Prehension synergy: use of mechanical advantage during multifinger torque production on mechanically fixed and free objects

The aim of this study was to test the mechanical advantage (MA) hypothesis in multifinger torque production tasks in humans: fingers with longer moment arms produce greater force magnitudes during torque production tasks. There were eight experimental conditions: two prehension types determined by different mechanical constraints (i.e., fixed- and free-object prehension) with two torque directions (supination and pronation) and two torque magnitudes (0.24 and 0.48 N·m). The subjects were asked to produce prescribed torques during the fixed-object prehension or to maintain constant position of the free hand-held object against external torques. The index of MA was calculated for agonist and antagonist fingers, which produce torques in the same and opposite directions to the target torques, respectively. Within agonist fingers, the fingers with longer moment arms produced greater grasping forces while within antagonist fingers, the fingers with shorter moment arms produced greater forces. The MA index was greater in the fixed-object condition as compared with the free-object condition. The MA index was greater in the pronation condition than in the supination condition. This study supports the idea that the CNS utilizes the MA of agonist fingers, but not of antagonist fingers, during torque production in both fixed- and free-object conditions.     

Force and torque production in static multifinger prehension: biomechanics and control. I. Biomechanics

 We studied the coordinated action of fingers during static tasks involving exertion of force and torque on a handheld object. Subjects were asked to keep a handle with an attachment that allowed for independent change of the suspended load (0.5–2.0 kg) and external torque (0.375–1.5 N m) in a vertical position while applying minimal effort. Normal and shear forces were measured from the thumb; normal forces only were measured from the four fingers. Experimental results: (1) the thumb shear force increased during supination efforts and decreased during pronation efforts; (2) the total moment of the normal finger forces only counterbalanced approximately 50% of the external torque, hence shear forces accounted for approximately one-half of the total torque exerted on the object; (3) the total normal force increased with external torque, and the total force magnitude did not depend on the torque direction; (4) the forces of the `peripheral' (index and little) fingers depended mainly on the torque while the forces exerted by the `central' (middle and ring) fingers depended both on the load and torque; (5) there was a monotonic relationship between the mechanical advantage of a finger (i.e., its moment arm during torque production) and the force produced by that finger; and (6) antagonist finger moments acting opposite to the intended direction of the total moment were always observed – at low torques the antagonist moments were as high as 40–60% of the agonist moments. Modeling: A three-zone model of coordinated finger action is suggested. In the first zone of load/torque combinations, activation of antagonist fingers (i.e., fingers that generate antagonist moments) is necessary to prevent slipping. In the second zone, the activity of agonist fingers is sufficient for preventing slips. In the third zone, the performer has freedom to choose between either activating the antagonist fingers or redistributing activities amongst the agonist fingers. The findings of this study provide the foundation for neural network and optimization modeling described in the companion paper [Zatsiorsky et al. (2002) Biol Cybern DOI 10.1007/s00422-002-0320-7]. Received: 8 August 2001 / Accepted in revised form: 7 February 2002     

Coordination of contact forces during multifinger static prehension

This study investigated the effects of modifying contact finger forces in one direction-normal or tangential-on the entire set of the contact forces, while statically holding an object. Subjects grasped a handle instrumented with finger force-moment sensors, maintained it at rest in the air, and then slowly: (1) increased the grasping force, (2) tried to spread fingers apart, and (3) tried to squeeze fingers together. Analysis was mostly performed at the virtual finger (VF) level (the VF is an imaginable finger that generates the same force and moment as the four fingers combined). For all three tasks there were statistically significant changes in the VF normal and tangential forces. For finger spreading/squeezing the tangential force neutral point was located between the index and middle fingers. We conclude that the internal forces are regulated as a whole, including adjustments in both normal and tangential force, instead of only a subset of forces (normal or tangential). The effects of such factors as EFFORT and TORQUE were additive; their interaction was not statistically significant, thus supporting the principle of superposition in human prehension.     

