Analysis of the effects of surface stiffness on the contact interaction between a finger and a cylindrical handle using a three-dimensional hybrid model

Contact interactions between the hand and handle, such as the contact surface softness and contact surface curvature, will affect both physical effort and musculoskeletal fatigue, thereby the comfort and safety of power tool operations. Previous models of hand gripping can be categorized into two groups: multi-body dynamic models and finite element (FE) models. The goal of the current study is to develop a hybrid FE hand gripping model, which combines the features of conventional FE models and multi-body dynamic models. The proposed model is applied to simulate hand-gripping on a cylindrical handle with covering materials of different softness levels. The model included three finger segments (distal, middle, and proximal phalanxes), three finger joints (the distal interphalangeal (DIP), proximal interphalangeal (PIP), and metacarpophalangeal (MCP) joint), and major anatomical substructures. The model was driven by joint moments, which are the net effects of all passive and active muscular forces acting about the joints. The finger model was first calibrated by using experimental data of human subject tests, and then applied to investigate the effects of surface softness on contact interactions between a finger and a cylindrical handle. Our results show that the maximal compressive stress and strain in the soft tissues of the fingers can be effectively reduced by reducing the stiffness of the covering material.     

The relation between force and movement when grasping an object with a precision grip

When reaching out for objects, the digits’ paths curve so that they approach their positions of contact moving more or less perpendicularly to the local surface orientation. This increases the accuracy of positioning the digits and ensures that any forces exerted at contact are nearly perpendicular to the surface, so that friction will prevent the digits from slipping along the surface. When lifting the object a similar force perpendicular to the surface is needed to prevent the object from slipping from one’s fingers. In order to determine whether these two issues are dealt with simultaneously we let subjects pick up a cube from three different starting positions and measured the digits’ movements and forces from before contact until the moment the cube started moving. The impact force was low. After impact, the digits spent about 200 ms in contact with the surface of the cube before the latter started to move. The digits first decelerated, and then they gradually built up the grip- and lift forces to move the cube upwards. We found no direct relationship between the control of the reaching movement towards the object and the force applied at the surface of the object to pick it up. We conclude that the reaching and lifting movements are quite independent.     

An object for an action,the same object for other actions: effects on hand shaping

Objects can be grasped in several ways due to their physical properties, the context surrounding the object, and the goal of the grasping agent. The aim of the present study was to investigate whether the prior-to-contact grasping kinematics of the same object vary as a result of different goals of the person grasping it. Subjects were requested to reach toward and grasp a bottle filled with water, and then complete one of the following tasks: (1) Grasp it without performing any subsequent action; (2) Lift and throw it; (3) Pour the water into a container; (4) Place it accurately on a target area; (5) Pass it to another person. We measured the angular excursions at both metacarpal-phalangeal (mcp) and proximal interphalangeal (pip) joints of all digits, and abduction angles of adjacent digit pairs by means of resistive sensors embedded in a glove. The results showed that the presence and the nature of the task to be performed following grasping affect the positioning of the fingers during the reaching phase. We contend that a one-to-one association between a sensory stimulus and a motor response does not capture all the aspects involved in grasping. The theoretical approach within which we frame our discussion considers internal models of anticipatory control which may provide a suitable explanation of our results.

Control of grip force during restraint of an object held between finger and thumb: responses of cutaneous afferents from the digits

