Kinematic variability of grasp movements as a function of practice and movement speed

Tradeoffs of estimating reaction time with absolute and relative thresholds

Measuring the duration of cognitive processing with reaction time is fundamental to several subfields of psychology. Many methods exist for estimating movement initiation when measuring reaction time, but there is an incomplete understanding of their relative performance. The purpose of the present study was to identify and compare the tradeoffs of 19 estimates of movement initiation across two experiments. We focused our investigation on estimating movement initiation on each trial with filtered kinematic and kinetic data. Nine of the estimates involved absolute thresholds (e.g., acceleration 1000 back to 200 mm/s2, micro push-button switch), and the remaining ten estimates used relative thresholds (e.g., force extrapolation, 5% of maximum velocity). The criteria were the duration of reaction time, immunity to the movement amplitude, responsiveness to visual feedback during movement execution, reliability, and the number of manually corrected trials (efficacy). The three best overall estimates, in descending order, were yank extrapolation, force extrapolation, and acceleration 1000 to 200 mm/s2. The sensitive micro push-button switch, which was the simplest estimate, had a decent overall score, but it was a late estimate of movement initiation. The relative thresholds based on kinematics had the six worst overall scores. An issue with the relative kinematic thresholds was that they were biased by the movement amplitude. In summary, we recommend measuring reaction time on each trial with one of the three best overall estimates of movement initiation. Future research should continue to refine existing estimates while also exploring new ones.

Articulatory Variability is Reduced by Repetition and Predictability

Repeating the movements associated with activities such as drawing or sports typically leads to improvements in kinematic behavior: these movements become faster, smoother, and exhibit less variation. Likewise, practice has also been shown to lead to faster and smoother movement trajectories in speech articulation. However, little is known about its effect on articulatory variability. To address this, we investigate the extent to which repetition and predictability influence the articulation of the frequent German word "sie" [zi] (they). We find that articulatory variability is proportional to speaking rate and the duration of [zi], and that overall variability decreases as [zi] is repeated during the experiment. Lower variability is also observed as the conditional probability of [zi] increases, and the greatest reduction in variability occurs during the execution of the vocalic target of [i]. These results indicate that practice can produce observable differences in the articulation of even the most common gestures used in speech.

Modulation of ellipses drawing by sonification

Most studies on the regulation of speed and trajectory during ellipse drawing have used visual feedback. We used online auditory feedback (sonification) to induce implicit movement changes independently from vision. The sound was produced by filtering a pink noise with a band-pass filter proportional to movement speed. The first experiment was performed in 2D. Healthy participants were asked to repetitively draw ellipses during 45 s trials whilst maintaining a constant sonification pattern (involving pitch variations during the cycle). Perturbations were produced by modifying the slope of the mapping without informing the participants. All participants adapted spontaneously their speed: they went faster if the slope decreased and slower if it increased. Higher velocities were achieved by increasing both the frequency of the movements and the perimeter of the ellipses, but slower velocities were achieved mainly by decreasing the perimeter of the ellipses. The shape and the orientation of the ellipses were not significantly altered. The analysis of the speed-curvature power law parameters showed consistent modulations of the speed gain factor, while the exponent remained stable. The second experiment was performed in 3D and showed similar results, except that the main orientation of the ellipse also varied with the changes in speed. In conclusion, this study demonstrated implicit modulation of movement speed by sonification and robust stability of the ellipse geometry. Participants appeared to limit the decrease in movement frequency during slowing down to maintain a rhythmic and not discrete motor regimen.

