106, and 87 mmHg, respectively. Closing load and opening load were significantly correlated with MAP(foot) (r =0.70, 0.72, respectively) and were significantly different (P < 0.001) from each other. The data suggest that decreased local arterial pressure in the foot lowers tolerance to external compression. Consequently, the human foot sole may be more prone to cutaneous ischemia during load bearing in microgravity than on Earth.

Simulated microgravity increases cutaneous blood flow in the head and leg of humans.

BACKGROUND: The cutaneous microcirculation vasodilates during acute 6 degrees head-down tilt (HDT, simulated microgravity) relative to upright conditions, more in the lower body than in the upper body. HYPOTHESIS: We expected that relative magnitudes of and differences between upper and lower body cutaneous blood flow elevation would be sustained during initial acclimation to simulated microgravity. METHODS: We measured cutaneous microvascular blood flow with laser-Doppler flowmetry at the leg (over the distal tibia) and cheek (over the zygomatic arch) of eight healthy men before, during, and after 24 h of HDT. Results were calculated as a percentage of baseline value (100% measured during pre-tilt upright sitting). RESULTS: Cutaneous blood flow in the cheek increased significantly to 165 +/- 37% (mean +/- SE, p < 0.05) at 9-12 h HDT, then returned to near baseline values by 24 h HDT (114 +/- 29%, NSD), despite increased local arterial pressure. Microvascular flow in the leg remained significantly elevated above baseline throughout 24 h HDT (427 +/- 85% at 3 h HDT and 215 +/- 142% at 24 h HDT, p < 0.05). During the 6-h upright sitting recovery period, cheek and leg blood flow levels returned to near pre-tilt baseline values. CONCLUSIONS: Because hydrostatic effects of HDT increase local arterial pressure at the carotid sinus, baroreflex-mediated withdrawal of sympathetic tone probably contributed to increased microvascular flows at the head and leg during HDT. In the leg, baroreflex effects combined with minimal stimulation of local veno-arteriolar and myogenic autoregulatory vasoconstriction to elicit relatively larger and more sustained increases in cutaneous flow during HDT. In the cheek, delayed myogenic vasoconstriction and/or humoral effects apparently compensated for flow elevation by 24 h of HDT. Therefore, localized vascular adaptations to gravity probably explain differences in acclimation of lower and upper body blood flow to HDT and actual microgravity.

Induced periodic hemodynamics in skeletal muscle of anesthetized rabbits, studied with multiple laser Doppler flow probes.

Periodic fluctuations, regular slow-wave flowmotion, were induced in the skeletal muscle of six mature anesthetized New Zealand White rabbits by acute femoral artery pressure reduction from a median control value of 78 to 36 mm Hg. This phenomenon was monitored simultaneously with four laser Doppler flowmetry (LDF) probes placed over the gastrocnemius muscle in a linear array with a spacing of 5 mm. The median relative peak-to-trough amplitude of the oscillatory flow patterns was 47%, white a frequency of approximately 2.5 cycles per minute (cpm), which remained relatively stable over an observation period of 1 h (+/- 15-20%). Application of two frequency analysis methods, Welch's FFT method and Prony Spectral Line Estimation yielded similar results and showed a correlation coefficient of r = +0.84. Spectral coherence between pairs of regular slow-wave flowmotion records decreased with increasing probe separation (0.28 at 5 mm; 0.01 at 15 mm). However, patterns of frequency fluctuation over time were significantly correlated between concurrent record pairs regardless of probe separation. These results suggest that the regular slow-wave flowmotion signal originates in regions that are independently regulated by local vasoactive sites, which may be called pacemakers. These sites may also be influenced by an additional common control mechanism, which may be myogenic/metabolic or central in nature.

Calf venous compliance measured with head-up tilt equals supine calf compliance.

