Altered precision grasping in stumptail macaques after fasciculus cuneatus lesions.

Patterns of precision grasp are described in stumptail macaques (Macaca arctoides) before and after lesions of the fasciculus cuneatus (FC). Three monkeys were videotaped while reaching for and grasping small food items. From these videotapes, records were made of the style and outcome of each grasp. Kinematic measurements were also made to describe grip formation and terminal grasp. During grip formation, grip aperture was measured as the distance between the tips of the index finger and the thumb. For terminal grasp, the joint angles of the index finger were measured. The majority of grasps by normal monkeys were of the precision type, in which the item was carried between the tips of the index finger and thumb. Each normal monkey approached objects with a highly consistent grip formation; that is, the fingertips formed a small grip aperture during the approach, and the aperture varied little on repeated grasps. To grasp an item, the forefinger moved in a multiarticular pattern, in which the proximal joint flexed and the distal joint extended. As a result of this combination of movements, the forefinger pad was placed directly onto the object. Following FC transection, the monkeys were studied for 10 months, beginning 1 month after the lesion, to allow for recovery from the acute effects of surgery. The monkeys could grasp the food items, but they rarely opposed the fingertips in precision grasp. Grip formation was altered and was characterized either by excessive grip aperture or by little to no finger opening. All of the monkeys used the table surface to help grasp items. Combined multiarticular patterns of flexion and extension were never observed postoperatively; they were replaced by flexion at all joints of the fingers. These results suggest that the FCs are more important for precision grasping than for other, less refined grasp forms (e.g., power grasps; Napier, 1956). The FCs provide critical proprioceptive feedback to cerebral areas involved in the planning and/or the execution of these movements.     

Manual Function in Cebus apella. Digital Mobility,Preshaping, and Endurance in Repetitive Grasping

Manual dexterity varies across species of primates in accord with hand morphology and degree of fine motor control of the digits. Platyrrhine monkeys achieve less direct opposition between thumb and index finger than that of catarrhine primates, and many of them typically whole-hand grip. However, tufted capuchins (Cebus apella), exhibit a degree of independent control of the digits and effective thumb–forefinger opposition. We report how capuchins prehended small objects, with particular attention to the form of sequential fine movements of the fingers, choice of hand, and differences between the two hands in the temporal properties of reaching and grasping. We compare these actions across tasks with differing demands for fine motor control. For tasks that required all the digits to flex in synchrony, capuchins displayed smooth, fast, and efficient reach-to-grasp movements and a higher endurance than for tasks requiring more complex digital coordination. These latter tasks induced a slightly differentiated preshaping of the hand when approaching the objects, indicating preparation for grasping in advance of contact with the object. A right-hand preponderance for complex digital coordination was evident. The monkeys coordinated their fingers rather poorly at the substrate, and they took longer to achieve control of the objects when complex coordination was required than when simultaneous flexion was sufficient. We conclude that precise finger coordination is more effortful and less well coordinated, and the coordination is less lateralized, in capuchins than in catarrhine primates.     

Grasping behavior in tufted capuchin monkeys (Cebus apella): grip types and manual laterality for picking up a small food item

This study investigates prehension in 20 tufted capuchins (Cebus apella) in a reaching task requiring individuals to grasp a small food item fixed to a tray. The aim was twofold: 1) to describe capuchins' grasping techniques in detail, focusing on digit movements and on different areas of contact between the grasping fingers; and 2) to assess the relationship between grip types and manual laterality in this species. Capuchins picked up small food items using a wide variety of grips. In particular, 16 precision grip variants and 4 power grip variants were identified. The most frequently used precision grip involved the distal lateral areas of the thumb and the index finger, while the most preferred kind of power grip involved the thumb and the palm, with the thumb being enclosed by the other fingers. Immature capuchins picked up small food items using power grips more often than precision grips, while adult individuals exhibited no significant preference for either grip type. The analysis performed on the time capuchins took to grasp the food and withdraw it from the tray hole revealed that 1) precision grips were as efficient as power grips; 2) for precision grips, the left hand was faster than the right hand; and 3) for power grips, both hands were equally quick. Hand preference analysis, based on the frequency for the use of either hand for grasping actions, revealed no significant hand bias at group level. Likewise, there was no significant relationship between grip type and hand preference.     

The grasping side of odours

Background

Research on multisensory integration during natural tasks such as reach-to-grasp is still in its infancy. Crossmodal links between vision, proprioception and audition have been identified, but how olfaction contributes to plan and control reach-to-grasp movements has not been decisively shown. We used kinematics to explicitly test the influence of olfactory stimuli on reach-to-grasp movements.