Finger force vectors in multi-finger prehension

In a majority of studies on grasp, only normal forces were measured and only when a zero torque was exerted on a hand-held object. This study concerns finger force vectors during the torque production tasks. Subjects (n=8) stabilized a handle with an attachment that allowed for change of external torque from -1.5 to 1.5 Nm. Forces and moments exerted by the digit tips on the object were recorded. At the large (>-0.375 Nm) supination torques the index/middle and ring/little pairs of fingers generated oppositely directed tangential forces. The index and middle finger produced forces in a downward direction and therefore did not support the load. At a zero torque and pronation torques, the middle, ring and little fingers produced forces along nearly the same direction. The vector of the index finger force was always directed differently from the vectors of other finger forces, the angles ranged from 19 degrees 30' to 47 degrees 40'. The points of force application were systematically displaced with the torque, with the exception of the little finger. Tangential finger forces contributed substantially to the total torque exerted on the hand-held object.     

Age-related changes in finger coordination in static prehension tasks.

We studied age-related changes in the performance of maximal and accurate submaximal force and moment production tasks. Elderly and young subjects pressed on six dimensional force sensors affixed to a handle with a T-shaped attachment. The weight of the whole system was counterbalanced with another load. During tasks that required the production of maximal force or maximal moment by all of the digits, young subjects were stronger than elderly. A greater age-related deficit was seen in the maximal moment production tests. During maximal force production tests, elderly subjects showed larger relative involvement of the index and middle fingers; they moved the point of thumb force application upward (toward the index and middle fingers), whereas the young subjects rolled the thumb downward. During accurate force/moment production trials, elderly persons were less accurate in the production of both total moment and total force. They produced higher antagonistic moments, i.e., moment by fingers that acted against the required direction of the total moment. Both young and elderly subjects showed negative covariation of finger forces across repetitions of a ramp force production task. In accurate moment production tasks, both groups showed negative covariation of two components of the total moment: those produced by the normal forces and those produced by the tangential forces. However, elderly persons showed lower values of the indexes of both finger force covariation and moment covariation. We conclude that age is associated with an impaired ability to produce both high moments and accurate time profiles of moments. This impairment goes beyond the well-documented deficits in finger and hand force production by elderly persons. It involves worse coordination of individual digit forces and of components of the total moment. Some atypical characteristics of finger forces may be viewed as adaptive to the increased variability in the force production with age.     

Force and torque production in static multifinger prehension: biomechanics and control. II. Control

 The coordination of digits during combined force/torque production tasks was further studied using the data presented in the companion paper [Zatsiorsky et al. Biol Cybern this issue, Part I]. Optimization was performed using as criteria the cubic norms of (a) finger forces, (b) finger forces normalized with respect to the maximal forces measured in single-finger tasks, (c) finger forces normalized with respect to the maximal forces measured in a four-finger task, and (d) finger forces normalized with respect to the maximal moments that can be generated by the fingers. All four criteria failed to predict antagonist finger moments when these moments were not imposed by the task mechanics. Reconstruction of neural commands: The vector of neural commands c was reconstructed from the equation c=W ⁻¹ F, where W is the finger interconnection weight matrix and F is the vector of finger forces. The neural commands ranged from zero (no voluntary force production) to one (maximal voluntary contraction). For fingers producing moments counteracting the external torque (`agonist' fingers), the intensity of the neural commands was well correlated with the relative finger forces normalized to the maximal forces in a four-finger task. When fingers produced moments in the direction of the external torque (`antagonist' fingers), the relative finger forces were always larger than those expected from the intensity of the corresponding neural commands. The individual finger forces were decomposed into forces due to `direct' commands and forces induced by enslaving effects. Optimization of the neural commands resulted in the best correspondence between actual and predicted finger forces. The antagonist moments are, at least in part, due to enslaving effects: strong commands to agonist fingers also activated antagonist fingers. Received: 8 August 2001 / Accepted in revised form: 7 February 2002     

Effect of object width on muscle and joint forces during thumb-index finger grasping

