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.     

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.     

Chimpanzee and human grips: A new classification with a focus on evolutionary morphology

We observed grips by the hand during locomotor and manipulative behavior of captive chimpanzees to improve our ability to interpret differences between chimpanzees and humans in hand morphology that are not easily explained by current behavioral data. The study generated a new classification of grips,which takes into account three elements of precision and power gripping that appear to distinguish between the chimpanzees and humans, and which have not been explored previously in relation to hand morphology. These elements are (1) the relative force of the precision grips (pinch versus hold), (2) the relative ability to translate and rotate objects by the thumb and fingers (precision handling), and (3) the relative ability to orient a cylindrical object so that it functions effectively as an extension of the forearm (power squeeze). We recommend that this classification be incorporated into protocols for field and laboratory studies of nonhuman primate manipulative behavior, in order to test our prediction that these three elements clearly distinguish humans from chimpanzees and other nonhuman primates. The results of this test will have direct bearing upon decisions as to which grips (with their associated behaviors) are most likely to guide us through kinematic and kinetic analysis to possible explanations for morphological differences between humans and other species. These explanations, in turn, are fundamental to our ability to discern evidence for potential grips and tool behaviors in the manual morphology of fossil hominids.     

Alterations of natural hand movements after interruption of fasciculus cuneatus in the macaque.

As part of a series of investigations on the control of fine finger movements in the macaque, spontaneous use of the hand in grooming, scratching, and manipulation was observed before and after interruption of fasciculus cuneatus (FC). Videotaped observations were made of four stumptail macaques (Macaca arctoides) living outdoors in social groups. The monkeys were followed for 1 to 3 years postoperatively. For the first 2 weeks following surgery, all monkeys neglected the affected hand and did not use it for support, locomotion, climbing, scratching, foraging, or grooming. Recovery of gross arm and hand movements occurred over a 1- to 3-month period. All the monkeys eventually used the hand for support, climbing, and object manipulation, but fine control of the fingers did not recover. Also, there was an apparent hypotonia of the fingers, imparting a "floppy" appearance to the hand. The animals coped with the loss of fine control by decreasing the frequency of some behaviors, eliminating others, and developing alternative strategies. Exploratory movements that were utilized for investigating the anogenital area or foraging for small food items were eliminated by FC interruption. There were obvious deficits in grip formation and grasp of small food objects (see Glendinning et al., this issue), but effects on similar movements during grooming only became obvious after repeated inspection of videotaped records. Self-scratching and sweeps of the hand in grooming were preserved but altered in form and frequency. The component movements in these behaviors were relatively uncoordinated, and the fingers were splayed (abducted). Often the hand was formed in a rigid posture throughout the sweeping motion, and the fingers did not stroke the skin individually. Frame-by-frame analysis of videotapes revealed that the morphology of the precision grip during grooming, in movements termed "plucks," was permanently altered. Preoperatively, the monkeys kept the index finger and thumb closely apposed and routinely made contact on the distal surfaces of the digits, as has been described for precision grip in humans. Postoperatively, this relationship was altered. The index finger frequently missed the thumb tip and made contact on the proximal part of the phalanx, or missed the thumb altogether. Thus, the dorsal column input is important for proprioceptive guidance of movements that achieve "tactile foveation," when objects or surfaces are actively contacted by the receptive areas of keenest sensitivity (on the fingertips).     

Was Australopithecus afarensis able to make the Lomekwian stone tools? Towards a realistic biomechanical simulation of hand force capability in fossil hominins and new insights on the role of the fifth digit

While no consensus allows explaining how and when human-like traits arose in fossil hominin hands, the recent discoveries of the Lomekwian stone tools (3.3 Ma) support the view that early hominins were able to use forceful grips in order to manipulate large-sized blocks for pounding activities. Then, assessing gripping abilities of contemporaneous hominin, i.e. Australopithecus afarensis, is necessary, particularly with regards to its unusual 5th ray morphology that has been deemed crucial to ensure forceful grips. Here, we present a musculoskeletal simulation based on the A. afarensis hand morphology that includes an original 5th carpometacarpal joint. Our first results suggest a limited influence of muscle parameters (e.g., PCSA) and support the value of simulations for studying extinct taxa even in absence of soft-tissue data. Given the inability for the pulp of the 5th ray to face the surface of a large-sized object, the A. afarensis hand would have had limited possibility to exert sufficient force to make the Lomekwian stone tools.     