Functional motor control deficits in older FMR1 premutation carriers

Individuals with fragile X mental retardation 1 (FMR1) gene premutations are at increased risk for fragile X-associated tremor/ataxia syndrome (FXTAS) during aging. However, it is unknown whether older FMR1 premutation carriers, with or without FXTAS, exhibit functional motor control deficits compared with healthy individuals. The purpose of this study, therefore, was to determine whether older FMR1 premutation carriers exhibit impaired ability to perform functional motor tasks. Eight FMR1 premutation carriers (age: 58.88 ± 8.36 years) and eight age- and sex-matched healthy individuals (60.13 ± 9.25 years) performed (1) a steady isometric force control task with the index finger at 20% of their maximum voluntary contraction (MVC) and; (2) a single-step task. During the finger abduction task, firing rate of multiple motor units of the first dorsal interosseous (FDI) muscle was recorded. Compared with healthy controls, FMR1 premutation carriers exhibited (1) greater force variability (coefficient of variation of force) during isometric force (1.48 ± 1.02 vs. 0.63 ± 0.37%; P = 0.04); (2) reduced firing rate of multiple motor units during steady force, and; (3) reduced velocity of their weight transfer during stepping (156.62 ± 26.24 vs. 191.86 ± 18.83 cm/s; P = 0.01). These findings suggest that older FMR1 premutation carriers exhibit functional motor control deficits that reflect either subclinical issues associated with premutations independent of FXTAS, or prodromal markers of the development of FXTAS.

Experimental and theoretical study of velocity fluctuations during slow movements in humans

Moving smoothly is generally considered as a higher-order goal of motor control and moving jerkily as a witness of clumsiness or pathology, yet many common and well-controlled movements (e.g., tracking movements) have irregular velocity profiles with widespread fluctuations. The origin and nature of these fluctuations have been associated with the operation of an intermittent process but in fact remain poorly understood. Here we studied velocity fluctuations during slow movements, using combined experimental and theoretical tools. We recorded arm movement trajectories in a group of healthy participants performing back-and-forth movements at different speeds, and we analyzed velocity profiles in terms of series of segments (portions of velocity between 2 minima). We found that most of the segments were smooth (i.e., corresponding to a biphasic acceleration) and had constant duration irrespective of movement speed and linearly increasing amplitude with movement speed. We accounted for these observations with an optimal feedback control model driven by a staircase goal position signal in the presence of sensory noise. Our study suggests that one and the same control process can explain the production of fast and slow movements, i.e., fast movements emerge from the immediate tracking of a global goal position and slow movements from the successive tracking of intermittently updated intermediate goal positions. NEW & NOTEWORTHY We show in experiments and modeling that slow movements could result from the brain tracking a sequence of via points regularly distributed in time and space. Accordingly, slow movements would differ from fast movement by the nature of the guidance and not by the nature of control. This result could help in understanding the origin and nature of slow and segmented movements frequently observed in brain disorders.

A Single, Continuously Applied Control Policy for Modeling Reaching Movements with and without Perturbation

It has been debated whether kinematic features, such as the number of peaks or decomposed submovements in a velocity profile, indicate the number of discrete motor impulses or result from a continuous control process. The debate is particularly relevant for tasks involving target perturbation, which can alter movement kinematics. To simulate such tasks, finite-horizon models require two preset movement durations to compute two control policies before and after the perturbation. Another model employs infinite- and finite-horizon formulations to determine, respectively, movement durations and control policies, which are updated every time step. We adopted an infinite-horizon optimal feedback control model that, unlike previous approaches, does not preset movement durations or use multiple control policies. It contains both control-dependent and independent noises in system dynamics, state-dependent and independent noises in sensory feedbacks, and different delays and noise levels for visual and proprioceptive feedbacks. We analytically derived an optimal solution that can be applied continuously to move an effector toward a target regardless of whether, when, or where the target jumps. This single policy produces different numbers of peaks and “submovements” in velocity profiles for different conditions and trials. Movements that are slower or perturbed later appear to have more submovements. The model is also consistent with the observation that subjects can perform the perturbation task even without detecting the target jump or seeing their hands during reaching. Finally, because the model incorporates Weber's law via a state representation relative to the target, it explains why initial and terminal visual feedback are, respectively, less and more effective in improving end-point accuracy. Our work suggests that the number of peaks or submovements in a velocity profile does not necessarily reflect the number of motor impulses and that the difference between initial and terminal feedback does not necessarily imply a transition between open- and closed-loop strategies.

Motor Output Variability Impairs Driving Ability in Older Adults

BACKGROUND

The functional declines with aging relate to deficits in motor control and strength. In this study, we determine whether older adults exhibit impaired driving as a consequence of declines in motor control or strength.