Methodology/Principal Findings

Subjects were requested to reach towards and grasp a small or a large visual target (i.e., precision grip, involving the opposition of index finger and thumb for a small size target and a power grip, involving the flexion of all digits around the object for a large target) in the absence or in the presence of an odour evoking either a small or a large object that if grasped would require a precision grip and a whole hand grasp, respectively. When the type of grasp evoked by the odour did not coincide with that for the visual target, interference effects were evident on the kinematics of hand shaping and the level of synergies amongst fingers decreased. When the visual target and the object evoked by the odour required the same type of grasp, facilitation emerged and the intrinsic relations amongst individual fingers were maintained.

Conclusions/Significance

This study demonstrates that olfactory information contains highly detailed information able to elicit the planning for a reach-to-grasp movement suited to interact with the evoked object. The findings offer a substantial contribution to the current debate about the multisensory nature of the sensorimotor transformations underlying grasping.     

Grips and hand movements of chimpanzees during feeding in Mahale Mountains National Park,Tanzania

It has long been assumed that stone tool making was a major factor in the evolution of derived hominin hand morphology. However, stresses on the hand associated with food retrieval and processing also have been recognized as relevant early hominin behaviors that should be investigated. To this end, chimpanzee food manipulation was videotaped in the Mahale Mountains National Park, Tanzania. Grips and hand movements by 39 chimpanzees were analyzed for arboreal and terrestrial feeding involving 10 food‐types and associated vegetation. It was predicted that (1) new grips would be found that had not been observed in captivity, (2) forceful precision grips would be absent from the repertoire, as in captivity, and (3) precision handling would be observed. New grips involving the full thumb and buttressed index finger, and a new integrated pattern of grips and forceful hand movements were discovered, associated with feeding on large fruits and meat. Participation of the full thumb in these grips, rather than the distal thumb and fingers, throws light on feeding behaviors that may have become increasingly significant factors in the evolution of derived hominin thumb morphology. The proximal thumb stabilizes food with the flexed index finger against the pull of the teeth and provides leverage in breaking food into portions. Isolated qualitative observations of possibly forceful pinch by the thumb and side of the index finger highlight the need for comparative quantitative data to test whether humans are unique in forceful precision gripping capability. Precision handling was not seen. Am J Phys Anthropol 156:317–326, 2015. © 2014 Wiley Periodicals, Inc.     

Comparison of the Effects of Flexion and Extension of the Thumb and Fingers on the Position and Cross-Sectional Area of the Median Nerve

Objective

To assess the separate effects of thumb and finger extension/flexion on median nerve position and cross-sectional area.

Methods

Ultrasonography was used to assess median nerve transverse position and cross-sectional area within the carpal tunnel at rest and its movement during volitional flexion of the individual digits of the hand. Both wrists of 165 normal subjects (11 men, 4 women, mean age, 28.6, range, 22 to 38) were studied.

Results

Thumb flexion resulted in transverse movement of the median nerve in radial direction (1.2±0.6 mm), whereas flexion of the fingers produced transverse movement in ulnar direction, which was most pronounced during flexion of the index and middle fingers (3.2±0.9 and 3.1±1.0 mm, respectively). Lesser but still statistically significant movements were noted with flexion of the ring finger (2.0±0.8 mm) and little finger (1.2±0.5 mm). Flexion of the thumb or individual fingers did not change median nerve cross-sectional area (8.5±1.1 mm²).

Conclusions

Volitional flexion of the thumb and individual fingers, particularly the index and middle fingers, produced significant transverse movement of the median nerve within the carpal tunnel but did not alter the cross-sectional area of the nerve. The importance of these findings on the understanding of the pathogenesis of the carpal tunnel syndrome and its treatment remains to be investigated.     