Comparative morphology of the pollical distal phalanx

Functional analysis of human pollical distal phalangeal (PDP) morphology is undertaken to establish a basis for the assessment of fossil hominid PDP morphology. Features that contribute to the effectiveness of grips involving the distal thumb and finger pulp areas include: 1) distal thumb interphalangeal joint morphology, facilitating PDP conjunct pronation with flexion; 2) differentiation of a proximal, mobile pulp region from a distal, stable pulp region, providing for firm precision pinch grips and precision handling of objects; and 3) asymmetric attachment of the flexor pollicis longus (FPL) tendon fibers, favoring PDP conjunct pronation. A proportionately larger size of the ulnar vs. radial ungual spine suggests differential loading intensity of the ulnar side of the proximal ungual pulp and supporting nail bed. Stresses at the distal interphalangeal joint are indicated by the presence of a sesamoid bone within the volar (palmar) plate, which also increases the length of the flexor pollicis longus tendon moment arm. Dissections of specimens from six nonhuman primate genera indicate that these human features are shared variably with individuals in other species, although the full pattern of features appears to be distinctively human. Humans share variably with these other species all metric relationships examined here. The new data identify a need to systematically review long-standing assumptions regarding the range of precision and power manipulative capabilities that might reasonably be inferred from morphology of the distal phalangeal tuberosity and from the FPL tendon insertion site on the PDP.     

Tool making,hand morphology and fossil hominins

Was stone tool making a factor in the evolution of human hand morphology? Is it possible to find evidence in fossil hominin hands for this capability? These questions are being addressed with increasingly sophisticated studies that are testing two hypotheses; (i) that humans have unique patterns of grip and hand movement capabilities compatible with effective stone tool making and use of the tools and, if this is the case, (ii) that there exist unique patterns of morphology in human hands that are consistent with these capabilities. Comparative analyses of human stone tool behaviours and chimpanzee feeding behaviours have revealed a distinctive set of forceful pinch grips by humans that are effective in the control of stones by one hand during manufacture and use of the tools. Comparative dissections, kinematic analyses and biomechanical studies indicate that humans do have a unique pattern of muscle architecture and joint surface form and functions consistent with the derived capabilities. A major remaining challenge is to identify skeletal features that reflect the full morphological pattern, and therefore may serve as clues to fossil hominin manipulative capabilities. Hominin fossils are evaluated for evidence of patterns of derived human grip and stress-accommodation features.     

EMG study of hand muscle recruitment during hard hammer percussion manufacture of Oldowan tools

The activity of 17 hand muscles was monitored by electromyography (EMG) in three subjects during hard hammer percussion manufacture of Oldowan tools. Two of the subjects were archaeologists experienced in the replication of prehistoric stone tools. Simultaneous videotapes recorded grips associated with the muscle activities. The purpose of the study was to identify the muscles most likely to have been strongly and repeatedly recruited by early hominids during stone tool-making. This information is fundamental to the identification of skeletal features that may reliably predict tool-making capabilities in early hominids. The muscles most frequently recruited at high force levels for strong precision pinch grips required to control the hammerstone and core are the intrinsic muscles of the fifth finger and the thumb/index finger regions. A productive search for skeletal evidence of habitual Oldowan tool-making behavior will therefore be in the regions of the hand stressed by these intrinsic muscles and in the joint configurations affecting the relative lengths of their moment arms. Am J Phys Anthropol 105:315–332, 1998. © 1998 Wiley-Liss, Inc.     

On the Evolution of Handedness: Evidence for Feeding Biases

Many theories have been put forward to explain the origins of right-handedness in humans. Here we present evidence that this preference may stem in part from a right hand advantage in grasping for feeding. Thirteen participants were asked to reach-to-grasp food items of 3 different sizes: SMALL (Cheerios®), MEDIUM (Froot Loops®), and LARGE (Oatmeal Squares®). Participants used both their right- and left-hands in separate blocks (50 trials each, starting order counterbalanced) to grasp the items. After each grasp, participants either a) ate the food item, or b) placed it inside a bib worn beneath his/her chin (25 trials each, blocked design, counterbalanced). The conditions were designed such that the outward and inward movement trajectories were similar, differing only in the final step of placing it in the mouth or bib. Participants wore Plato liquid crystal goggles that blocked vision between trials. All trials were conducted in closed-loop with 5000 ms of vision. Hand kinematics were recorded by an Optotrak Certus, which tracked the position of three infrared diodes attached separately to the index finger, thumb, and wrist. We found a task (EAT/PLACE) by hand (LEFT/RIGHT) interaction on maximum grip aperture (MGA; the maximum distance between the index finger and thumb achieved during grasp pre-shaping). MGAs were smaller during right-handed movements, but only when grasping with intent to eat. Follow-up tests show that the RIGHT-HAND/EAT MGA was significantly smaller than all other hand/task conditions. Because smaller grip apertures are typically associated with greater precision, our results demonstrate a right-hand advantage for the grasp-to-eat movement. From an evolutionary perspective, early humans may have preferred the hand that could grasp food with more precision, thereby maximizing the likelihood of retrieval, consumption, and consequently, survival.     

