Initiation and development of fingertip forces during whole-hand grasping

The present study examined the initiation of digit contact and fingertip force development during whole-hand grasping. Sixteen healthy subjects grasped an object instrumented with force transducers at each digit and lifted it 10 cm. The grip (normal) and load (tangential) forces and the position of the object were recorded. Twenty-five lifts were performed with various object weights (300 g, 600 g, 900 g) and surface textures (sandpaper and rayon). Despite the large number of degrees of freedom, grip initiation with an object using the whole hand was characterized by stereotypical contact patterns, which are idiosyncratic to each subject across all object weights and textures. However, in spite of the initial asymmetric control, the forces were mainly synchronized by the occurrence of the peak grip and load force rates. The contribution of each digit to the total grip force decreased from radial to ulnar digits. The final force distribution was generally established already at the onset of load forces. Only subtle adjustments were seen thereafter, suggesting a fairly fixed force distribution pattern throughout the grasp. The findings suggest that, despite the large number of degrees of freedom in terms of contact initiation and force distribution in whole-hand grasping: (1) subjects employ preferred movement patterns to establish object contact with their digits, and (2) synchronize the subsequent force development and temporal coordination of the task. Thus while the complexity of the task requires control mechanisms beyond those seen in two-finger precision grasping, there are strategies to simplify the complex task of the initiation and development of fingertip forces in whole-hand grasping.     

Bimanual coupling during the specification of isometric forces

The present study investigated the generalizability of the hypothesis of transient coupling during the preparation of bimanual movements (Spijkers and Heuer 1995) to the specification of isometric forces. In the first experiment we used the timed response paradigm (TRP) to examine the time course of the specification process. Subjects had to generate bimanual isometric force pulses while preparation time was controlled by the TRP. Target forces were weak (20% of maximal voluntary force, MVF) or strong (40% MVF) and assigned randomly to each hand. The first experiment revealed the predicted pattern of correlations between the peak forces but, because the subjects tended to delay responding when time for preparation was very brief, the time course of the specification process did not fully match expectations. In the second experiment we improved force–trajectory feedback and presented two initial cues that were expected to induce better preparation of the default force (30% MVF). Both changes were successful and the results further corroborate the transient-coupling hypothesis. Received: 15 October 1998 / Accepted: 04 June 1999     

Myo-electric signals from two extrinsic hand muscles and force tremor during isometric handgrip

Summary The objective of the present study was to investigate the myo-electric signs of muscle fatigue and the isometric force tremor of two extrinsic hand muscles, extensor digitorum communis and flexor digitorum superficialis, during isometric power grip. In addition, the synergy between flexor and extensor muscles and hand differences in a right-handed population have been studied. During isometric hand-dynamometry the myo-electric signal was recorded using surface electrodes and isometric force tremor was recorded using a special load cell. Eight subjects participated in this study and contractions were performed at 20%, 40%, 60% and 80% of maximal voluntary contraction (MVC) with left and right hands. The decrease of mean power frequency (MPF) with duration of contraction was greater in the left extensor as compared to the ipsilateral flexor muscle. No differences in the decrease in MPF with the duration of the contraction were found between the right extensor and flexor muscles. Isometric force tremor root mean square did not change during contractions at a given contraction level. Isometric tremor amplitude increased from 20% to 60% MVC and decreased at higher contraction levels. Tremor amplitude was higher in the left hand at all contraction levels but 60% MVC. These data would suggest differences in fatiguability and muscle fibre composition between the dominant and nondominant hand, which may be due to preferred use. The significance of force tremor for the evaluation of recruitment order and muscle fatigue is discussed.     

