Moving weightless objects

When we move grasped objects, our grip force precisely anticipates gravitational and inertial loads. We analysed the control of grip forces during very substantial load changes induced by parabolic flights. During these flight manoeuvres, the gravity varies between hypergravity associated with a doubling of normal terrestrial gravity and a 20-s period of microgravity. Accordingly, the contribution of the object's weight to the load changed from being twice the normal value to being absent. Two subjects continuously performed vertical and horizontal movements of an object equipped with grip force and acceleration sensors. Whereas, during vertical movements performed under normal and hypergravity, a load force maximum occurred at the lower turning point and a minimum at the upper turning point, the load force pattern was completely changed under microgravity. In particular, the upper turning point was also associated with a load force maximum. Analysis of the grip forces produced by the two subjects revealed that the grip forces underwent the same characteristic changes as the load forces. Thus, subjects were able to adjust grip forces in anticipation of arm movement-induced fluctuations in load force under different and novel load conditions. Adaptation to changing levels of gravity was also obvious when the vertical and horizontal movements were compared: grip forces depended heavily on movement direction during normal and hypergravity but not during microgravity. The predictive coupling of grip force and load force was observed even during transitions between gravity levels, indicating rapid adaptation to changing load conditions. To account for the striking preservation of the normal characteristics of grip force control, we suggest that a highly automatized, extremely flexible sensorimotor mechanism firmly implemented within the central nervous system can cope with even massive changes in the environmental conditions.     

Control of grasp stability during pronation and supination movements

 We analyzed the control of grasp stability during a major manipulative function of the human hand: rotation of a grasped object by pronation and supination movement. We investigated the regulation of grip forces used to stabilize an object held by a precision grip between the thumb and index finger when subjects rotated it around a horizontal axis. Because the center of mass was located distal to the grip axis joining the fingertips, destabilizing torque tangential to the grasp surfaces developed when the grip axis rotated relative to the field of gravity. The torque load was maximal when the grip axis was horizontal and minimal when it was vertical. An instrumented test object, with a mass distribution that resulted in substantial changes in torque load during the rotation task, measured forces and torques applied by the digits. The mass distribution of the object was unpredictably changed between trials. The grip force required to stabilize the object increased directly with increasing torque load. Importantly, the grip force used by the subjects also changed in proportion to the torque load such that subjects always employed adequate safety margins against rotational slips, i.e., some 20–40% of the grip force. Rather than driven by sensory feedback pertaining to the torque load, the changes in grip force were generated as an integral part of the motor commands that accounted for the rotation movement. Subjects changed the grip force in parallel with, or even slightly ahead of, the rotation movement, whereas grip force responses elicited by externally imposed torque load changes were markedly delayed. Moreover, blocking sensory information from the digits did not appreciably change the coordination between movement and grip force. We thus conclude that the grip force was controlled by feedforward rather than by feedback mechanisms. These feedforward mechanisms would thus predict the consequences of the rotation movement in terms of changes in fingertip loads when the orientation of the grip axis changed in the field of gravity. Changes in the object’s center of mass between trials resulted in a parametric scaling of the motor commands prior to their execution. This finding suggests that the sensorimotor memories used in manipulation to adapt the motor output for the physical properties of environmental objects also encompass information related to an object’s center of mass. This information was obtained by somatosensory cues when subjects initially grasped the object with the grip axis vertical, i.e., during minimum tangential torque load. Received: 16 October 1998 / Accepted: 1 February 1999     

Effect of slow,small movement on the vibration-evoked kinesthetic illusion

In this experiment we examined the coupling between grip force and load force observed during cyclic vertical arm movements with a hand-held object, performed in different gravitational environments. Six subjects highly experienced in parabolic flight participated in this study. They had to continuously move a cylindrical object up and down in the different gravity fields (1g, 1.8g and 0g) induced by parabolic flights. The imposed movement frequency was 1 Hz, the object mass was either 200 or 400 g, the amplitude of movement was either 20 or 40 cm and an additional mass of 200 g could be wound around the forearm. Each subject performed the task during 15 consecutive parabolas. The coordination between the grip force normal to the surface and the tangential load force was examined in nine loading conditions. We observed that the same normal grip force was used for equivalent loads generated by changes of mass, gravity or acceleration despite the fact that these loads required different motor commands to move the arm. Moreover, our results suggest that the gravitational and inertial components of the load are treated adequately and independently by the internal models used to predictively control the required grip force. These results indicate that the forward internal models used to control precision grip take into account the dynamic characteristics of the upper limb, the object and the environment to predict the objects acceleration and, in turn, the load force acting at the fingertips.     