METHODS

Young and older adults performed the following tasks: (i) maximum voluntary contractions of ankle dorsiflexion and plantarflexion; (ii) sinusoidal tracking with isolated ankle dorsiflexion; and (iii) a reactive driving task that required responding to unexpected brake lights of the car ahead. We quantified motor control with ankle force variability, gas position variability, and brake force variability. We quantified reactive driving performance with a combination of gas pedal error, premotor and motor response times, and brake pedal error.

RESULTS

Reactive driving performance was ~30% more impaired (t = 3.38; p < .01) in older adults compared with young adults. Older adults exhibited greater motor output variability during both isolated ankle dorsiflexion contractions (t = 2.76; p < .05) and reactive driving (gas pedal variability: t = 1.87; p < .03; brake pedal variability: t = 4.55; p < .01). Deficits in reactive driving were strongly correlated to greater motor output variability (R ² = .48; p < .01) but not strength (p > .05).

CONCLUSIONS

This study provides novel evidence that age-related declines in motor control but not strength impair reactive driving. These findings have implications on rehabilitation and suggest that interventions should focus on improving motor control to enhance driving-related function in older adults.

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.

Online kinematic regulation by visual feedback for grasp versus transport during reach-to-pinch

PURPOSE

This study investigated novel kinematic performance parameters to understand regulation by visual feedback (VF) of the reaching hand on the grasp and transport components during the reach-to-pinch maneuver. Conventional metrics often signify discrete movement features to postulate sensory-based control effects (e.g., time for maximum velocity to signify feedback delay). The presented metrics of this study were devised to characterize relative vision-based control of the sub-movements across the entire maneuver.

METHODS

Movement performance was assessed according to reduced variability and increased efficiency of kinematic trajectories. Variability was calculated as the standard deviation about the observed mean trajectory for a given subject and VF condition across kinematic derivatives for sub-movements of inter-pad grasp (distance between thumb and index finger-pads; relative orientation of finger-pads) and transport (distance traversed by wrist). A Markov analysis then examined the probabilistic effect of VF on which movement component exhibited higher variability over phases of the complete maneuver. Jerk-based metrics of smoothness (minimal jerk) and energy (integrated jerk-squared) were applied to indicate total movement efficiency with VF.

RESULTS/DISCUSSION

The reductions in grasp variability metrics with VF were significantly greater (p<.05) compared to transport for velocity, acceleration, and jerk, suggesting separate control pathways for each component. The Markov analysis indicated that VF preferentially regulates grasp over transport when continuous control is modeled probabilistically during the movement. Efficiency measures demonstrated VF to be more integral for early motor planning of grasp than transport in producing greater increases in smoothness and trajectory adjustments (i.e., jerk-energy) early compared to late in the movement cycle.

CONCLUSIONS

These findings demonstrate the greater regulation by VF on kinematic performance of grasp compared to transport and how particular features of this relativistic control occur continually over the maneuver. Utilizing the advanced performance metrics presented in this study facilitated characterization of VF effects continuously across the entire movement in corroborating the notion of separate control pathways for each component.

Affordances as Probabilistic Functions: Implications for Development, Perception, and Decisions for Action

We propose a new way to describe affordances for action. Previous characterizations of affordances treat action possibilities as binary categories-either possible or impossible-separated by a critical point. Here, we show that affordances are probabilistic functions, thus accounting for variability in motor performance. By measuring an affordance function, researchers can describe the likelihood of success for every unit of the environment. We demonstrate how to fit an affordance function to performance data using established psychophysical procedures and illustrate how the threshold and variability parameters describe different possibilities for action. Finally, we discuss the implications of probabilistic affordances for development, perception, and decision-making.

Influence of index finger proximal interphalangeal joint arthrodesis on precision pinch kinematics

PURPOSE

To evaluate the impact of proximal interphalangeal (PIP) joint arthrodesis on the kinematics of precision pinch.

METHODS

Eleven healthy subjects performed index finger-thumb pinch motions under 4 conditions: unrestricted thumb and index finger (CONTROL) and fusion of the PIP joint of the index finger in flexion of 30° (PIP30), 40° (PIP40), and 50° (PIP50). Fusion was simulated with metallic splints. Kinematics of the thumb and index finger were recorded with a motion capture system.