Effects of object shape and visual feedback on hand configuration during grasping

Normal subjects gradually preshape their hands during a grasping movement in order to conform the hand to the shape of a target object. The evolution of hand preshaping may depend on visual feedback about arm and hand position as well as on target shape and location at specific times during the movement. The present study manipulated object shape in order to produce differentiable patterns of finger placement along two orthogonal "dimensions" (flexion/extension and abduction/adduction), and manipulated the amount of available visual information during a grasp. Normal subjects were asked to reach to and grasp a set of objects presented in a randomized fashion at a fixed spatial location in three visual feedback conditions: Full Vision (both hand and target visible), Object Vision (only the object was visible but not the hand) and No Vision (vision of neither the hand nor the object during the movement). Flexion/extension angles of the metacarpophalangeal and proximal interphalangeal joints of the index, ring, middle and pinkie fingers as well as the abduction/adduction angles between the index-middle and middle-ring fingers were recorded. Kinematic analysis revealed that as visual feedback was reduced, movement duration increased and time to peak aperture of the hand decreased, in accord with previously reported studies. Analysis of the patterns of joint flexion/extension and abduction/adduction per object shape revealed that preshaping based on the abduction/adduction dimension occurred early during the reach for all visual feedback conditions (~45% of normalized movement time). This early preshaping across visual feedback conditions suggests the existence of mechanisms involved in the selection of basic hand configurations. Furthermore, while configuration changes in the flexion/extension dimension resulting in well-defined hand configurations occurred earlier during the movement in the Object Vision and No Vision conditions (45%), those in the Full Vision condition were observed only after 75% of the movement, as the moving hand entered the central region of the visual field. The data indicate that there are at least two control mechanisms at work during hand preshaping, an early predictive phase during which grip selection is attained regardless of availability of visual feedback and a late responsive phase during which subjects may use visual feedback to optimize their grasp.     

Dyscoordination of pinch and lift forces during grasp in patients with cerebellar lesions

Effects of cerebellar lesions on the production of isometric pinch force and the coordination of pinch and lift force were examined. Twenty-one patients, mostly with degenerative cerebellar disorders, and ten healthy controls lifted an instrumented test object using the precision grip of thumb and index finger. The load of the object could be varied to study the adaptation of pinch force generation. The results were: (1) Cerebellar patients were able to adapt their pinch force levels to the different object loads. (2) Patients showed a longer latency between the onset of pinch force and onset of lift force than controls. The level of pinch force at the start of lift force was elevated. (3) Patients were able to use sensori-motor memory about object load to adapt force output based on previous experience through repetitive testing, but they were significantly less efficient than healthy controls. (4) The temporal profile of pinch force rate of change featured an irregular pattern characteristic for a lack of sufficient anticipatory parameterization.     

Forced ventilation increases variability of isometric finger forces

The study aimed to assess the effects of forced ventilation on variability of the index finger force at the submaximal levels. Fourteen (6 men, 8 women) healthy subjects were instructed to perform self-initiated forced inspiration and forced expiration, the Valsalva maneuver and normal breathing while sustaining 15%, 30%, and 45% of maximal voluntary contraction (MVC) by the index finger. Standard deviation (S.D.) of finger forces increased significantly with the level of force. At each level of force, the mean force was not significantly changed under different breathing conditions. S.D. and coefficient variation (CV) during forced inspiration and expiration was significantly greater than S.D. and CV during normal breathing and the Valsalva maneuver at each force level. No significant differences in S.D. and CV were found between forced inspiration and expiration or between normal breathing and the Valsalva maneuver. Force variability synchronized with the initiation of forced inspiration and expiration, but not with the ventilation data during the Valsalva maneuver or normal breathing. These findings demonstrate clearly that finger force variability is affected by specific ventilation patterns at submaximal force levels. Therefore, assessment of force variability should consider the influence of ventilation.     

Asymmetric control of bilateral isometric finger forces

We examined the ability to match the voluntary isometric finger flexion forces of the dominant and nondominant hand in humans, as well as the influence of unilateral visual feedback during this task. Right-and left-handed subjects were trained to produce a low force level (50±25 g) and a high force level (200±50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the isometric forces of both fingers simultaneously within the required range (either low or high) so that they were perceived to be identical. The results showed an asymmetry, whereby greater forces were exerted with the index finger of the dominant hand. The asymmetry was independent of the subjects' maximum finger flexion strength. When unilateral visual feedback represented the force output of the dominant hand, the asymmetry was no longer present. In contrast, when it represented the force output of the nondominant hand, the asymmetry was not compensated. We hypothesize that these findings are the result of anatomical or physiological asymmetries inherent in the motor system controlling the production of force.     