Toward a realistic optoelectronic-based kinematic model of the hand: representing the transverse metacarpal arch reduces accessory rotations of the metacarpophalangeal joints

A kinematic model representing the versatility of the human hand is needed to evaluate biomechanical function and predict injury risk in the workplace. We improved upon an existing optoelectronic-based kinematic hand model with grouped metacarpals by defining segmented metacarpals and adding the trapeziometacarpal joint of the thumb. Eight participants performed three static postures (neutral pose, cylinder grip, cap grip) to evaluate kinematic performance of three different models, with one, two, and four metacarpal segment(s). Mean distal transverse metacarpal arch angles in the four-segment metacarpal model were between 22.0° ± 3.3° (neutral pose) and 32.1° ± 3.7° (cap grip). Representation of the metacarpals greatly influenced metacarpophalangeal joint rotations. Both the two- and four-segment metacarpal models displayed significantly lower metacarpophalangeal joint ‘supination’ angles (than the one-segment model) for the fourth and fifth fingers. However, the largest reductions were for the four- versus one-segment models, with mean differences ranging from 9.3° (neutral pose) to 17.0° (cap grip) for the fourth finger and 16.3° (neutral pose) to 33.0° (cylinder grip) for the fifth finger. MCP joint abduction/adduction angles of the fourth and fifth fingers also decreased with segmentation of the metacarpals, although the lowest magnitudes generally occurred in the four-segment model. Overall, the four-segment metacarpal model produced the lowest accessory rotations in non-dominant axes, and best matched previous radiological studies that found MCP joint pronation/supination angles were typically less than 10°. The four-segment metacarpal model, with improved anatomic fidelity, will better serve future studies of detailed actions of the hand in clinical or work applications.     

Precision grips,hand morphology,and tools

This study asks whether there are discernable links between precision gripping, tool behaviors,

1 The term “tool behavior” has been variously used in the literature, in some cases implying exclusively tool making distinctive of humans (Susman, 1991) and in others referring variably to tool using and/or tool-making abilities, some shared with us by other animals (Susman, 1988a,b, 1994). In this paper the term is used to include both tool using and tool making behaviors of humans and non-humans; the term “tool making” is used in place of “tool behavior” whenever the discussion is focused upon distinguishing a capacity for removing flakes from stone preforms from a more general capacity to manipulate stone tools.

and hand morphology in modern hominoids, which may guide functional interpretation of early hominid hand morphology. Findings from a three-pronged investigation answer this question in the affirmative, as follows. (1) Experimental manufacture of early prehistoric tools provides evidence of connections between distinctive human precision grips and effective tool making. (A connection is not found between the “fine” thumb/index finger pad precision grip and early tool making.) (2) Manipulative behavior studies of chimpanzees, hamadryas baboons, and humans show that human precision grips are distinguished by the greater force with which objects may be secured by the thumb and fingers of one hand (precision pinching) and the ability to adjust the orientation of gripped objects through movements at joints distal to the wrist (precision handling). (3) Morphological studies reveal eight features distinctive of modern humans which facilitate use of these grips. Among these features are substantially larger moment arms for intrinsic muscles that stabilize the proximal thumb joints. Examination of evidence for these reveals that three of the eight features occur in Australopithecus afarensis, but limited thumb mobility would have compromised tool making. Also, Olduvai hand morphology strongly suggests a capacity for stone tool making. However, functional and behavioral implications of Sterkfontein and Swartkrans hand morphology are less clear. At present, no single skeletal feature can be safely relied upon as an indicator of distinctively human capabilities for precision gripping or tool making in fossil hominids. Am J Phys Anthropol 102:91–110, 1997. © 1997 Wiley-Liss, Inc.     

One Digit Interruption: The Altered Force Patterns during Functionally Cylindrical Grasping Tasks in Patients with Trigger Digits

Most trigger digit (TD) patients complain that they have problems using their hand in daily or occupational tasks due to single or multiple digits being affected. Unfortunately, clinicians do not know much about how this disease affects the subtle force coordination among digits during manipulation. Thus, this study examined the differences in force patterns during cylindrical grasp between TD and healthy subjects. Forty-two TD patients with single digit involvement were included and sorted into four groups based on the involved digits, including thumb, index, middle and ring fingers. Twelve healthy subjects volunteered as healthy controls. Two testing tasks, holding and drinking, were performed by natural grasping with minimal forces. The relations between the force of the thumb and each finger were examined by Pearson correlation coefficients. The force amount and contribution of each digit were compared between healthy controls and each TD group by the independent t test. The results showed all TD groups demonstrated altered correlation patterns of the thumb relative to each finger. Larger forces and higher contributions of the index finger were found during holding by patients with index finger involved, and also during drinking by patients with affected thumb and with affected middle finger. Although no triggering symptom occurred during grasping, the patients showed altered force patterns which may be related to the role of the affected digit in natural grasping function. In conclusion, even if only one digit was affected, the subtle force coordination of all the digits was altered during simple tasks among the TD patients. This study provides the information for the future studies to further comprehend the possible injuries secondary to the altered finger coordination and also to adopt suitable treatment strategies.     