Elevated calf compliance may contribute to orthostatic intolerance following space flight and bed rest. Calf venous compliance is measured conventionally with venous occulusion plethysmography in supine subjects. With this well-established technique, subjects undergo inflation of a pressure cuff around the thigh just above the knee, which increases calf venous pressure. A plethysmograph simultaneously measures calf volume elevation. Compliance equals calf volume elevation per mm Hg thigh occlusion (calf venous) pressure in relaxed legs of the supine subjects. Compliance may also be measured during stepwise head-up tilt (HUT) as calf volume elevation per mm Hg gravitational venous pressure elevation produced by HUT. However, during HUT on a tilt table with a footplate, calf muscles activate to counteract gravity: this is an obvious and natural response to gravitational force. Such muscle activation conceivably could reduce calf compliance, yet relatively little calf muscle activation occurs during HUT and orthostasis (<10% of maximal voluntary levels). Also, this activation produces minimal calf volume change (<0.3%). Therefore, we hypothesized that calf compliance measured with HUT equals that measured with supine venous occlusion.

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.(ABSTRACT TRUNCATED AT 250 WORDS)

A modeling cross-spectral analysis technique based on the Prony Spectral Line Estimator (PSLE).

The Cross-Prony Spectral Line Estimator (XPSLE) is proposed for spectral comparison of short data records. Basic theory is discussed. The XPSLE method is tested on pairs of synthetic data records and is shown to be sensitive to disparity of spectral content. Application to analysis of arteriolar vasomotion is discussed.

Acute cutaneous microvascular flow responses to whole-body tilting in humans.

The transition from upright to head-down tilt (HDT) posture in humans increases blood pressure superior to the heart and decreases pressure inferior to the heart. Consequently, above heart level, myogenic arteriolar tone probably increases with HDT, in opposition to the withdrawal of baroreceptor-mediated sympathetic tone. We hypothesized that due to antagonism between central and local controls, the response of the facial cutaneous microcirculation to acute postural change will be weaker than that in the leg, where these two mechanisms reinforce each other. Cutaneous microvascular flow was measured by laser Doppler flowmetry simultaneously at the shin and the neck of 7 male and 3 female subjects. Subjects underwent a stepwise tilt protocol from standing control to 54 degrees head-up tilt (HUT), 30 degrees, 12 degrees, 0 degrees, -6 degrees (HDT), -12 degrees, -6 degrees, 0 degrees, 12 degrees, 30 degrees, 54 degrees, and standing, for 30-sec periods with 10-sec transitions between postures. Flows at the shin and the neck increased significantly (P < 0.05) from standing baseline to 12 degrees HUT (252 +/- 55 and 126 +/- 9% (means +/- SE) of baseline, respectively). From 12 degrees to -12 degrees tilt, flows continued to increase at the shin (509 +/- 71% of baseline) but decreased at the neck to baseline levels (100 +/- 15% of baseline). Cutaneous microvascular flow recovered at both sites during the return to standing posture with significant hysteresis. Flow increases from standing to near-supine posture are attributed at both sites to baroreceptor-mediated vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow velocity in humans exposed to 24 h of head-down tilt.

This study investigates cerebral blood flow (CBF) velocity in humans before, during, and after 24 h of 6 degree head-down tilt (HDT), which is a currently accepted experimental model to simulate microgravity. CBF velocity was measured by use of the transcranial Doppler technique in the right middle cerebral artery of eight healthy male subjects. Mean CBF velocity increased from the pre-HDT upright seated baseline value of 55.5 +/- 3.7 (SE) cm/s to 61.5 +/- 3.3 cm/s at 0.5 h of HDT (P < 0.05), reached a peak value of 63.2 +/- 4.1 cm/s at 3 h of HDT, and remained significantly above the pre-HDT baseline for > or = 6 h of HDT. During upright seated recovery (1-5 h post-HDT), mean CBF velocity decreased to 87% of the pre-HDT baseline value (P < 0.05). Mean CBF velocity correlated well with calculated intracranial arterial pressure (IAP) (r = 0.54, P < 0.001). As analyzed by linear regression, mean CBF velocity = 29.6 + 0.32IAP. These results suggest that HDT increases CBF velocity by increasing IAP during several hours after the onset of microgravity. Importantly, the decrease in CBF velocity after HDT may be responsible, in part, for the increased risk of syncope observed in subjects after prolonged bed rest and also in astronauts returning to Earth.