Object size effects on initial lifting forces under microgravity conditions

Individuals usually report for two objects of equal mass but different volume that the larger object feels lighter. This so-called size-weight illusion has been investigated for more than a century. The illusion is accompanied by increased forces, used to lift the larger object, resulting in a higher initial lifting speed and acceleration. The illusion holds when subjects know that the mass of the two objects is equal and it is likely that this also counts for the enlarged initial effort in lifting a larger box. Why should this happen? Under microgravity, subjects might be able to eliminate largely the weight-related component of the lifting force. Then, if persistent upward scaling of the weight-related force component had been the main cause of the elevated initial lifting force under normal gravity, this elevated force might disappear under microgravity. On the other hand, the elevated initial lifting effort in the large box would be preserved if it had been caused mainly by a persistent upward scaling of the force component, necessary to accelerate the object. To test whether the elevated initial lifting effort either persists or disappears under microgravity, a lifting experiment was carried out during brief periods of microgravity in parabolic flights. Subjects performed whole-body lifting movements with their feet strapped to the floor of the aircraft, using two 8-kg boxes of different volume. The subjects were aware of the equality of the box masses. The peak lifting forces declined almost instantaneously with approx. a factor 9 in the first lifting movements under microgravity compared with normal gravity, suggesting a rapid adaptation to the loss of weight. Though the overall speed of the lifting movement decreased under microgravity, the mean initial acceleration of the box over the first 200 ms of the lifting movement remained higher (P=0.030) in the large box (1.87±0.127 m/s²) compared with the small box (1.47±0.122 m/s²). Under normal gravity these accelerations were 3.30±0.159 m/s² and 2.67±0.159 m/s², respectively (P=0.008). A comparable trend was found in the initial lifting forces, being significant in the pooled gravity conditions (P=0.036) but not in separate tests on the normal gravity (P=0.109) and microgravity (P=0.169) condition. It is concluded that the elevated initial lifting effort with larger objects holds during short-term exposure to microgravity. This suggests that upward scaling of the force component, required to accelerate the larger box, is an important factor in the elevated initial lifting effort (and the associated size-weight illusion) under normal gravity.     

Precision-grip force changes in the anatomical and prosthetic limb during predictable load increases

This study examined precision-grip force applied to an instrumented test object held aloft while the weight of the object was predictably varied by transporting and placing loads (50, 100, or 200 g) atop the test object. Transport of the loads was performed either by the subject or the experimenter. Grip force was examined in four non-amputee control subjects and in the anatomical and prosthetic hand of a subject with a prosthetic device. As subjects transported the load, anticipatory grip-force changes occurred in the anatomical hands and prosthetic hand, which were scaled in relation to the load. When the experimenter transported the load to the anatomical hands of control subjects or the prosthetic user, anticipatory increases in grip force occurred that also were scaled in relation to load. However, when the experimenter transported the load to the prosthetic hand, anticipatory grip-force adjustments were absent. During the phase in which the load was being assumed by the postural hand, grip forces in the anatomical hands and prosthetic hand were further scaled to load demands. Ability to adapt grip force in the prosthetic hand during this phase suggested that the subject was utilizing sensory information from the residual limb to adjust grip force. Thus, while anticipatory changes precede the process of adaptation to load changes, actual sensory consequences resulting from added weight remain necessary to fully adapt grip force to load demands, even for the prosthetic user.     