The objective of this study was to identify the impact of modifying the object width on muscle and joint forces while gripping objects. The experimental protocol consisted to maintain horizontally five objects of different widths (3.5, 4.5, 5.5, 6.5, and 7.5 cm) with a thumb-index finger grip. Subjects were required to grasp spontaneously the object without any instruction regarding the grip force (GF) to apply. A biomechanical model of thumb-index finger pinch was developed to estimate muscle and joint forces. This model included electromyography, fingertip force, and kinematics data as inputs. The finger joint postures and the GF varied across the object widths. The estimated muscle forces also varied significantly according to the object width. Interestingly, we observed that the muscle force/GF ratios of major flexor muscles remain particularly stable with respect to the width whereas other muscle ratios differed largely. This may argue for a control strategy in which the actions of flexors were preserved in spite of change in joint postures. The estimated joint forces tended to increase with object width and increased in the distal-proximal sense. Overall, these results are of importance for the ergonomic design of handheld objects and for clinical applications.     

Characteristics of human fingertips in the shearing direction

The shearing strain of the human fingertip plays an important role in the determination of the optimal grasping force and in the perception of texture. Most research concerned with the mechanical impedance of the human fingertips has treated the orthogonal direction to the tip surface, and little attention has been paid to the tangential direction. This paper describes impedance characteristics of the human fingertips in the tangential directions to the tip surface. In the experiment, step and ramp shearing forces were individually applied to the tips of the thumb, middle finger, and little finger. Dynamics of the fingertips were represented by the Kelvin model. Experimental results show that each fingertip had different properties with respect to the shearing strain versus the applied force, and that the thumb had the strongest shearing stiffness among these three digits. Moreover, the shearing stiffness depended on the direction of the applied force, and the stiffness in the pointing direction was stronger than that in the perpendicular direction. As the contact force in the orthogonal direction to the fingertip surface was increased, the shearing stiffness and viscosity increased without regard to the load speed of the shearing force. Furthermore, it is shown that the average strain rate of the fingertip in the tangential direction to the fingertip surface became slower and converged to a constant value with higher contact forces.     

Effect of Tendon Vibration on Hemiparetic Arm Stability in Unstable Workspaces

Sensory stimulation of wrist musculature can enhance stability in the proximal arm and may be a useful therapy aimed at improving arm control post-stroke. Specifically, our prior research indicates tendon vibration can enhance stability during point-to-point arm movements and in tracking tasks. The goal of the present study was to investigate the influence of forearm tendon vibration on endpoint stability, measured at the hand, immediately following forward arm movements in an unstable environment. Both proximal and distal workspaces were tested. Ten hemiparetic stroke subjects and 5 healthy controls made forward arm movements while grasping the handle of a two-joint robotic arm. At the end of each movement, the robot applied destabilizing forces. During some trials, 70 Hz vibration was applied to the forearm flexor muscle tendons. 70 Hz was used as the stimulus frequency as it lies within the range of optimal frequencies that activate the muscle spindles at the highest response rate. Endpoint position, velocity, muscle activity and grip force data were compared before, during and after vibration. Stability at the endpoint was quantified as the magnitude of oscillation about the target position, calculated from the power of the tangential velocity data. Prior to vibration, subjects produced unstable, oscillating hand movements about the target location due to the applied force field. Stability increased during vibration, as evidenced by decreased oscillation in hand tangential velocity.     

Effect of resistance training on skeletal muscle-specific force in elderly humans.