Unexpected pulling and pushing loads exerted by an object held with a precision grip evoke automatic and graded increases in the grip force (normal to the grip surfaces) that prevent escape of the object; unloading elicits a decrease in grip force. Anesthesia of the digital nerves has shown that these grip reactions depend on sensory signals from the digits. In the present study we assessed the capacity of tactile afferents from the digits to trigger and scale the evoked grip responses. Using tungsten microelectrodes inserted percutaneously into the median nerve of awake human subjects, unitary recordings were made from ten FA I and 13 FA II rapidly adapting afferents, and 12 SA I and 18 SA II slowly adapting afferents. While the subject held a manipulandum between a finger and the thumb, tangential load forces were applied to the receptor-bearing digit (index, middle, or ring finger or thumb) as trapezoidal load-force profiles with a plateau amplitude of 0.5 – 2.0 N and rates of loading and unloading at 2 – 8 N/s, or as step-loads of 0.5 N delivered at 32 N/s. Such load trials were delivered in both the distal (pulling) and proximal (pushing) direction. FA I afferents responded consistently to the load forces, being recruited during the loading and unloading phases. During the loading ramp the ensemble discharge of the FA I afferents reflected the first time-derivative of the load force (i.e., the load-force rate). These afferents were relatively insensitive to the subject's grip force responses. However, high static finger forces appeared to suppress excitation of these afferents during the unloading phase. The FA II afferents were largely insensitive to the load trials: only with the step-loads did some afferents respond. Both classes of SA afferents were sensitive to load force and grip force, and discharge rates were graded by the rate of loading. The firing of the SA I afferents appeared to be relatively more influenced by the subject's grip-force response than the discharge of the SA II afferents, which were more influenced by the load-force stimulus. The direction in which the tangential load force was applied to the skin influenced the firing of most afferents and in particular the SA II afferents. Individual afferents within each class (except for the FA IIs) responded to the loading ramp before the onset of the subject's grip response and may thus be responsible for initiating the automatic increase in grip force. However, nearly half of the FA I afferents recruited by the load trials responded to the loading phase early enough to trigger the subject's gripforce response, whereas only ca. one-fifth of the SA Is and SA IIs did so. These observations, together with the high density of FA I receptors in the digits, might place the FA I afferents in a unique position to convey the information required to initiate and scale the reactive gripforce responses to the imposed load forces.     

An analysis of spatiotemporal variability during prehension movements: effects of object size and distance

 Human prehension movements have been studied with regard to the parallel processing of motor control and sensorimotor coordination. Temporal aspects of the movement (e.g., onset time and duration) have been studied extensively, while spatial aspects have not been studied systematically. Thus, the purpose of this study was to examine spatiotemporal variability of the transport (wrist trajectory) and grasp (grip aperture between the index finger and the thumb) components. In this experiment, the extrinsic (e.g., distance) and intrinsic object properties (e.g., object size) were manipulated. Subjects were required to pick up an aluminum cylinder as quickly and accurately as possible using the index finger and the thumb. It was found that object size significantly affected both transport and grasp components. Distance mainly affected the transport component. These kinematic results were consistent with the findings of earlier studies. Furthermore, the distribution of mean within-subject variability across normalized movement time for the transport component was not the same as that of the grasp component, suggesting that the different motor control processes exist. The peak amplitudes in variability of the wrist trajectory and the grip aperture were obtained at similar points throughout movement time. Furthermore, the peak of wrist variability depended on distance not object size, while that of aperture variability depended on both distance and object size. These results strongly support the hypothesis that the grasp component is adjusted using dynamic information provided from the transport component as the wrist moves toward the object. We also found that wrist variability converged to the target point, while aperture variability was biphasic: it converged, at least, around the point of maximum aperture in the first phase and then remained constant in the second phase. This result suggests that the two components are under different control processes. We hypothesize that the transport component can be modeled as a single feedforward system, while the grasp component can be divided into two separate mechanisms. Received: 4 March 1996 / Accepted: 29 January 1997     

Control of grip force during restraint of an object held between finger and thumb: responses of muscle and joint afferents from the digits