RESULTS

Proximal interphalangeal joint fusion at 30°, 40°, and 50° restricted maximal pinch span between the thumb tip and index finger tip by 6%, 10%, and 14%, respectively. At the time of pulp contact, PIP fusion led to an increase in index metacarpophalangeal joint flexion angle for the PIP30 condition and an increase in variability of thumb tip location for the PIP50 condition. Furthermore, the dynamic coordination between joint angles throughout the movement was affected by PIP fusion.

CONCLUSIONS

This study reports impairment in the kinematics of precision pinch associated with index finger PIP joint fusion. A PIP joint fusion at 40° to 50° leads to a more natural precision pinch posture, but it restricts the aperture and reduces pinch precision.

Computational motor control: feedback and accuracy

Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDN(m)) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDN(s)) and SDN(m), a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability.

Variability of reciprocal aiming movements during standing: the effect of amplitude and frequency

This study investigated the variability of the center of pressure (COP) trajectory during voluntary whole-body oscillations. While standing upright on a force platform, eight subjects leaned forward and backward so as to perform reciprocal aiming movements with their COP at a prescribed frequency (F) and amplitude (A) using online visual feedback of their COP location. A total of 25 F-A combinations were tested for each subject (3 < A < 9 cm, and 0.35 < F < 1.35 Hz). Spatial and temporal variability of the COP was assessed by computing the standard deviation (SD) and coefficient of variation (CV) of the amplitude (trough to peak) and frequency (peak to peak) of the COP cycles within each trial, respectively. The results revealed that all variability indices depended on the prescribed F and A. Concerning the effect of spatial constraints on spatial variability, SD spatial increased as a function of A, while CV spatial decreased as function of A. A similar pattern was observed with respect to the effect of temporal constraints on temporal variability (SD temporal increased as a function of F, while CV temporal decreased). As for "cross-over" effects, there was an effect of F on spatial variability, such that SD spatial and CV spatial were minimal at 0.6 Hz. For the "cross over" effect of A on temporal variability, both SD spatial and CV spatial decreased as a function of A. Across the experimental conditions, there were weak or no correlations between variability in the time and space domain. Comparisons with an earlier study on human gait (Danion F, Varraine E, Bonnard M, Pailhous J. Stride variability in human gait: the effect of stride frequency and stride length. Gait Posture 2003;18:69-77) suggest that the effects of spatial constraints are relatively task independent, whereas the effects of temporal constraints depend on the nature of the motor task that is performed.

The effects of observational feedback and verbal cues on the motor learning of an aimed reach-and-point task

PURPOSE

Current technology allows the recording of movement for both motion analysis and providing observational feedback. The most effective type of observational feedback is under debate. We compared a child's reach-and-point performance after viewing a videotaped playback of a model's performance and after viewing a split-screen comparison of the model's and child's performances while simultaneously receiving verbal cues.

METHODS

A PTVision system provided observational feedback and recorded spatial trajectory, target accuracy, movement time, and joint angles while a 13 year-old boy with cerebral palsy reached for three targets.

RESULTS

The split-screen comparison had the largest effect on reach performance, including slower-yet-more-accurate movements and a more extended wrist, curved spatial trajectories, and an ulnar-deviated wrist.

CONCLUSIONS

Feedback using split-screen comparison between a model's and the child's performance with verbal cues appears to promote motor learning. When using technology to augment therapy, the intervention should be designed considering current motor learning principles.

Kinematic and dynamic synergies of human precision-grip movements

We analyzed the adaptability of human thumb and index finger movement kinematics and dynamics to variations of precision grip aperture and movement velocity. Six subjects performed precision grip opening and closing movements under different conditions of movement velocity and movement aperture (thumb and index finger tip-to-tip distance). Angular motion of the thumb and index finger joints was recorded with a CyberGlove and a three-dimensional biomechanical model was used for solving the inverse dynamics problem during precision grip movements, i.e., for calculating joint torques from experimentally obtained angular variations. The time-varying joint angles and joint torques were analyzed by principal-component analysis to quantify the contributions of individual joints in kinematic and dynamic synergies. At the level of movement kinematics, we found subject-specific angular contributions. However, the adaptation to large aperture, achieved by an increase of the relative contribution of the proximal joints, was subject-invariant. At the level of movement dynamics, the adaptation of thumb-index finger movements to task constraints was similar among all subjects and required the linear scaling of joint torques, the synchronization of joint torques under high velocity conditions, and a flexible redistribution of joint torques between the proximal joint of the thumb and that of the index finger. This work represents one of the first attempts at calculating the joint torques during human precision-grip movements and indicates that the dynamic synergies seem to be remarkably simple compared with the synergies found for movement kinematics.