Force development during target-directed isometric force production in Parkinson's disease

Performing various daily activities requires precise application and control of forces, which has been well addressed in neurologically healthy individuals. Recent experiments have demonstrated that in young, normal subjects generating rapid force pulses over various force amplitudes was accomplished by linearly increasing the rate of force development while keeping time to peak force approximately constant (i.e., a pulse-height control strategy). Using Parkinson's disease (PD) patients the present study examined whether PD patients use a pulse-height control strategy during rapid and accurate isometric force production. Subjects were instructed to produce force pulses to three different target amplitudes (15, 35, and 55% of their maximal voluntary contractions) at their preferred speed and as fast as possible. When the task was performed as fast as possible, PD patients differed from controls by producing reduced rates of force development and prolonged times to peak force as a function of force amplitude. During performance of the task at preferred speed, which leaves the rate of force production unconstrained, PD patients did not show improved regulation of time to peak force in scaling different force amplitudes compared to controls. These results suggest that PD patients have a difficulty in utilizing a pulse-height control strategy and that such impairments are not dependent on speed.     

A comparison of isometric force,maximum power and isometric heat rate as a function of sarcomere length in mouse skeletal muscle

The isometric force, maximum power and isometric heat rate have been measured at different sarcomere lengths (SL) between 1.40 and 3.63 m in two types of mouse muscle, soleus and omohyoideus, at 25°C. The SL force relationship is different in the two muscles. At a SL above optimum filament overlap, 2.44 m in omohyoideus muscles, maximum power declined while isometric force remained high. In soleus muscles this occurred above a SL of 2.33 m. In parallel experiments, the isometric heat rate declined linearly with increasing SL above 2.33 m in soleus muscles, while isometric force remained closer to its maximum. At short SL, between 2.33–1.75 m in soleus and 2.44–2.15 m in omohyoideus, maximum power remained at or near its maximum value as did heat rate (soleus) while isometric force fell. In both muscles at SL greater than optimum for force development maximum power output (unlike force) is proportional to filament overlap. The variation in heat rate over this SL range can be described as the sum of a constant rate and a rate proportional to filament overlap. These observations are compatible with the idea that maximum power and heat rate are less affected by non-uniformities in SL than is force.     

Measurement of forearm blood flow by venous occlusion plethysmography: Influence of hand blood flow during sustained and intermittent isometric exercise

Summary The requirement for using an arterial occlusion cuff at the wrist when measuring forearm blood flows by plethysmography was tested on a total of 8 subjects at rest and during and after sustained and intermittent isometric exercise. The contribution of the venous effluent from the hand to the forearm flow during exercise was challenged by immersing the arm in water at 20, 34, and 40 C. Occlusion of the circulation to the hand reduced the blood flow through the resting forearm at all water temperatures. There was an inverse relationship between the temperature of the water and the proportion in the reduction of forearm blood flow upon inflation of the wrist-cuff, ranging from 45 to 19% at 20 to 40 C, respectively. However, during sustained isometric exercise at 10% of the subjects maximum voluntary contraction (MVC) there was no reduction in the measured forearm flow when an arterial occlusion cuff was inflated around the wrist. Similarly, there was no alteration in the blood flow measured 2 s after each of a series of intermittent isometric contractions exerted at 20% or 60% MVC for 2 s whether or not circulation to the hand was occluded nor of the post-exercise hyperemia following 1 min of sustained contraction at 40% MVC. These results indicate that a wrist-cuff is not required for accurate measurement of forearm blood flows during or after isometric exercise.This work was supported by N.I.H. training grant HLO 7050-03, H.E.W. contract 210-77-0044 and Air Force grant AFOSR-76-3084 B     

Muscular sound and force relationship during isometric contraction in man

Summary The contracting muscle generates a low frequency sound detectable at the belly surface, ranging from 11 to 40 Hz. To study the relationship between the muscular sound and the intensity of the contraction a sound myogram (SMG) was recorded by a contact sensor from the biceps brachii of seven young healthy males performing 4-s isometric contractions from 10% to 100% of the maximal voluntary contraction (MVC), in 10% steps. Simultaneously, the electromyogram (EMG) was recorded as an index of muscle activity. SMG and EMG were integrated by conventional methods (iSMG and iEMG). The relationship between iSMG and iEMG vs MVC% is described by parabolic functions up to 80% and 100% MVC respectively. Beyond 80% MVC the iSMG decreases, being about half of its maximal value at 100% MVC. Our results indicate that the motor unit recruitment and firing rate affect the iSMG and iEMG in the same way up to 80% MVC. From 80% to 100% MVC the high motor units' discharge rate and the muscular stiffness together limit the pressure waves generated by the dimensional changes of the active fibres. The muscular sound seems to reflect the intramuscular visco-elastic characteristics and the motor unit activation pattern of a contracting muscle.     