Pulling or pushing forces applied to an object gripped between finger and thumb excite tactile afferents in the digits in a manner awarding these afferents probable roles in triggering the reactive increases in grip force and in scaling the changes in grip force to the changes in applied load-force. In the present study we assessed the possible contributions from slowly adapting afferents supplying muscles involved in the generation of grip forces and from digital joint afferents. Impulses were recorded from single afferents via tungsten microelectrodes inserted percutaneously into the median or ulnar nerves of awake human subjects. The subject held a manipulandum with a precision grip between the receptor-related digit (index finger, middle finger, ring finger or thumb) and an opposing digit (thumb or index finger). Ramp-and-hold load forces of various amplitudes (0.5–2.0 N) and ramp rates (2–32 N/s) were delivered tangential to the parallel grip surfaces in both the distal (pulling) and the proximal (pushing) directions. Afferents from the long flexors of the digits (n=19), regardless of their muscle-spindle or tendon-organ origin, did not respond to the load forces before the onset of the automatic grip response, even with the fastest ramp rates. Their peak discharge closely followed the peak rate of increase in grip force. During the hold phase of the load stimulus, the afferents sustained a tonic discharge. The discharge rates were significantly lower with proximally directed loads despite the mean grip-force being similar in the two directions. This disparity could be explained by the differing contributions of these muscles to the finger-tip forces necessary to restrain the manipulandum in the two directions. Most afferents from the short flexors of the digits (n=17), including the lumbricals, dorsal interossei, opponens pollicis, and flexor pollicis brevis, did not respond at all, even with the fastest ramps. Furthermore, the ensemble pattern from the joint afferents (n=6) revealed no significant encoding of changes in finger-tip forces before the onset of the increase in grip force. We conclude that mechanoreceptors in the flexors of the digits and in the interphalangeal joints cannot be awarded a significant role in triggering the automatic changes in grip force. Rather, their responses appeared to reflect the reactive forces generated by the muscles to restrain the object. Hence, it appears that tactile afferents of the skin in contact with the object are the only species of receptor in the hand capable of triggering and initially scaling an appropriate change in grip force in response to an imposed change in load force, but that muscle and joint afferents may provide information related to the reactive forces produced by the subject.     

An electrophysiological analysis of animacy effects in the processing of object relative sentences.

Event-related brain potentials (ERPs) were used to investigate how and when a semantic factor (animacy) affects the early analysis of a difficult syntactic structure, namely, object relative sentences. We contrasted electrophysiological and behavioral responses to two object relative types that were syntactically and lexically identical and varied only in the order of the component animate and inanimate nouns [Inanimate (Animate) vs. Animate (Inanimate)]. ERPs were recorded from 40 subjects to each word of 30 I(A) and 30 A(I) sentences that occurred randomly among a set of various other sentence types read for comprehension. ERP effects to the early noun animacy manipulation were observed beginning with the initial noun and extending past the main clause verbs. We interpret the timing and multitude of electrophysiological effects, including the N400, P600, and left-anterior negativity, as evidence that both semantic and syntactic, and perhaps other types of information, are used early during structural analysis and message-level computations as needed for comprehension.     

Impaired innate immunity of ocular surface is the key bridge between extended contact lens wearing and occurrence of Acanthamoeba keratitis

Acanthamoeba keratitis is a progressive, sight-threatening corneal disease. Extended wearing contact lens is one of predisposed factors. Early studies mostly focused on "improper contact-lens hygiene", which described that contact lens wearers have more opportunities to contact with pathogens directly and prone to get A. keratitis. However, improper contact-lens hygiene can not explain the phenomenon that Acanthamoeba protozoon were found in normal individuals' lens-cases. So there might be other factors related with A. keratitis. Recently, more attention has been paid on the influence of extended wearing contact lens on the innate immunity of ocular surface. It has been proven that in contact lens wearers the reactivity of polymorphonuclear leucocytes (PMNs) and the concentration of certain inflammatory mediators were significantly altered compared with that in non-lens wearers. Moreover, other studies showed the important contributions of innate immunity on occurrence and development of A. keratitis. With the contribution of extended wearing contact lens on immunity and the relation between innate immunity and Acanthamoeba, we suggest that the impaired innate immunity of ocular surface may be a key bridge between extended wearing contact lens and A. keratitis. With the impaired innate immunity caused by extended contact-lens wearing, the Acanthamoeba trophozoites and cysts could not be easily killed, therefore A. keratitis was occurred and aggravated. Understanding the immunological mechanism of extended contact lens wearing on the A. keratitis may give more contributions on the research of the disease, and facilitate the production of contact lens with much higher biocompatibility.     