This study assessed muscle-specific force in vivo following strength training in old age. Subjects were assigned to training (n = 9, age 74.3 +/- 3.5 yr; mean +/- SD) and control (n = 9, age 67.1 +/- 2 yr) groups. Leg-extension and leg-press exercises (2 sets of 10 repetitions at 80% of the 5 repetition maximum) were performed three times/wk for 14 wk. Vastus lateralis (VL) muscle fascicle force was calculated from maximal isometric voluntary knee extensor torque with superimposed stimuli, accounting for the patella tendon moment arm length, ultrasound-based measurements of muscle architecture, and antagonist cocontraction estimated from electromyographic activity. Physiological cross-sectional area (PCSA) was calculated from the ratio of muscle volume to fascicle length. Specific force was calculated by dividing fascicle force by PCSA. Fascicle force increased by 11%, from 847.9 +/- 365.3 N before to 939.3 +/- 347.8 N after training (P < 0.05). Due to a relatively greater increase in fascicle length (11%) than muscle volume (6%), PCSA remained unchanged (pretraining: 30.4 +/- 8.9 cm(2); posttraining: 29.1 +/- 8.4 cm(2); P > 0.05). Activation capacity and VL muscle root mean square electromyographic activity increased by 5 and 40%, respectively, after training (P < 0.05), indicating increased agonist neural drive, whereas antagonist cocontraction remained unchanged (P > 0.05). The VL muscle-specific force increased by 19%, from 27 +/- 6.3 N/cm(2) before to 32.1 +/- 7.4 N/cm(2) after training (P < 0.01), highlighting the effectiveness of strength training for increasing the intrinsic force-producing capacity of skeletal muscle in old age.     

The role of co-activation in strength and force modulation in the elbow of children with unilateral cerebral palsy

To study the role of coactivation in strength and force modulation in the elbow joint of children and adolescents with cerebral palsy (CP), we investigated the affected and contralateral arm of 21 persons (age 8-18) with spastic unilateral CP in three tasks: maximal voluntary isokinetic concentric contraction and passive isokinetic movement during elbow flexion and extension, and sub-maximal isometric force tracing during elbow flexion. Elbow flexion-extension torque and surface electromyography (EMG) of the biceps brachii (BB) and triceps brachii (TB) muscles were recorded. During the maximal contractions, the affected arm was weaker, had decreased agonist and similar antagonist EMG amplitudes, and thus increased antagonist co-activation (% of maximal activity as agonist) during both elbow flexion and extension, with higher coactivation levels of the TB than the BB. During passive elbow extension, the BB of the affected arm showed increased resistance torque and indication of reflex, and thus spastic, activity. No difference between the two arms was found in the ability to modulate force, despite increased TB coactivation in the affected arm. The results indicate that coactivation plays a minor role in muscle weakness in CP, and does not limit force modulation. Moreover, spasticity seems particularly to increase coactivation in the muscle antagonistic to the spastic one, possibly in order to increase stability.     

Radial force distribution changes associated with tangential force production in cylindrical grasping, and the importance of anatomical registration

Radial force (F(r)) distributions describe grip force coordination about a cylindrical object. Recent studies have employed only explicit F(r) tasks, and have not normalized for anatomical variance when considering F(r) distributions. The goals of the present study were (i) to explore F(r) during tangential force production tasks, and (ii) to examine the extent to which anatomical registration (i.e. spatial normalization of anatomically analogous structures) could improve signal detectability in F(r) data. Twelve subjects grasped a vertically oriented cylindrical handle (diameter=6 cm) and matched target upward tangential forces of 10, 20, and 30 N. F(r) data were measured using a flexible pressure mat with an angular resolution of 4.8°, and were registered using piecewise-linear interpolation between five manually identified points-of-interest. Results indicate that F(r) was primarily limited to three contact regions: the distal thumb, the distal fingers, and the fingers' metatacarpal heads, and that, while increases in tangential force caused significant increases in F(r) for these regions, they did not significantly affect the F(r) distribution across the hand. Registration was found to substantially reduce between-subject variability, as indicated by both accentuated F(r) trends, and amplification of the test statistic. These results imply that, while subjects focus F(r) primarily on three anatomical regions during cylindrical grasp, inter-subject anatomical differences introduce a variability that, if not corrected for via registration, may compromise one's ability to draw anatomically relevant conclusions from grasping force data.     