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.     

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.     

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.     

Cranio-dental evidence of a hominin-like hyper-masticatory apparatus in Oreopithecus bambolii. Was the swamp ape a human ancestor?

The phylogeny of Oreopithecus bambolii has been controversial since Johannes Hurzeler first argued that the Late Miocene (Tortonian) primate was a fossil hominin. While most paleontologist currently exclude Oreopithecus from human ancestry, recent postcranial evidence of hominin-like bipedalism and power precision grips in Oreopithecus has rekindled interest in the fossil Italian hominoid.In this study, a comparative review of hominoid cranio-dental morphology indicates that Oreopithecus possessed a suite of hominin-like characteristics that were apparently functionally related to powerful folivorous mastication. Since the oreopithecine dentition exhibited exceptional adaptations for folivory relative to most other extant and extinct hominoids, the accessory development of a hominin-like hyper-masticatory cranio-mandibular apparatus to further enhance plant comminution and digestibility is not surprising. However, the combination of hominin-like locomotive, manual, and masticatory functional attributes appears to substantiate Hurzeler's designation of O. bambolii as a Late Miocene hominin. Additionally, an extensive compilation of hominoid cranio-dental and postcranial characteristics strongly supports a close phylogenetic relationship between Oreopithecus and the earliest African hominins Sahelanthropus and Australopithecus.The wetland paleoecology of the Tortonian island of Tuscany–Sardinia suggests that Oreopithecus was a specialized semiaquatic folivore who apparently waded bipedally into freshwater swamps to feed on aquatic plants. However, the extensive wear on the oreopithecine canines and incisors along with their manual precision grips may indicate that freshwater invertebrates were also included in their diets. Such specialized aquatic feeding behavior by these ancient Italian swamp apes seems to support Alister Hardy's hypothesis that human bipedalism and power precision grips were inherited from Late Miocene hominin ancestors who originally utilized such functional attributes for aquatic foraging in shallow water environments. Additionally, the remarkably hominin-like cranio-dental morphology of O. bambolii suggests that modern omnivorous humans probably inherited a significant number of their cranio-dental characteristics from these highly specialized aquatic plant eating hominins.     

Measuring 3D Hand and Finger Kinematics—A Comparison between Inertial Sensing and an Opto-Electronic Marker System

Objective analysis of hand and finger kinematics is important to increase understanding of hand function and to quantify motor symptoms for clinical diagnosis. The aim of this paper is to compare a new 3D measurement system containing multiple miniature inertial sensors (PowerGlove) with an opto-electronic marker system during specific finger tasks in three healthy subjects. Various finger movements tasks were performed: flexion, fast flexion, tapping, hand open/closing, ab/adduction and circular pointing. 3D joint angles of the index finger joints and position of the thumb and index were compared between systems. Median root mean square differences of the main joint angles of interest ranged between 3.3 and 8.4deg. Largest differences were found in fast and circular pointing tasks, mainly in range of motion. Smallest differences for all 3D joint angles were observed in the flexion tasks. For fast finger tapping, the thumb/index amplitude showed a median difference of 15.8mm. Differences could be explained by skin movement artifacts caused by relative marker movements of the marker system, particularly during fast tasks; large movement accelerations and angular velocities which exceeded the range of the inertial sensors; and by differences in segment calibrations between systems. The PowerGlove is a system that can be of value to measure 3D hand and finger kinematics and positions in an ambulatory setting. The reported differences need to be taken into account when applying the system in studies understanding the hand function and quantifying hand motor symptoms in clinical practice.     

Early Origin for Human-Like Precision Grasping: A Comparative Study of Pollical Distal Phalanges in Fossil Hominins

Background

The morphology of human pollical distal phalanges (PDP) closely reflects the adaptation of human hands for refined precision grip with pad-to-pad contact. The presence of these precision grip-related traits in the PDP of fossil hominins has been related to human-like hand proportions (i.e. short hands with a long thumb) enabling the thumb and finger pads to contact. Although this has been traditionally linked to the appearance of stone tool-making, the alternative hypothesis of an earlier origin—related to the freeing of the hands thanks to the advent of terrestrial bipedalism—is also possible given the human-like intrinsic hand proportion found in australopiths.

Methodology/Principal Findings

We perform morphofunctional and morphometric (bivariate and multivariate) analyses of most available hominin pollical distal phalanges, including Orrorin, Australopithecus, Paranthropous and fossil Homo, in order to investigate their morphological affinities. Our results indicate that the thumb morphology of the early biped Orrorin is more human-like than that of australopiths, in spite of its ancient chronology (ca. 6 Ma). Moreover, Orrorin already displays typical human-like features related to precision grasping.