Prediction of object contact during grasping

The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects’ adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the ‘timing’ of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.     

Contribution of tactile afferent information to the control of isometric finger forces

The ability to match the voluntary isometric force output of the right and left index fingers when the contact surfaces differ in shape was examined. Before the experiment, subjects were trained to produce both a low force level (50±25 g) and a high force level (200±50 g) with the right and left index finger, separately. Following the training session, subjects were instructed to match the forces of both fingers simultaneously within the required range (either low or high) so that the forces were perceived to be identical. One of the index fingers pushed against a conical contact pad, while the other pushed against a flat contact pad. Midway through the experiment, the two contact pads were reversed. Subjects consistently produced less force with the finger pressing against the conical pad. This asymmetry could already be seen during the beginning of the ramp increase in force and continued throughout the trial, independent of the target force levels (low or high). These findings suggest tactile afferent information at the fingertip is important for determining the voluntary force exerted by the finger. It must be properly integrated with other peripheral information as well as with the central motor command, otherwise the perception of force is distorted. Furthermore, the perception of the force produced seemingly is dependent on the extent to which the skin of the fingertip is indented rather than the local pressure exerted at the skin.     

No graded responses of finger muscles to TMS during motor imagery of isometric finger forces

Previous studies of motor imagery have shown that the same neural correlates for actual movement are selectively activated during motor imagery of the same movement. However, little is known about motor imagery of isometric force. The aim of the present study was to investigate the neural correlates involved in motor imagery of isometric finger forces. Ten subjects were instructed to produce a finger flexion or extension force ranging from 10% to 60% of maximal isometric force and to mentally reproduce the force after an eight-second delay period. Transcranial magnetic stimulation (TMS) was applied over the hand motor area during imagining the force. We measured the amplitude of motor evoked potentials (MEPs) from the flexor digitorum superfialis (FDS) and the extensor digitorum communis (EDC) muscles and TMS-induced forces from the proximal phalanxes. The results showed that, as compared to the rest condition, the MEP amplitude was greater in the FDS during imagining flexion forces, whereas it was greater in the EDC during imagining extension forces. MEP amplitudes were similar for motor imagery of graded flexion or extension forces. Also, TMS produced flexion forces during imagining flexion forces, whereas it produced extension forces during imagining extension forces. There was no change in the amplitude of TMS-induced forces across graded motor imagery task. These results support the notion that the same neural correlates for actual movement could be selectively activated during motor imagery of the same movement, but demonstrated that the magnitude of isometric force could not be mentally simulated.     

Electrical and mechanical response of finger flexor muscles during voluntary isometric contractions in elite rock-climbers

To determine the differences between rock-climbers and controls in finger flexor (FF) motor units (MUs) features and activation strategy, eleven climbers and ten controls volunteered for the study. After maximal voluntary contraction (MVC) assessment, five levels of isometric contractions at 20, 40, 60, 80 and 100% MVC were performed. During contractions, electromyogram (EMG) and mechanomyogram (MMG) were recorded, from which the root mean square (RMS) and mean frequency (MF) were calculated. Climbers showed significantly higher MVC. EMG RMS was statistically higher in climbers than in controls from 60 to 100% MVC. In climbers MMG RMS increased up to 80% MVC, whereas in controls it increased only up to 60% MVC. MMG MF was higher in climbers than in controls from 60 to 100% MVC (P < 0.05). EMG–MMG combined analysis revealed significant differences in MU activation strategy between the two groups. The results are compatible with a shift of climbers’ muscles toward faster MUs.     