Impact when receiving a diagnosis: Additive and multiplicative effects between illness severity and perception of control

Based on Lazarus' transactional model of stress, this study examined how the information provided in a medical diagnosis (the severity of the illness, the effectiveness of the treatment and the self-efficacy to follow this treatment) are combined to generate perception of stress. Twenty-seven scenarios were presented to 152 volunteer students and their level of perceived stress was recorded. Results revealed that the lack of efficacy of the treatment raises the perception of stress proportionally to the different degrees of illness severity, but having a low perceived ability to follow the treatment triggers high levels of stress, particularly when the diagnosis is serious.     

An analysis of the effects of some types of anti-lymphocyte sera on contact hypersensitivity and certain models of inflammation

The effect of anti-thymocyte serum, anti-lymph node cell serum and anti-lymph node permeability factor serum have been compared on the peripheral manifestation of contact hypersensitivity in the guinea-pig and certain models of acute inflammation in the guinea-pig and the rat. Anti-lymph node permeability factor serum has a similar effect to the anti-lymphocyte sera on contact hypersensitivity. However, its effect on non-specific inflammation is very much less. The mechanism of action of anti-lymphocyte sera on the models of non-specific inflammation has been studied more thoroughly to attempt to work out the level at which inhibition occurs.

The central effect of these different antisera on lymph nodes reacting during the development of sensitivity to a chemical sensitizer has also been studied. Anti-serum prepared against cells derived from lymph nodes has the same specific effect on the thymus-dependent area of the lymph node as antiserum prepared against cells derived from the thymus. These changes are compared with those produced by anti-lymph node permeability factor serum and anti-epidermal cell serum.

     

An object-centred reference frame for control of grasping: effects of grasping a distractor object on visuomotor control

Previous evidence based on perceptual integration and arbitrary responses suggests extensive cross-modal links in attention across the various modalities. Attention typically shifts to a common location across the modalities, despite the vast differences in their initial coding of space. An issue that remains unclear is whether or not these effects of multisensory coding occur during more natural tasks, such as grasping and manipulating three-dimensional objects. Using kinematic measures, we found strong effects of the diameter of a grasped distractor object on the aperture used to grasp a target object at both coincident and non-coincident locations. These results suggest that interference effects can occur between proprioceptive and visuomotor signals in grasping. Unlike other interference effects in cross-modal attention, these effects do not depend on the spatial relation between target and distractor, but occur within an object-based frame of reference.     

Mallet finger: a simulation and analysis of hyperflexion versus hyperextension injuries

The goal of this study was to simulate the mechanisms of hyperflexion and hyperextension injuries of the distal interphalangeal (DIP) joint of the hand and to analyze the resulting extensor tendon injury patterns. The hypotheses were raised that hyperflexion trauma leads to a plastic deformation of the extensor tendon aponeurosis, with or without a small bony avulsion fragment but without joint surface involvement, and that hyperextension injuries can create a shear fracture of the dorsal lip of the distal phalanx, without injury to the extensor tendon aponeurosis. Loading was applied with a swinging pendulum impacting the distal phalanx in 103 human specimens in either an extended or flexion position. After loading, injury patterns were analyzed radiologically and histologically. There was evidence that hyperflexion trauma leads to a plastic deformation or rupture of the extensor tendon. Bony tendon avulsion was evident in 12.2 % of cases. With hyperextension, the extensor tendon remained intact in all cases, but there were large fracture fragments involving the articular surface in 4.1 % of cases. The results of the study show that force on the flexed joint leads to overstretching of the extensor tendon, and to an associated dorsal bony avulsion with intact joint line. Force applied to the joint in extension can lead to a bony dorsal edge fracture with articular involvement and with it, a palmar DIP joint capsule rupture. The results illuminate a direct correlation between the mechanism of injury and the pattern of injury in the clinical picture of mallet finger.     