Adaptation of center of mass control under microgravity in a whole-body lifting task

 Human balance in stance is usually defined as the preservation of the vertical projection of the center of mass (COM) on thesupport area formed by the feet. Under microgravity conditions, the control of equilibrium seems to be no longer required. However, several reports indicate preservation of COM control in tasks such as arm or leg raising, tiptoe standing, or trunk bending. It is still unclear whether COM control is also maintained in complex multijoint movements during short term exposure to microgravity. In the current study, the dynamics of equilibrium control were studied in four subjects performing two series of seven whole-body lifting movements under microgravity during parabolic flights. The aims of the study were to examine whether the trajectory of horizontal COM motion during lifting movements changes in short-term exposure to microgravity and whether there is any sign of recovery after several lifting movements. It was found that, compared with control movements under normal gravity, the horizontal position of the COM was shifted backward during the entire lifting movement in all subjects. In the second series of lifting movements under microgravity, a partial recovery of the COM trajectory toward the normal gravity situation was found. Under microgravity, angles of the ankle, knee, hip, and lumbar joints differed significantly from the angles found under normal gravity. Recovery of joint angular trajectories in the second series of lifting movementsmainly occurred for those angles that could contribute to a reduction of the backward COM shift. It is to be pointed out that COM control under microgravity is not redundant but functional. Persisting COM control under microgravity may be required for pure mechanical reasons, since rotational movements of the body are dependent on adequate control of the COM position with respect to external forces. It is shown that, from a mechanical perspective, subjects can benefit from a backward displacement of the COM in the downward as well as the upward phase of the lifting movement under microgravity. Received: 3 November 1998 / Accepted: 8 November 1998     

Increased brain cortical activity during parabolic flights has no influence on a motor tracking task

Previous studies showed that changing forces of gravity as they typically occur during parabolic flights might be responsible for adaptional processes of the CNS. However, until now it has not been differentiated between primary influences of weightlessness and secondary influences due to psycho-physiological factors (e.g., physical or mental strain). With the aim of detecting parabolic flight related changes in central cortical activity, a resting EEG was deduced in 16 subjects before, during and after parabolic flights. After subdividing EEG into α-, β-,δ- and θ-wave bands, an increase in β-power was noticeable inflight, whereas α₁-power was increased postflight. No changes could be observed for the control group. To control possible effects of cortical activation, a manual tracking task with mirror inversion was performed during either the phase of weightlessness or during the normal gravity phase of a parabolic flight. No differences in performance nor in adaptation could be observed between both groups. A third group, performing under normal and stress-free conditions in a lab showed similar tracking values. We assume that the specific increase in brain activity is a sign of an increase in arousal inflight. This does support previous assumptions of non-specific stressors during parabolic flights and has to be considered as a relevant factor for experiments on central nerve adaptation. Although no influences of stress and/or weightlessness on motor perfromance and adaptation could be observed, we suggest that an “inflight” control group seems to be more adequate than a laboratory control group to investigate gravity-dependent changes in motor control.     

Effect of gravity-like torque on goal-directed arm movements in microgravity

Gravitational force level is well-known to influence arm motor control. Specifically, hyper- or microgravity environments drastically change pointing accuracy and kinematics, particularly during initial exposure. These modifications are thought to partly reflect impairment in arm position sense. Here we investigated whether applying normogravitational constraints at joint level during microgravity episodes of parabolic flights could restore movement accuracy equivalent to that observed on Earth. Subjects with eyes closed performed arm reaching movements toward predefined sagittal angular positions in four environment conditions: normogravity, hypergravity, microgravity, and microgravity with elastic bands attached to the arm to mimic gravity-like torque at the shoulder joint. We found that subjects overshot and undershot the target orientations in hypergravity and microgravity, respectively, relative to a normogravity baseline. Strikingly, adding gravity-like torque prior to and during movements performed in microgravity allowed subjects to be as accurate as in normogravity. In the former condition, arm movement kinematics, as notably illustrated by the relative time to peak velocity, were also unchanged relative to normogravity, whereas significant modifications were found in hyper- and microgravity. Overall, these results suggest that arm motor planning and control are tuned with respect to gravitational information issued from joint torque, which presumably enhances arm position sense and activates internal models optimally adapted to the gravitoinertial environment.     