The Force Synergy of Human Digits in Static and Dynamic Cylindrical Grasps

This study explores the force synergy of human digits in both static and dynamic cylindrical grasping conditions. The patterns of digit force distribution, error compensation, and the relationships among digit forces are examined to quantify the synergetic patterns and coordination of multi-finger movements. This study recruited 24 healthy participants to perform cylindrical grasps using a glass simulator under normal grasping and one-finger restricted conditions. Parameters such as the grasping force, patterns of digit force distribution, and the force coefficient of variation are determined. Correlation coefficients and principal component analysis (PCA) are used to estimate the synergy strength under the dynamic grasping condition. Specific distribution patterns of digit forces are identified for various conditions. The compensation of adjacent fingers for the force in the normal direction of an absent finger agrees with the principle of error compensation. For digit forces in anti-gravity directions, the distribution patterns vary significantly by participant. The forces exerted by the thumb are closely related to those exerted by other fingers under all conditions. The index-middle and middle-ring finger pairs demonstrate a significant relationship. The PCA results show that the normal forces of digits are highly coordinated. This study reveals that normal force synergy exists under both static and dynamic cylindrical grasping conditions.     

Effect of muscle model parameter scaling for isometric plantar flexion torque prediction

This paper uses a EMG-driven Hill-type muscle model to estimate individual muscle forces of the triceps surae in isometric plantar flexion contractions. A uniform group of 20 young physical-active adult males was instructed to follow a specific contraction protocol with low (20%MVC) and medium-high (60%MVC) contractions, separated by relaxing intervals. The torque calculated by summing the individual muscle forces multiplied by the respective moment arms was compared to the torque measured by a dynamometer. Musculoskeletal parameters from the literature were used. Then, three different “correction factors” or bias have been applied on some of the muscle model parameters. These factors were based on anthropometric and dynamometric measurements: moment arm scaled by bimalleolar diameter, tendon slack length by leg length and optimal force by the maximum torque. Model torque agreement with dynamometer was recalculated with the parameter scales. It was observed that the relative torque estimation error decreased slightly but significantly when all factors were applied simultaneously (12.92±4.94% without scaling to 10.12±1.73%), which resulted mainly from the correction of the maximal muscle force parameter.     

Biomechanical outcomes and neural correlates of cutaneous reflexes evoked during rhythmic arm cycling

During walking cutaneous stimulation of the foot yields neural and mechanical reflexes that serve a functional purpose to correct or assist the ongoing movement. Concurrently, while cutaneous stimulation of the hand during rhythmic arm movement parallel the neural responses observed in the legs, studies of rhythmic arm movement have only limited mechanical measurements. Therefore it is difficult to determine whether reflex responses in the arms during rhythmic arm movement serve a functional purpose similar to those seen in the lower limbs. The purpose of this study was to explore the mechanical outcomes of stimulating a cutaneous nerve innervating the hand during arm cycling. We hypothesized that there would be measurable mechanical effects to cutaneous stimulation during arm cycling that function to correct or assist the task of arm cycling. Specifically, kinetic responses measured at the handle would be considered assistive if they were tangential to the arm cycling path in the direction of forward progression. Also, limb kinematic responses would be considered corrective if they allowed limb movement that would result in removal of the limb from stimulus while not altering the kinetic profile at the handle necessary for arm cycling progression. Participants performed seated arm cycling while EMG was recorded from the arm and trunk muscles, kinematic data was recorded from the right arm, and kinetic data was recorded from the handle. Cutaneous reflexes were evoked by stimulating the superficial radial nerve. The results show that there are observable mechanical responses to cutaneous stimulation of the hand during arm cycling. Subjects responded to cutaneous stimulation of the hand during arm cycling with significant changes in backward and lateral forces at the handle as well as wrist abduction/adduction and wrist flexion/extension kinematics. These responses, related to the task and phase of movement, are consistent with the anatomical location of the stimulus and are correlated to the neural responses. Therefore, these responses are comparable to functionally relevant responses in the legs during rhythmic movement. However, while there is a single observation of a kinematic corrective strategy, the kinetics measured at the handle are not tangential to the arm cycling path and therefore not considered an assistive response. Therefore, unlike the observations in the lower limbs, the mechanical responses during arm cycling are not clearly related to the functional context of the ongoing task.     