Effects of Modulations of the Visual Conditions on Subjects’ Perception of Their Own and Another Person’s Hand

According to current concepts, the execution of expedient actions well-coordinated in space becomes possible owing to the creation of a system for internal representation, which includes a body coordinate system, in the central nervous system. The goal of this study was to assess the effects induced by the exclusion of vision and a left-right inversion in visual space on the accuracy in the internal representation of hands and on aimed arm movements. The study cohort included 16 participants aged from 18 to 25 years. The experiment consisted of two test series. In the first series, a subject placed his/her left hand under a transparent plexiglass screen. Upon the experimenter’s command, the subject had to indicate the position of his/her left wrist and the terminal phalanges of the thumb, middle and little fingers with his/her right index finger on the plexiglass, which was accompanied by the corresponding marks displayed on the screen. The positional accuracy in the subject’s perception of his/her own hand position was recorded in the conditions of a leftright inversion of visual space, which were created by wearing prismatic spectacles and the exclusion of visual control. In the second case, the subject’s left hand was replaced on the table under the transparent screen by a similarly shaped left hand belonging to another person. It has been shown that the positions of the middle fingertip and the wrist were sufficiently precisely perceived by the subject through prismatic spectacles. However, the position of the tips of the thumb and little finger relative to the axis connecting the wrist and the terminal phalanx of the middle finger (the hand axis) was perceptually inverted. The accuracy of the indication was reduced for all fingers when the eyes were closed. In testing another person’s hand, a left–right inversion in the visual space created an illusory 90° turn of the hand’s axis and an illusory bias relative to the wrist towards elongation in the marker points corresponding to another person’s fingers. We can suggest that when the alien hand replaced the subject’s own hand, in accordance with the modulations in the motor task conditions, the egocentric system of coordinates was replaced by the allocentric system. The role of vision in the execution of spatially oriented and accurate hand movements increased in this case.     

Precentral neuron activity associated with ipsilateral forelimb movements in monkeys.

Unit activity was recorded in motor cortex on one side, while monkeys were moving left or right fingers, wrists, or arms. On hundred and eighty five movement-related neurons were obtained from two monkeys. Of these, 122 were related to contralateral movements, 50 were to movements of both sides, and the remaining 13 to ipsilateral movements. It was found that ipsilateral-movement-related neurons tended to appear in groups of neurons that were related more to arm movements than to finger and/or wrist movements.     

Oreopithecus bambolii: an unlikely case of hominid-like grip capability in a Miocene ape

Oreopithecus bambolii, an ape from the late Miocene of Italy, is said to possess a hand capable of a precision grip like that of humans. Relative hand length, proportions of the thumb, and morphological features of the thumb and wrist were adduced to support the idea that Oreopithecus had a hand that closely matched the pattern in Australopithecus. A reappraisal of earlier arguments and comparisons of Oreopithecus with humans, apes, and Old World monkeys, reveals that Oreopithecus had an essentially ape-like hand that emphasized ape-like power grasping over human-like precision grasping.     

Fixation Biases towards the Index Finger in Almost-Natural Grasping

We use visual information to guide our grasping movements. When grasping an object with a precision grip, the two digits need to reach two different positions more or less simultaneously, but the eyes can only be directed to one position at a time. Several studies that have examined eye movements in grasping have found that people tend to direct their gaze near where their index finger will contact the object. Here we aimed at better understanding why people do so by asking participants to lift an object off a horizontal surface. They were to grasp the object with a precision grip while movements of their hand, eye and head were recorded. We confirmed that people tend to look closer to positions that a digit needs to reach more accurately. Moreover, we show that where they look as they reach for the object depends on where they were looking before, presumably because they try to minimize the time during which the eyes are moving so fast that no new visual information is acquired. Most importantly, we confirmed that people have a bias to direct gaze towards the index finger’s contact point rather than towards that of the thumb. In our study, this cannot be explained by the index finger contacting the object before the thumb. Instead, it appears to be because the index finger moves to a position that is hidden behind the object that is grasped, probably making this the place at which one is most likely to encounter unexpected problems that would benefit from visual guidance. However, this cannot explain the bias that was found in previous studies, where neither contact point was hidden, so it cannot be the only explanation for the bias.     