Anticipatory postural adjustments in stance and grip

 The reactive forces and torques associated with moving a hand-held object between two points are potentially destabilising, both for the object’s position in the hand and for body posture. Previous work has demonstrated that there are increases in grip force ahead of arm motion that contribute to object stability in the hand. Other studies have shown that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm movement. This paper documents the concurrent evolution of grip force and postural adjustments in anticipation of dynamic and static loads. Subjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizontally towards or away from the body (the grasp axis was orthogonal to the line of the load force). A force plate measured ground reaction torques, and force transducers in the manipulandum measured the load (tangential) and grip (normal) forces acting on the thumb and finger. In all conditions, increases in grip force and ground reaction torque preceded any detectable rise in load force. Rates of change of grip force and ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and ground reaction torque rates at the onset of load force were correlated. These results imply the operation of motor planning processes that include anticipation of the dynamic consequences of voluntary action. Received: 28 June 1996 / Accepted: 4 February 1997     

Predictive and reactive control of grasping forces: on the role of the basal ganglia and sensory feedback

We comparatively investigated predictive and reactive grip force behaviour in 12 subjects with basal ganglia dysfunction (six subjects with Parkinson’s disease, six subjects with writer’s cramp), two subjects chronically lacking all tactile and proprioceptive sensory feedback and 16 sex- and age-matched control subjects. Subjects held an instrumented receptacle between the index finger and thumb. A weight was dropped into the receptacle either unexpectedly from the experimenter’s hand with the subject being blindfolded or expectedly from the subject’s opposite hand. This paradigm allowed us to study predictive and reactive modes of grip force control. All patients generated an overshoot in grip force, irrespective of whether the weight was dropped expectedly or unexpectedly. When the weight was dropped from the experimenter’s hand, a reactive grip force response lagged behind the load perturbation at impact in patients with basal ganglia dysfunction and healthy controls. When the weight was dropped expectedly from the subject’s opposite hand, patients with basal ganglia dysfunction and healthy subjects started to increase grip force prior to the release of the weight, indicating a predictive mode of control. We interpret these data to support the notion that the motor dysfunction in basal ganglia disorders is associated with deficits of sensorimotor integration. Both deafferented subjects did not show a reactive mode of force control when the weight was dropped unexpectedly, underlining the importance of sensory feedback to initiate reactive force responses. Also in the predictive mode, grip force processing was severely impaired in deafferented subjects. Thus, at least intermittent sensory information is necessary to establish and update predictive modes of grasping force control.     

The effects of a change in gravity on the dynamics of prehension

Investigating cyclic vertical arm movements with an instrumented hand-held load in an airplane undergoing parabolic flight profiles allowed us to determine how humans modulate their grip force when the gravitational and the inertial components of the load force are varied independently. Eight subjects participated in this study; four had already experienced parabolic flights and four had not. The subjects were asked to move the load up and down continuously at three different gravitational conditions (1 g, 1.8 g, and 0 g). At 1 g, the grip force precisely anticipated the fluctuations in the load force, which was maximum at the bottom of the object trajectory and minimum at the top. When gravity changed, the temporal coupling between grip force and load force persisted for all subjects from the first parabola. At 0 g, the grip force was accurately adjusted to the two load force peaks occurring at the two opposite extremes of the trajectory due to the absence of weight. While the experienced subjects exerted a grip force appropriate to a new combination of weight and inertia since their first trial, the inexperienced subjects dramatically increased their grip when faced with either high or low force levels for the first time. Then they progressively released their grip until a continuous grip-load force relationship with regard to 1 g was established after the fifth parabola. We suggest that a central representation of the new gravitational field was rapidly acquired through the incoming vestibular and somatic sensory information. Electronic Publication     

Grip-force responses to unanticipated object loading: load direction reveals body- and gravity-referenced intrinsic task variables