Regional cutaneous microvascular flow responses during gravitational and LBNP stresses.

The most significant cardiovascular event during the transition to microgravity is the redistribution of vascular transmural pressures that results from the loss of hydrostatic gradients along the length of the body. The well-documented effects of this redistribution include facial venous engorgement, headache, and a significant decrease in leg volume. These effects predominantly represent bulk fluid volume shifts, especially in the venous macro- and microcirculation, where volume is a direct function of pressure, related by the mechanical compliance of the vascular compartment. When considering the effect of gravitational pressure alterations on microcirculatory blood flow and volume, however, this direct monotonic relationship no longer applies. Regional microvascular perfusion is largely a function of local arteriolar tone, which is subject to a variety of central and local controls. Lower body venous pooling during application of footward gravitational stress unloads arterial and cardiopulmonary baroreceptors, increasing sympathetic arteriolar tone to elicit vasoconstriction and a general decrease in microvascular perfusion. The same stimulus also triggers an increase in the levels of circulating vasoactive hormones, such as norepinephrine and angiotensin II, further augmenting arteriolar tone. Vasomotor tone is also mediated by local mechanisms such as myogenic autoregulation and veno-arteriolar reflexes, which enhance microvascular tone in response to elevated local arteriolar and venular pressure, respectively. Due to the regional variability of local hydrostatic pressures, microvascular flow responses to gravitational stress probably vary along the length of the body. Although these differences in local autoregulation have been observed previously during whole-body tilting, they have not been investigated during application of artificial gravitational stresses, such as lower body negative pressure (LBNP) or +Gz centrifugation. Although these stresses can create equivalent G-levels at the feet, they result in distinct distributions of vascular transmural pressure along the length of the body, and should consequently elicit different magnitudes and distributions of microvascular response. In the present study, the effects of whole-body tilting and LBNP on the level and distribution of microvascular flows within skin along the length of the body were compared.

Four window differential capillary velocimetry.

A video red blood cell velocimeter was implemented with four photometric windows in such a fashion that the upstream and downstream signals are the difference between spatially separated window pairs. The performance of this system was compared with that of a conventional dual window photometric video velocimeter. Tests were made with artificial patterns of red blood cells that simulated long trains of contiguous cells, or large plasma gaps. It was found that the four window system produces a correlogram that is better suited for delay to maximum cross-correlation detection. Similarly, when the responses of the two methods were compared in terms of ability to detect changes of velocity, the time constant for a test step velocity change was found to be 1.2 +/- 0.7 sec for the four window system vs 2.3 +/- 0.6 sec for the two window system. It is concluded that this modification of the capillary red blood cell velocimetry methodology is better suited for detecting the spontaneous flow variations due to vasomotion.

Effects of hemodilution on skin microcirculation.

Red blood cell (RBC) velocity, capillary hematocrit (Hctcap), RBC flux, and arteriolar diameter were studied in the subcutaneous connective tissue of the Syrian hamster skinfold window preparation during successive normovolemic hemodilutions with 6% solutions of Dextran 70. The experiments were carried out in the unanesthetized animal. Heart rate (HR), mean systemic arterial pressure (Psys), and systemic hematocrit (Hctsys) were monitored throughout the procedure to ensure that normovolemia was maintained and hemodilution caused no adverse effects. The changes of RBC flux in the hemodiluted state, up to 50% hemodilution (Hctsys = 25% +/- 4), were not statistically significant when compared with control (t test, P less than 0.5). At this Hctsys, capillary RBC velocity increased by 60% and Hctcap decreased by 30%. Both of these changes were statistically significant relative to control. The arteriolar diameter did not change significantly during the reduction of Hctsys. The animals were studied during subsequent days to determine the chronic effects of hemodilution. Hctsys increased by approximately 12% per day after the experiment, and the systemic parameters returned to the prehemodiluted state in direct proportion to the reestablishment of the Hctsys.