Decomposition of variability in the execution of goal-oriented tasks: three components of skill improvement

A central ability of the motor system is to achieve goals with great reliability, although never with zero variability. It is argued that variability is reduced with practice by 3 separate means: reduction of stochastic noise (N), exploitation of task tolerance (T), and covariation (C) between central variables. A method is presented that decomposes variability into these components in relation to task space that is defined by the execution variables. Successful variable combinations form the solution manifold. In a virtual skittles task, it is demonstrated that participants' improvement over repetitions, indicated by increasing accuracy, is accounted for by N, T, and, to a lesser degree, C. The relative contribution of these components changes over the course of practice and task variations.

Examining movement variability in the basketball free-throw action at different skill levels

The analysis of variability both within and between performers can reveal important information about how athletes satisfy situational constraints. Transitory changes in the basketball free-throw shot were examined across different stages in skill development. Six female basketball players were selected, representing a range of playing expertise (pretest: 0-90% baskets scored). Each participant was video recorded performing 30 shots. Contrary to predictions, there was not a clear pattern of a reduction in trajectory variability with increasing skill level. However, improvements in skill level were associated with an increasing amount of intertrial movement consistency from the elbow and wrist joints. It is suggested that the angular motions of the elbow and wrist joints were compensated for each other toward the end of each throw to adapt to subtle changes in release parameters of the ball.

Dependence of finger flexion force on the posture of the nonperforming fingers during key pressing tasks

The influence of different positions of the nonperforming (idle) fingers on the maximal force contraction of flexion (master) fingers during key pressing tasks was investigated. Ten participants performed maximal voluntary flexion contractions with various combinations of the index, middle, ring, and little fingers while the idle fingers rested on or were lifted away from the supporting surface. The effect of idle finger posture on total finger force production of master fingers was dependent on finger combination. In general, force production by master fingers was higher when the idle fingers were lifted away from the supporting surface than when they rested on it. The average increase in total force production by master fingers caused by the lifting of idle fingers was +12.4% (from -8.3% to +30.2%). Force-production capability of individual master fingers can be facilitated (as high as 34.1%), unchanged, or depressed (as high as -29.0%) by lifting the idle fingers. The effect of idle finger posture on finger force production of master fingers led to changes in force deficit. Neural, anatomical, and mechanical factors might account for the dependence of finger flexion force of master fingers on the posture of the idle fingers.

Manual control and tracking--a human factor analysis relevant for beating heart surgery

Manual control and tracking are fundamental to human factors and define a metric framework which determines the limits of surgical precision. This review provides a brief analysis of factors that are relevant for targeted motions. Knowing and accepting the limitations of human performance may help to optimize performance in off-pump surgery.

Inter-digit co-ordination and object-digit interaction when holding an object with five digits

The current study investigated inter-digit co-ordination and object-digit interaction during sustained object holding tasks by using five, six-component force/torque sensors. The sum of the individual finger normal forces and the thumb normal force showed a parallel variation with a mean median correlation coefficient of 0.941. The normal force traces demonstrated the lowest coefficient of variation (about 9% as averaged across digits) as compared with other force/torque traces. The sum for the variances of the normal forces of the index, middle, ring, and little fingers was about 50% of the variance of the summed normal force of the four fingers. Of the five digits, the thumb, index, middle, ring and little fingers accounted for 50.0, 15.4, 14.6, 11.7 and 7.3% of the total normal force; and 39.4, 9.9, 19.3, 14.0 and 17.5% of the total vertical shear force (i.e. the load), respectively. The ratios of the normal force to the resultant shear force were 2.6, 4.5, 1.8, 2.2 and 1.3 for the thumb, index, middle, ring and little finger, respectively. The centre of pressure migration area of a single digit at the object-digit surface during object holding ranged from 0.30 to 1.21 mm(2). The current study reveals a number of detailed object-digit mechanics and multiple digits co-ordination principle. The results of this study may help to improve ergonomic designs that involve the usage of multiple digits.