Humans preserve grasp stability by automatically regulating the grip forces when loads are applied tangentially to the grip surfaces of a manipulandum held in a precision grip. The effects of the direction of the load force in relation to the palm, trunk, and gravity were investigated in blindfolded subjects. Controlled, tangential load-forces were delivered in an unpredictable manner to the grip surface in contact with the index finger either in the distal and proximal directions (away from and toward the palm) or in the ulnar and radial directions (transverse to the palm). The hand was oriented in: (1) a standard position, with the forearm extended horizontally and anteriorly in intermediate pronosupination; (2) an inverted position, reversing the direction of radial and ulnar loads in relation to gravity; and (3) a horizontally rotated position, in which distal loads were directed toward the trunk. The amplitude of the grip-force responses (perpendicular to the grip surface) varied with the direction of load in a manner reflecting frictional anisotropies at the digit-object interface; that is, the subjects automatically scaled the grip responses to provide similar safety margins against frictional slips. For all hand positions, the time from onset of load increase to start of the gripforce increase was shorter for distal loads, which tended to pull the object out of the hand, than for proximal loads. Furthermore, this latency was shorter for loads in the direction of gravity, regardless of hand position. Thus, shorter latencies were observed when frictional forces alone opposed the load, while longer latencies occurred when gravity also opposed the load or when the more proximal parts of the digits and palm were positioned in the path of the load. These latency effects were due to different processing delays in the central nervous system and may reflect the preparation of a default response in certain critical directions. The response to loads in other directions would incur delays required to implement a new frictional scaling and a different muscle activation pattern to counteract the load forces. We conclude that load direction, referenced to gravity and to the hand's geometry, represents intrinsic task variables in the automatic processes that maintain a stable grasp on objects subjected to unpredictable load forces. In contrast, the grip-force safety margin against frictional slips did not vary systematically with respect to these task variables. Instead, the magnitude of the grip-force responses varied across load direction and hand orientation according to frictional differences providing similar safety margins supporting grasp stability.     

Nondigital afferent input in reactive control of fingertip forces during precision grip

Sensory inputs from the digits are important in initiating and scaling automatic reactive grip responses that help prevent frictional slips when grasped objects are subjected to destabilizing load forces. In the present study we analyzed the contribution to grip-force control from mechanoreceptors located proximal to the digits when subjects held a small manipulandum between the tips of the thumb and index finger. Loads of various controlled amplitudes and rates were delivered tangential to the grip surfaces at unpredictable times. Grip forces (normal to the grip surfaces) and the position of the manipulandum were recorded. In addition, movements of hand and arm segments were assessed by recording the position of markers placed at critical points. Subjects performed test series during normal digital sensibility and during local anesthesia of the index finger and thumb. To grade the size of movements of tissues proximal to the digits caused by the loadings, three different conditions of arm and hand support were used; (1) in the hand-support condition the subjects used the three ulnar fingers to grasp a vertical dowel support and the forearm was supported in a vacuum cast; (2) in the forearm-support condition only the forearm was supported; finally, (3) in the no-support condition the arm was free. With normal digital sensibility the size of the movements proximal to the digits had small effects on the grip-force control. In contrast, the grip control was markedly influenced by the extent of such movements during digital anesthesia. The poorest control was observed in the hand-support condition, allowing essentially only digital movements. The grip responses were either absent or attenuated, with greatly prolonged onset latencies. In the forearm and no-support conditions, when marked wrist movements took place, both the frequency and the strength of grip-force responses were higher, and the grip response latencies were shorter. However, the performance never approached normal. It is concluded that sensory inputs from the digits are dominant in reactive grip control. However, nondigital sensory input may be used for some grip control during impaired digital sensibility. Furthermore, the quality of the control during impaired sensibility depends on the extent of movements evoked by the load in the distal, unanesthetized parts of the arm. The origin of these useful sensory signals is discussed.     

Is writer's cramp caused by a deficit of sensorimotor integration?

Writer's cramp is a highly specific movement disorder in which handwriting is impaired while most other manual skills are often unaffected. On the basis of abnormal findings in experiments measuring the control of grip forces, it has been suggested that writer's cramp is caused by a deficit of sensorimotor integration. The aim of our study was to determine whether there is a functional link between sensory deficits, abnormalities in the control of grip force, and handwriting disorders. We compared the grip force and handwriting performance of writer's cramp patients with that of control subjects and with that of a stroke patient suffering a purely somatosensory deficit of his dominant hand (patient S1). We found that: (1) writer's cramp patients and patient S1 had elevated grip-force levels; (2) training reduced the grip force to near-normal levels in all writer's cramp patients but not in S1; (3) effortful writing performance also induced increased grip-force levels in healthy subjects; and (4) patient S1 had normal handwriting movements. These findings suggest that the elevated pretraining gripforce levels of writer's cramp patients might be a consequence of their effortful writing style and do not reflect a deficit of sensorimotor integration. Moreover, the good handwriting performance of patient S1 shows that a severe somatosensory deficit is not a sufficient condition for a handwriting disorder. These findings disagree with the sensorimotor explanation of writer's cramp.     