Modelling torque generation by the mero-carpopodite joint of the american lobster and the snow crab

The torque generated by a rotating joint comprises the useful force exerted by the joint on the external environment, and both the magnitude and distribution of torque through the step cycle during walking are important variables in understanding the mechanics of walking. The mechanics of the American lobster (Homarus americanus) and snow crab (Chionoecetes opilio) during walking were modelled to examine the relative roles of flexor versus extensor apodeme-muscle complexes, investigate which legs of these decapods likely contribute the greatest to locomotion, determine scaling effects of torque generation, and assess the relative roles of various model variables on torque production. Force generated along the length of the apodeme by the muscle was modelled based on apodeme surface area, muscle stress, and muscle fibre pinnation angle. Torque was then calculated from this estimated force and the corresponding moment arm. The flexor apodeme-muscle complex is calculated to generate consistently greater forces than the extensor, and generally this results in flexor torque being larger than extensor, though the snow crab does illustrate the opposite in two of its legs. This greater torque generation in flexion suggests that, in addition to the pushing of the trailing legs, the pulling action of the leading legs may play a significant role, at least during lateral walking. Leg 4 of both species appears to generate greater torques and thus provide the greatest forces for locomotion. Torque generation as a function of body size shows a second order response due to the increase in apodeme surface area. The pinnation angle of the muscle fibre is found to be insignificant in force generation, apodeme surface area (representing muscle cross sectional area) likely plays the most influential role in total force production, and moment arm controls the distribution of this force through the step cycle. Muscle stress remain a largely unknown quantity however, and may significantly affect both magnitude and distribution through step cycle of forces, and thus torque. Despite the uncertainty associated with the muscle stress parameter, the modelled results fit well with previously published force measurements.     

Prehension synergies during nonvertical grasping,I: experimental observations

The mechanical complexities of rotating an object through the gravity field present a formidable challenge to the human central nervous system (CNS). The current study documents the finger force patterns selected by the CNS when performing one-, two-, and four-finger grasping while holding an object statically at various orientations with respect to vertical. Numerous mechanically unnecessary behaviors were observed. These included: nonzero tangential forces for horizontal handle orientations, large internal forces (i.e., those in excess of equilibrium requirements) for all orientations, and safety margins between 50 and 90%. Additionally, none of the investigated measures were constant across orientations or could be represented as a simple trigonometric function of orientation. Nonetheless, all measures varied in systematic (and sometimes symmetric) ways with orientation. The results suggest that the CNS selects force patterns that are based on mechanical principles but also that are not simply related to object orientation. This study is complemented by a second paper that provides an in-depth analysis of the mechanics of nonvertical grasping and accounts for many of the observed results with numerical optimization (see Part II – current issue). Together, the papers demonstrate that the CNS is likely to utilize optimization processes when controlling prehensile actions.Supported in part by NIH grants AR-48563, AG-018751 and NS-35032.     

The effects of the antagonist muscle force on intersegmental loading during isokinetic efforts of the knee extensors

Hamstrings activation when acting as antagonists is considered very important for knee joint stability. However, the effect of hamstring antagonist activity on knee joint loading in vivo is not clear. Therefore, the purpose of this study was to examine the differences in antagonistic muscle force and their effect on agonist muscle and intersegmental forces during isokinetic eccentric and concentric efforts of the knee extensors. Ten males performed maximum isokinetic eccentric and concentric efforts of the knee extensors at 30 degrees s(-1). The muscular and tibiofemoral joint forces were then estimated using a two-dimensional model with and without including the antagonist muscle forces. The antagonist moment was predicted using an IEMG-moment model. The predicted antagonist force reached a maximum of 2.55 times body weight (BW) and 1.16 BW under concentric and eccentric conditions respectively. Paired t-tests indicated that these were significantly different (p<0.05). A one-way analysis of variance indicated that when antagonist forces are included in the calculations the patella tendon, compressive and posterior shear joint forces are significantly higher compared to those calculated without including the antagonist forces. The anterior shear force was not affected by antagonist activity. The antagonists produce considerable force throughout the range of motion and affect the joint forces exerted at the knee joint. Further, it appears that the antagonist effect depends on the type of muscle action examined as it is higher during concentric compared to eccentric efforts of the knee extensors.