Sixty-four cases of thumb and finger reconstruction using transplantation of the big toe skin-nail flap combined with the second toe or the second and third toes

The purpose of this article is to introduce the results of thumb and finger reconstruction using transplantation of the big toe wraparound flap combined with the second toe or the second and third toes. Between August of 1981 and December of 1998, in a series of 64 cases involving 58 patients with digitless hands, either (1) the thumb and index fingers were reconstructed by transplantation of a big toe wraparound flap combined with the adjacent second toe harvested from the ipsilateral foot; or (2) the thumb, index, and long fingers were reconstructed by transplantation of an ipsilateral big toe wraparound flap combined with the adjacent second and third toes. The phalanx of the new thumb was usually an iliac block. The success rate of this series was 92.2 percent. At long-term follow-up, the average static 2-point discrimination was less than 10 mm. The distance between the tip of the new thumb and the new index finger ranged from 6 to 10 cm (average, 8 cm). Opposition action was nimble and forceful. The patients could lift a 6- to 12-kg weight with their reconstructed digits. All patients were satisfied with their new hands and were able to use them in their daily activities. The transplants for reconstructing the thumb and fingers are harvested from the same foot in a procedure known as one-foot donation. Function of the bilateral digitless hand can be recovered with this procedure.     

The role of cutaneous feedback for anticipatory grip force adjustments during object movements and externally imposed variation of the direction of gravity

Grip force adjustments to changes of object loading induced by external changes of the direction of gravity during discrete arm movements with a grasped object were analyzed during normal and anesthetized finger sensibility. Two subjects were seated upright in a rotatable chair and rotated backwards into a horizontal position during discrete movements with a hand-held instrumented object. The movement direction varied from vertical to horizontal inducing corresponding changes in the direction of gravity, but the orientation of the movement in relation to the body remained unaffected. During discrete vertical movements a maximum of load force occurs early in upward and late in downward movements; during horizontal movements two load force peaks result from both acceleratory and deceleratory phases of the movement. During performance with normal finger sensibility grip force was modulated in parallel with fluctuations of load force during vertical and horizontal movements. The grip force profile adopted to the varying load force profile during the transition from the vertical to the horizontal position. The maximum grip force occurred at the same time of maximum load force irrespective of the movement plane. During both subjects' first experience of digital anesthesia the object slipped from the grasp during rotation to the horizontal plane. During the following trials with anesthetized fingers subjects substantially increased their grip forces, resulting in elevated force ratios between maximum grip and load force. However, grip force was still modulated with the movement-induced load fluctuations and maximum grip force coincided with maximum load force during vertical and horizontal movements. This implies that the elevated force ratio between maximum grip and load force does not alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits seems to be important for the economic scaling of the grip force magnitude according to the actual loading conditions and for reactive grip force adjustments in response to load perturbations. However, it plays a subordinate role for the precise anticipatory temporal coupling between grip and load forces during voluntary object manipulation.     

The role of cutaneous feedback for anticipatory grip force adjustments during object movements and externally imposed variation of the direction of gravity

Grip force adjustments to changes of object loading induced by external changes of the direction of gravity during discrete arm movements with a grasped object were analyzed during normal and anesthetized finger sensibility. Two subjects were seated upright in a rotatable chair and rotated backwards into a horizontal position during discrete movements with a hand-held instrumented object. The movement direction varied from vertical to horizontal inducing corresponding changes in the direction of gravity, but the orientation of the movement in relation to the body remained unaffected. During discrete vertical movements a maximum of load force occurs early in upward and late in downward movements; during horizontal movements two load force peaks result from both acceleratory and deceleratory phases of the movement. During performance with normal finger sensibility grip force was modulated in parallel with fluctuations of load force during vertical and horizontal movements. The grip force profile adopted to the varying load force profile during the transition from the vertical to the horizontal position. The maximum grip force occurred at the same time of maximum load force irrespective of the movement plane. During both subjects' first experience of digital anesthesia the object slipped from the grasp during rotation to the horizontal plane. During the following trials with anesthetized fingers subjects substantially increased their grip forces, resulting in elevated force ratios between maximum grip and load force. However, grip force was still modulated with the movement-induced load fluctuations and maximum grip force coincided with maximum load force during vertical and horizontal movements. This implies that the elevated force ratio between maximum grip and load force does not alter the feedforward system of grip force control. Cutaneous afferent information from the grasping digits seems to be important for the economic scaling of the grip force magnitude according to the actual loading conditions and for reactive grip force adjustments in response to load perturbations. However, it plays a subordinate role for the precise anticipatory temporal coupling between grip and load forces during voluntary object manipulation.     