Coordinated isometric muscle commands adequately and erroneously programmed for the weight during lifting task with precision grip

Summary Small objects were lifted from a table, held in the air, and replaced using the precision grip between the index finger and thumb. The adaptation of motor commands to variations in the object's weight and sensori-motor mechanisms responsible for optimum performance of the transition between the various phases of the task were examined. The lifting movement involved mainly a flexion of the elbow joint. The grip force, the load force (vertical lifting force) and the vertical position were measured. Electromyographic activity (e.m.g.) was recorded from four antagonist pairs of hand/arm muscles primarily influencing the grip force or the load force. In the lifting series with constant weight, the force development was adequately programmed for the current weight during the loading phase (i.e. the phase of parallel increase in the load and grip forces during isometric conditions before the lift-off). The grip and load force rate trajectories were mainly single-peaked, bell-shaped and roughly proportional to the final force. In the lifting series with unexpected weight changes between lifts, it was established that these force rate profiles were programmed on the basis of the previous weight. Consequently, with lifts programmed for a lighter weight the object did not move at the end of the continuous force increase. Then the forces increased in a discontinous fashion until the force of gravity was overcome. With lifts programmed for a heavier weight, the high load and grip force rates at the moment the load force overcame the force of gravity caused a pronounced positional overshoot and a high grip force peak, respectively. In these conditions the erroneous programmed commands were automatically terminated by somatosensory signals elicited by the start of the movement. A similar triggering by somatosensory information applied to the release of programmed motor commands accounting for the unloading phase (i.e. the parallel decrease in the grip and load forces after the object contacted the table following its replacement). These commands were always adequately programmed for the weight.     

Holding an object: neural activity associated with fingertip force adjustments to external perturbations

When you hold an object, a sudden unexpected perturbation can threaten the stability of your grasp. In such situations grasp stability is maintained by fast reflexive-like grip-force responses triggered by the somatosensory feedback. Here we use functional magnetic resonance imaging (fMRI) to investigate the neural mechanisms involved in the grip-force responses associated with unexpected increases (loading) and decreases (unloading) in the load force. Healthy right-handed subjects held an instrumented object (of mass 200 g) between the tips of right index finger and thumb. At some time during an interval of 8 to 45 s the weight of the object was suddenly increased or decreased by 90 g. We analyzed the transient increases in the fMRI signal that corresponded precisely in time to these grip-force responses. Activity in the left primary motor cortex was associated with the loading response, but not with unloading, suggesting that sensorimotor processing in this area mediates the sensory-triggered reflexive increase in grip force during loading. Both the loading and the unloading events activated the cingulate motor area and the medial cerebellum. We suggest that these regions could participate in the updating of the sensorimotor representations of the fingertip forces. Finally, the supplementary somatosensory area located on the medial wall of the parietal lobe showed an increase in activity only during unloading, indicating that this area is involved in the sensorimotor processing generating the unloading response. Taken together, our findings suggest different central mechanisms for the grip-force responses during loading and unloading.     

Grip-force modulation in multi-finger prehension during wrist flexion and extension

Extrinsic digit muscles contribute to both fingertip forces and wrist movements (FDP and FPL–flexion, EDC–extension). Hence, it is expected that finger forces depend on the wrist movement and position. We investigated the relation between grip force and wrist kinematics to examine whether and how the force (1) scales with wrist flexion–extension (FE) angle and (2) can be predicted from accelerations induced during FE movement. In one experiment, subjects naturally held an instrumented handle using a prismatic grasp and performed very slow FE movements. In another experiment, the same movement was performed cyclically at three prescribed frequencies. In quasistatic conditions, the grip force remained constant over the majority of the wrist range of motion. During the cyclic movements, the grip force changed. The changes were described with a linear regression model that represents the thumb and virtual finger (VF = four fingers combined) normal forces as the sum of the effects of the object’s tangential and radial accelerations and an object-weight-dependent constant term. The model explained 99 % of the variability in the data. The independence of the grip force from wrist position agrees with the theory that the thumb and VF forces are controlled with two neural variables that encode referent coordinates for each digit while accounting for changes in the position dependence of muscle forces, rather than a single neural variable like referent aperture. The results of the cyclical movement study extend the principle of superposition (some complex actions can be decomposed into independently controlled elemental actions) for a motor task involving simultaneous grip-force exertion and wrist motion with significant length changes of the grip-force-producing muscles.     