Effects of dynamic and static handgrip exercises on hand and wrist volume

It is not known how the mode of exercise, dynamic and static exercises, affects the limb volume. Therefore, the purpose of this study was to investigate hand and wrist volume (HWV) after dynamic and static handgrip exercise. Nine healthy subjects (age 31.8 ± 7.3 years; height 172.0 ± 5.7 cm; body mass 66.9 ± 8.1 kg, mean ± SD) volunteered for this study. HWV was measured with a hand and wrist volumeter before and immediately after dynamic and static exercises. Initially during rest, HWV was measured after the hand was passively hung for 5 min. Handgrip exercises with an ergonomic hand exerciser were performed at 20% of maximum voluntary contraction in right and left hands by static and dynamic exercises, respectively. Both dynamic and static handgrip exercises consisted of six sets of 30-s contractions with 10-s rest intervals between exercise bouts. The dynamic handgrip exercise was performed by repetitive contraction and relaxation of the hand at a maximum frequency. In order to determine intensity of handgrip exercises, maximum isometric handgrip strength of the right and left hand was measured with a handgrip dynamometer. Data are presented as mean ± SD. After dynamic and static handgrip exercises, HWV increased significantly, and these increases represent 2.2 ± 0.7% (P < 0.001) and 1.4 ± 0.8% (P < 0.001) of resting HWV, respectively. The elevation of HWV after dynamic exercise was significantly higher than that after static exercise (P < 0.05). These results suggest that the higher HWV after dynamic exercise may be caused by higher increased interstitial fluid volume, capillary volume and venous volume in hand and wrist tissues.     

Endpoint accuracy for a small and a large hand muscle in young and old adults during rapid,goal-directed isometric contractions

The minimum variance theory proposes that stronger (larger) muscles produce less variable trajectories compared with weaker (smaller) muscles and thus can accomplish more accurate contractions. The purpose of the study was to determine the influence of muscle size and trajectory variability on the endpoint accuracy of goal-directed isometric contractions. Twelve young (25 ± 5 years) and 12 old adults (76 ± 6 years) performed 100 trials with each of two muscles in both hands. Subjects were instructed to match the peak of a force trajectory to a target force by controlling either the abduction (first dorsal interosseus muscle; FDI) or adduction force (second palmar interosseus muscle; SPI) exerted by the index finger of each hand. The time to peak force was 150 ms and the peak force required was 25% of the maximal force that could be achieved in 150 ms. Endpoint accuracy and variability in force and time along with intramuscular EMG activity of the agonist muscle (FDI and SPI) involved in each task were quantified for each set of 100 trials. The MVC force was less for the SPI muscle, and the force endpoint error and variance were greater in the SPI muscle compared with the FDI muscle. Conversely, endpoint measures that included timing were similar for the two muscles. Trajectory variability was greater for the FDI muscle, but did not influence endpoint error for either muscle. The young and old adults had similar strength values, but the old adults were less accurate and more variable than the young subjects. Nonetheless, the accuracy and variability displayed by the old adults for the two muscles was the same as that observed for the young adults. The force accuracy and variability findings are consistent with the predictions of the minimum variance theory that motor-output variability is inversely related to muscle size, strength, and motor unit number.     

Effects of grasping force magnitude on the coordination of digit forces in multi-finger prehension