Grip pattern and finger coordination differences between pianists and non-pianists

The aim of this study was to assess differences of grip pattern and finger coordination in pianists and non-pianists, using hand tasks that were unrelated to pianistic practice. Eleven pianists with more than 10 years of intensive practice were compared to 14 non-pianists. Both groups performed four tasks with their right hand: (1) gross grip at fast velocity; (2) gross grip at slow velocity; (3) hook grip at fast velocity; and (4) hook grip at slow velocity. The three-dimensional coordinates were reconstructed using a kinematic analysis system, and the flexion and extension angles of the metacarpophalangeal joints were calculated. The phase diagrams were qualitatively and quantitatively appraised in order to identify differences between the groups. Principal component analysis was used to assess differences between pianists and non-pianists in terms of the reproducibility and regularity of palmar grip cycles. Coefficients of correlation between the joint angles were used to analyze finger coordination during the tasks. The pianists showed better reproducibility and regularity in the palmar grip pattern, as well as finger movements that were more coordinated when performing different hand tasks.     

Effect of dental tool surface texture and material on static friction with a wet gloved fingertip

Hand injuries are an important cause of pain and disability among dentists and dental hygienists and may be due to the high pinch forces involved in periodontal work. The pinch forces required to perform scaling may be reduced by increasing the friction between the tool and fingers. The purpose of this study was to determine whether modifying the tool material, surface texture, or glove type altered the coefficient of static friction for a wet gloved finger. Seven tools with varying surface topography were machined from 13 mm diameter stainless steel and Delrin and mounted to a 6-component force plate. The textures tested were a fine, medium and coarse diamond knurled pattern and a medium and fine annular pattern (concentric rings). Thirteen subjects pulled their gloved, wet thumb pad along the long axis of the tool while maintaining a normal force of 40 N. Latex and nitrile gloves were tested. The coefficient of static friction was calculated from the shear force history. The mean coefficients of static friction ranged from 0.20 to 0.65. The coefficient of static friction was higher for a smooth tool of Delrin than one of stainless steel. Differences in the coefficient of static friction were observed between the coarse and medium knurled patterns and the fine knurled and annular patterns. Coefficients of static friction were higher for the nitrile glove than the latex glove for tools with texture. These findings may be applied to the design of hand tools that require fine motor control with a wet, gloved hand.     

Hand locomotor functions, body structure, and epidermal ridge patterns: preliminary study

The purpose of this study was to determine whether any relationships exist between hand locomotor functions and dermatoglyphic characteristics and body structure. The pilot sample consisted of 71 adult normal individuals (30 males and 41 females). The locomotor function tests included a power grip and two precision grips; dermatoglyphic features were represented by finger and palmar pattern intensities and ridge counts, and body structure by 35 head, face, trunk, and limb anthropometric measurements. Univariate and multivariate correlation analyses reveal that on average half of the variance in the locomotor hand-function tests can be accounted for by a set of body and/or dermatoglyphic variables in males; this contribution is appreciably lower in females. Body longitudinal measurements and some facial measures, such as jaw length, were found to be the main correlates of either a power grip or a simple thumb-index squeeze, especially in males; head and face measurements and the size and intensity of patterns on fingers 1 and 2 were the main correlates of more complex precision tests involving complicated manipulation of objects using the thumb and index finger. These preliminary results identify some previously unknown sources of variation in dermatoglyphic patterns and contribute to a better understanding of the evolutionary aspects of the relationships between specific functional and morphologic traits in humans.     

Manipulabilities of the index finger and thumb in three tip-pinch postures

Tip-pinch, in which the tips of the index finger and thumb pick up and hold a very fine object, plays an important role in the function of the hand. The objective of this study was to investigate how human subjects affect manipulabilities of the tips of the index finger and thumb within the flexion/extension plane of the finger in three different tip-pinch postures. The index finger and thumb of twenty male subjects, were modeled as linkages, based on measurement results obtained using two three-dimensional position measurement devices. The manipulabilities of the index finger and thumb were investigated in three tip-pinch postures, using three criteria indicating the form and posture of the manipulability ellipse of the linkage model. There were no significant differences (p > 0.05, ANOVA) in each criterion of each digit across the subjects, except for two criteria of the thumb. The manipulabilities of the index finger and thumb were separately similar across all subjects in tip-pinch postures. It was found that the manipulability for the cooperation of the index finger and thumb of all the subjects in tip-pinch depended on the posture of the index finger, but not on the posture of the thumb. In two-dimensional tip-pinch, it was possible that the index finger worked actively while the thumb worked passively to support the manipulation of the index finger.