Kinematic synergy adaptation to microgravity during forward trunk movement

The aim of the present investigation was to see whether the kinematic synergy responsible for equilibrium control during upper trunk movement was preserved in absence of gravity constraints. In this context, forward trunk movements were studied during both straight-and-level flights (earth-normal gravity condition: normogravity) and periods of weightlessness in parabolic flights (microgravity). Five standing adult subjects had their feet attached to a platform, their eyes were open, and their hands were clasped behind their back. They were instructed to bend the trunk (the head and the trunk together) forward by approximately 35 degrees with respect to the vertical in the sagittal plane as fast as possible in response to a tone, and then to hold the final position for 3 s. The initial and final anteroposterior center of mass (CM) positions (i.e., 200 ms before the onset of the movement and 400 ms after the offset of the movement, respectively), the time course of the anteroposterior CM shift during the movement, and the electromyographic (EMG) pattern of the main muscles involved in the movement were studied under both normo- and microgravity. The kinematic synergy was quantified by performing a principal components analysis on the hip, knee, and ankle angle changes occurring during the movement. The results indicate that 1) the anteroposterior position of the CM remains minimized during performance of forward trunk movement in microgravity, in spite of the absence of equilibrium constraints; 2) the strong joint coupling between hip, knee, and ankle, which characterizes the kinematic synergy in normogravity and which is responsible for the minimization of the CM shift during movement, is preserved in microgravity. It represents an invariant parameter controlled by the CNS. 3) The EMG pattern underlying the kinematic synergy is deeply reorganized. This is in contrast with the invariance of the kinematic synergy. It is concluded that during short-term microgravity episodes, the kinematic synergy that minimizes the anteroposterior CM shift is surprisingly preserved due to fast adaptation of the muscle forces to the new constraint.     

Factors affecting grip force: anatomy,mechanics, and referent configurations

The extrinsic digit muscles naturally couple wrist action and grip force in prehensile tasks. We explored the effects of wrist position on the steady-state grip force and grip-force change during imposed changes in the grip aperture [apparent stiffness (AS)]. Subjects held an instrumented handle steady using a prismatic five-digit grip. The grip aperture was changed slowly, while the subjects were instructed not to react voluntarily to these changes. An increase in the aperture resulted in an increase in grip force, and its contraction resulted in a proportional drop in grip force. The AS values (between 4 and 6 N/cm) were consistent across a wide range of wrist positions. These values were larger when the subjects performed the task with eyes open as compared to eyes-closed trials. They were also larger for trials that started from a larger initial aperture. After a sequence of aperture increase and decrease to the initial width, grip force dropped by about 25 % without the subjects being aware of this. We interpret the findings within the referent configuration hypothesis of grip-force production. The results support the idea of back-coupling between the referent and actual digit coordinates. According to this idea, the central nervous system defines referent coordinates for the digit tips, and the difference between the referent and actual coordinates leads to force production. If actual coordinates are not allowed to move to referent ones, referent coordinates show a relatively slow drift toward the actual ones.     

Isometric force production during changed-Gz episodes of parabolic flight

Changes of the normal gravitational acceleration are known to affect sensorimotor performance. For example, subjects exposed to three times the normal terrestrial acceleration (+3 Gz) in a centrifuge will produce exaggerated isometric force. The present study compares the effects of high-Gz to that of micro-Gz on isometric force production. Twelve right-handed human subjects were tested during the microgravity (micro-Gz), high gravity (high-Gz) and normal gravity (normal-Gz) episodes of parabolic flight. Holding an isometric joystick in their right hand, they produced forces of 5, 15, or 25 N in different directions, according to visually prescribed vectors. In some parabolas, 70 Hz vibration was applied to the flexor- and extensor-side of the wrist, in order to reduce spinal segmental activity. If compared to normal-Gz, produced forces were higher in high-Gz, and still higher in micro-Gz. Vibration reduced the magnitude of produced forces independent of the Gz-level. We confirm that isometric force increases in high-Gz, and document that it increases in micro-Gz as well. This increase is probably not related to degraded segmental excitability and proprioception, since it was not modified by vibration and manifests before proprioception becomes effective.