The study addresses three main questions: (1) Does the magnitude of the grasping force affect the prehension synergies, i.e., conjoint changes of finger forces and moments? (2) Do individual finger forces scale with the total grasping forces (‘scale-invariance hypothesis’)? (3) How specification of the grasping force magnitude affects the inverse optimization of digit forces. Subjects (n = 7) grasped with minimal force an instrumented handle and maintained it at rest in the air. Then, the subjects doubled the initial grasping force. The forces and moments exerted by individual digits were recorded with six-component sensors. External torques that the subjects should resist (9 in total) varied among the trials from 0 to 0.46 Nm both in clockwise and counterclockwise directions. After the force doubling, the moments of the normal forces (M ⁿ) increased in the pronation effort tasks (PR-tasks) and decreased in the supination effort tasks (SU-tasks). The changes in the moments of the tangential forces (M ^t) were opposite to the M ⁿ changes; the moments increased in the SU-tasks and decreased in the PR-tasks. The opposite effects of force doubling on the M ^ts in the SU-tasks and PR-tasks were a consequence of the unidirectional changes of the thumb tangential forces: in all the tasks the contribution of the thumb tangential force to the total tangential force increased after the grasping force doubling (and the total contribution of the four fingers decreased). The decrease of the virtual finger (VF) tangential force was mainly due to the decrease of the index finger force (VF is an imagined finger that exerts the same force and moment as all the fingers together). In the non-zero torque tasks the individual finger forces did not scale proportionally with the grasping force, the sharing percentage of the individual finger forces in the VF normal force changed with the grasping force increase. The root mean square differences between the actual finger sharing percentages in the VF force and the sharing percentages predicted from optimization procedures in which different cost functions were used were in all cases smaller after the doubling than before the doubling. Hence the answers to the three questions formulated above are: (1) the alteration of the grasping force magnitude induces complex coordinated changes of all digit forces and moments; (2) the scale invariance hypothesis is confirmed only for the zero-torque tasks and rejected for the non-zero tasks, and (3) the specification of the grasping force magnitude at the level of twice the initial grasping force—which essentially restricts the control task to the object tilt prevention—improves the accuracy of the employed optimization procedures.     

Role of nitric oxide in isometric contraction properties of rat diaphragm during hypoxia

Hypoxia disturbs Ca²⁺ regulation and increases the intracellular Ca²⁺ concentration ([Ca²⁺]_i), which may in turn activate the nitric oxide synthase (NOS) regulated by [Ca²⁺]_i. Since nitric oxide (NO) reduces the isometric contractility of rat diaphragm in vitro, we hypothesized that NO contributes to the impaired force generation of an hypoxic diaphragm. The effects of different concentrations of the NOS inhibitor, N^G-monomethyl-L-arginine (L-NMMA), the NO scavenger haemoglobin (150 μmol·l^–1) and the NO donor spermine NONOate (Sp-NO; 1 mmol·l^–1) were determined on isometric contractility during hypoxia [partial pressure of oxygen, PO₂, about 7 kPa (about 54 mmHg)] and hyperoxia [PO₂ about 83 kPa (about 639 mmHg)]. Hypoxia significantly reduced maximal twitch force (F _t), and submaximal tetanic force (30 Hz, F ₃₀) in all L-NMMA groups. A low concentration of L-NMMA (30 μmol·l^–1) increased F ₃₀ but a high concentration (1,000 μmol·l^–1) reduced F ₃₀ during hypoxia. The effects of L-NMMA on force generation were more pronounced during hypoxia compared to hyperoxia. Peak increases in F ₃₀ and F _t were observed at a concentration of 30 μmol·l^–1 L-NMMA during hypoxia, but with 10 μmol·l^–1 L-NMMA during hyperoxia. The same concentration of haemoglobin increased F ₃₀ and F _t less during hypoxia compared to hyperoxia. The Sp-NO reduced F _t, F ₃₀ and maximal tetanic force (F ₀) during hypoxia; these effects were abolished in the presence of haemoglobin. The Sp-NO did not alter F _t, F ₃₀ and F ₀ during hyperoxia. We conclude that NO plays a more prominent role during hypoxia and that NO contributes to the depression of force generation in the hypoxic rat diaphragm in vitro. This change may be related to an elevated NO generation within the hypoxic diaphragm. Electronic Publication     

Effect of arm position on cardiovascular responses during isometric handgrips

Summary Sixteen subjects (eight women and eight men, age 20–25 years) carried out in the seated position, isometric contractions sustained until exhaustion of the digital flexors. The subject's arm was placed in two positions, high and low. The muscle tensions used were 30, 40 and 50% of maximum voluntary contraction (MVC). Under these conditions, for a given relative force, the duration of contraction (limit-time) was not modified by the arm position. In the male subjects, increases in heart rate (HR) and systolic blood pressure (SBP) were slightly more pronounced in the low than the high position, but the differences were not significant. Limit times in the high position were similar to those in the low position, and, in the absence of an increase in HR and SBP, this seemed to be due to an increase in cardiac output consequent upon a transient improvement in venous return together with an increase in the coefficient of oxygen utilization.