Conclusions

These results reinforce previous hypotheses relating the origin of refined manipulation of natural objects–not stone tool-making–with the relaxation of locomotor selection pressures on the forelimbs. This suggests that human hand length proportions are largely plesiomorphic, in the sense that they more closely resemble the relatively short-handed Miocene apes than the elongated hand pattern of extant hominoids. With the advent of terrestrial bipedalism, these hand proportions may have been co-opted by early hominins for enhanced manipulative capabilities that, in turn, would have been later co-opted for stone tool-making in the genus Homo, more encephalized than the previous australopiths. This hypothesis remains may be further tested by the finding of more complete hands of unequivocally biped early hominins.     

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.     

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.     

Compound grips in tufted capuchin monkeys (Sapajus spp and Sapajus libidinosus)

An experimental study with captive individuals and study of video recordings of wild monkeys explored whether and how tufted capuchin monkeys use onehand to hold one or more objects with multiple grips (compound grips). A task designed to elicit compound grip was presented to five captive tufted capuchin monkeys (Sapajus spp). The monkeys held one to four balls in onehand and dropped the balls individually into a vertical tube. Multiple simple grips and independent digit movements enabled separate control of multiple objects in one hand. Monkeys always supported the wrist on the horizontal edge of the tube before releasing the ball. Increasing the number of balls decreased the likelihood that the monkeys managed the task. Wild bearded capuchins (Sapajus libidinosus) used compound grips spontaneously to store multiple food items. Compound grips have been described in macaques, gorillas, chimpanzees, and humans, and now in a New World primate. We predict that any primate species that exhibits precision grips and independent digit movement can perform compound grips. Our findings suggest many aspects of compound grip that await investigation.     

Directional Coordination of Thumb and Finger Forces during Precision Pinch

The human opposable thumb enables the hand to perform dexterous manipulation of objects, which requires well-coordinated digit force vectors. This study investigated the directional coordination of force vectors generated by the thumb and index finger during precision pinch. Fourteen right-handed, healthy subjects were instructed to exert pinch force on an externally stabilized apparatus with the pulps of the thumb and index finger. Subjects applied forces to follow a force-ramp profile that linearly increased from 0 to 12 N and then decreased to 0 N, at a rate of ±3 N/s. Directional relationships between the thumb and index finger force vectors were quantified using the coordination angle (CA) between the force vectors. Individual force vectors were further analyzed according to their projection angles (PAs) with respect to the pinch surface planes and the shear angles (SAs) within those planes. Results demonstrated that fingertip force directions were dependent on pinch force magnitude, especially at forces below 2 N. Hysteresis was observed in the force-CA relationship for increasing and decreasing forces and fitted with exponential models. The fitted asymptotic values were 156.0±6.6° and 150.8±9.3° for increasing and decreasing force ramps, respectively. The PA of the thumb force vector deviated further from the direction perpendicular to the pinching surface planes than that of the index finger. The SA showed that the index finger force vector deviated in the ulnar-proximal direction, whereas the thumb switched its force between the ulnar-proximal and radial-proximal directions. The findings shed light on the effects of anatomical composition, biomechanical function, and neuromuscular control in coordinating digit forces during precision pinch, and provided insight into the magnitude-dependent force directional control which potentially affects a range of dexterous manipulations.     

Precision grips in young chimpanzees

A precision grip, thumb-finger opposition, has been regarded as an uniquely human trait. Napier's conclusion that chimpanzees were incapable of precision grip was based on two subjects and prehension of a single object (i.e., a grape). The purpose of the present study was to specify grip type and hand use by 13 young chimpanzees to prehend three different-sized food objects. The subjects were laboratory raised (eight males and five females) and ranged in age from 27 to 58 months. An ethogram was devised that comprised 43 different grip types: ten configurations of precision grips were found, in addition to imprecise or inefficient grip types (nine types), thumb-to-finger opposition (10 types), power grips (two types), and a variety of other grips (12 types). Subjects most often prehended were very small-sized (5 mm × 5 mm × 3 mm) or small-sized (10 mm × 10 mm × 3 mm) food objects with precision and imprecise grips. An analysis of latency to prehend, i.e., efficiency, revealed (1) precision grips were equally efficient for all object sizes; (2) power grips were most efficient with the largest object (a grape); (3) with imprecise grips, the left hand was more efficient than the right with small objects, and with power grips the right hand was more efficient than the left for medium-sized objects. No population handedness was observed, but individual handedness was seen in nine subjects for some grip types and some object sizes. This study provides evidence that young chimpanzees preferentially use a true precision grip to prehend small and very small objects. © 1996 Wiley-Liss, Inc.