An analysis of the relationship between radiosensitivity and volume effects in tumor control probability modeling

The dependence of local tumor control probability (tcp) on tumor volume is analyzed and discussed with the help of radiobiological modeling; in particular the impact of possible correlations between mean tumor radiosensitivity and tumor dimensions on the tcp volume dependence is explored. The linear-quadratic Poissonian tumor control probability (tcp) model was modified to account for the possible dependence of clonogenic cell density and radiosensitivity parameters on tumor volume; then the original and modified versions of the model were fitted to published clinical and laboratory tumor control data. These different versions of the tcp model often fitted tumor control data equally well, because of the high degree of correlation between the parameters. Nevertheless the results were very different from a physical point of view and we suggest that sometimes it is possible to choose between equally good fits on the basis of physical considerations. Possible links between the volume dependence of the mean radiosensitivity and the degree of tumor hypoxia were also analyzed through a comparison of the results of the tcp fit to published measurements of oxygen tension in tumors.     

Distributing vertical forces between the digits during gripping and lifting: the effects of rotating the hand versus rotating the object

During pinch grip we partition the vertical tangential forces at the digits according to the friction at the grip surfaces, and the mass distribution of the object. However, we cannot predictively partition the vertical forces to adjust to new frictional conditions after viewing a 180-deg rotation of an object with different textures at each grip surface. Hence, the processes that lead to predictive force partitioning may not access object representations, thereby suggesting that these processes are digit-specific. If this is true, then we should fail to predictively partition our fingertip forces when we rotate our hand. We tested this prediction by comparing the effects of object rotation with hand rotation for repeated lifts of an object that had one slippery grip surface and one rough grip surface. Subjects did not predictively redistribute the vertical tangential forces upon grasping the rotated object. Following object rotation, the vertical tangential force trajectories during the first 100 ms after contact indicated that 12/15 subjects failed to anticipate the reversed digit-friction relationships. All subjects appropriately partitioned the vertical tangential forces between the digits by the second lift after object rotation, confirming previous reports that sensory signals update the memory associated with lifting the object. In contrast, after hand rotation, 13/15 subjects anticipated the new digit-friction relationships and upon grasping the object immediately generated a steep rise in the vertical force trajectory at the rough surface. They also delayed the initial rise in vertical tangential force at the digit encountering the low-friction surface by approximately 65 ms. Thus, anticipatory partitioning of vertical fingertip forces is not strictly digit-specific. Internally driven motor plans can access the relevant memories or internal models for predictively partitioning the vertical tangential forces. It is not clear if this process involves rotating internal representations of fingertip force directly, or if the forces are derived after internally rotating a representation of the object. In contrast to the robust effects of vision on reach kinematics, or on wrist and finger configuration, visual signals about object rotation and orientation apparently do not influence vertical tangential fingertip forces.     

Eye contact and arousal: the effects of stimulus duration

The present study investigated the effect of stimulus duration on skin conductance responses (SCRs) evoked by different gaze directions of a live person. In two separate parts of the experiment, either two fixed stimulus durations (2 s and 5 s) or a participant-controlled stimulus duration was used. The results showed that the eye contact evoked enhanced SCRs compared to averted gaze or closed eyes conditions irrespective of the presentation time. Subjective evaluations of approach-avoidance-tendencies indicated that the direct gaze elicited either approach or avoidance, depending on the participant. Participants who had evaluated a direct gaze-condition as approachable were found to be more emotionally stabile than those who had evaluated the same condition as avoidable. In the self-timing condition, averted gaze was looked at longer than direct gaze. Our results suggest that direct gaze, also when encountered only briefly like in every-day social encounterings, increases autonomic sympathetic arousal.     

Finger skin cooling on contact with cold materials: an investigation of male and female responses during short-term exposures with a view on hand and finger size

This study investigated the independent effects of hand anthropometry and gender upon contact cooling responses. Subjects were selected for matching hand/finger size between genders, with equal variation between individuals of each group. Fourteen volunteers (7 male, 7 female) participated, touching blocks of aluminium and stainless steel using the first phalanx of the index finger with a contact force of 1.0 N and 9.8 N, at surface temperatures of –2°C and –10°C. Conditions were selected in order to elicit varying rates of skin cooling upon contact. Contact temperature (T _C) of the finger-pad was measured over time using a T-type thermocouple. Overall, no significant difference was found between the cooling responses of males and females. In order to investigate whether differences in hand anthropometry correlated with contact cooling response, a multiple regression approach was used. Analyses of the residual variance in contact cooling data, after the effects of material type, surface temperature and finger contact force had been accounted for, showed that, under slow cooling conditions (>45 s to reach T _C=1°C), hand size correlated significantly with contact cooling response only when represented by index finger volume (P<0.05), but gender did not. Whilst under fast cooling conditions (<25 s to reach T _C=1°C), hand size did not correlate significantly with contact cooling response at all, but gender had a significant effect (P<0.001). Under slow cooling conditions, a larger finger (and in most cases hands) provides a higher heat content, thus giving a slower skin cooling speed. Under fast cooling conditions, the significantly longer time required for males to reach a T _C of 1°C, despite matching hand and finger size, is attributed to higher epidermal insulation provided by the thicker stratum corneum, combined with the higher starting skin temperature observed in the slightly cool environment.     

Relationship between stiffness,internal cell pressure and shape of outer hair cells isolated from the guinea-pig hearing organ

The mechanical properties of outer hair cells are of importance for normal hearing, and it has been shown that damage of the cells can lead to a reduction in the hearing sensitivity. In this study, we measured the stiffness of isolated outer hair cells in hyper- and hypotonic conditions, and examined the change in stiffness in relation to the corresponding changes in internal cell pressure and cell shape. The results showed that the axial stiffness of isolated outer hair cells (30–90 μm in length, 8–12 μm in diameter), ranging from 0.13–5.39 mN m^?1, was inversely related to cell length. Exposure to hyper- and hypotonic external media with a small percentage change in osmolality caused a similar magnitude of change in cell length and cell diameter, but an average 60% change in cell stiffness. Therefore, a moderate osmotic change in the external medium can lead to a significant alteration in cell stiffness. The findings thus indicate an important contribution of internal cell pressure to cell stiffness.     

Prehension movements in the macaque monkey: effects of object size and location

Prehension movements were examined in freely behaving monkeys and compared with the well-known characteristics of human movements. The degree of independence of the components of movements (i.e., reaching and grasping) was investigated in animals trained to reach for and grasp three-dimensional objects. To this aim, the kinematics of prehension movements was recorded using an Optotrak system in two tasks. In one task, monkeys grasped a small or a large object (size task), in the other, they grasped an object of constant size placed at three different spatial locations (location task). We found that object size and its location affected both reaching and grasping. In particular, in the size task, we found that the maximum grip aperture strongly depended on the selection of the grip and not only on the size of an object. Our results also revealed that, in monkeys as well as in humans, the reaching parameters are highly sensitive to task-related constraints such as accuracy demands. The results of the location task showed a difference between rightward and leftward movements, a pattern of grip aperture that varied across animals, and a large degree of coordination between the two components. These findings argue against a strict postulate of independence between the visuo-motor channels, favoring instead the idea of variable degrees of coordination between the reach and grasp components depending on the task demands. Finally, this work emphasizes the relevance of studying monkey's prehension movements as a useful step to the understanding of visuo-motor control in humans.