Single motor unit activity in relation to pulsatile motor output in human finger movements

1. Forty-six single motor units in the common finger extensor, superficial finger flexor, and first dorsal interosseus muscles were recorded with intramuscular wire electrodes while subjects made voluntary flexion and extension finger movements at a single metacarpo-phalangeal joint. 2. Motor unit firing was analysed in relation to the 8-10 Hz discontinuities which previously have been shown to characterize these movements. Statistical time- and frequency-domain analyses of the activity of individual motor units in relation to the discontinuities showed that when the muscle was the agonist, all motor units in the common finger extensor muscle, and all units except one in the flexor muscles exhibited significant frequency modulation of their discharge in close temporal association with the joint acceleration. On the other hand, motor unit firing rate was not related to the frequency of the discontinuities. When the muscle recorded from was the antagonist, 21 of the 25 active units exhibited a similar frequency modulation. 3. When angular movement velocity was increased from 4 to 25 deg s-1, the strength of motor unit frequency modulation increased. Peak coherence between motor unit activity and acceleration increased by 74 %, on average, in the common finger extensor units. 4. The findings rule out a tentative mechanism attributing the discontinuities to newly recruited motor units firing at circa 8-10 Hz. Instead, a coherent 8-10 Hz input to the agonist and antagonist motoneurone pools is implied.

An analysis of human comfort when entering and exiting the rear seat of an automobile

This paper describes a study of human motion and human comfort when entering and exiting the rear seat of an automobile. A simulator was used to test several possible door frame configurations, and various positions of the front and rear seats. Thirty-six human subjects were asked to enter and exit the simulator five times for each configuration and to answer a subjective questionnaire. The motion performed by each test subject was recorded by means of a VHS recorder and an ELITE motion measurement system. A statistical analysis was performed on the data from the questionnaires and comfort rankings were produced for the various configurations. The most influential design parameters were identified and iso-comfort surfaces were defined and fitted which provided a simple means of quantifying the effect of one of the main parameters.

Velocity profiles of lip protrusion across changes in speaking rate

The effects of speaking rate manipulation were examined in the velocity profiles of anticipatory lip protrusion gestures. Systematic changes in the shape, symmetry, and smoothness of the velocity profiles were observed as speaking rate was modulated across a wide range of self-selected rates, from fast to slow. Velocity profiles of movements produced at slower than normal speaking rates demonstrated greater asymmetry, irregularity, and differences in geometric form, compared to a normal and faster-than-normal rates. Subjects evidenced both inter- and intrasubject variability in the accomplishment of lip protrusion and rate manipulations. These results indicate that the velocity profiles of lip protrusion gestures do not necessarily remain invariant across changes in speaking rate. Rather, the data suggest that distinct movement patterns may be generated for slow speaking rates, with select characteristics of the movement pattern being maintained across normal and fast speaking rates.

Dextrous hands: human, prosthetic, and robotic

The sensory and motor capacities of the human hand are reviewed in the context of providing a set of performance characteristics against which prosthetic and dextrous robot hands can be evaluated. The sensors involved in processing tactile, thermal, and proprioceptive (force and movement) information are described, together with details on their spatial densities, sensitivity, and resolution. The wealth of data on the human hand's sensory capacities is not matched by an equivalent database on motor performance. Attempts at quantifying manual dexterity have met with formidable technological difficulties due to the conditions under which many highly trained manual skills are performed. Limitations in technology have affected not only the quantifying of human manual performance but also the development of prosthetic and robotic hands. Most prosthetic hands in use at present are simple grasping devices, and imparting a "natural" sense of touch to these hands remains a challenge. Several dextrous robot hands exist as research tools and even though some of these systems can outperform their human counterparts in the motor domain, they are still very limited as sensory processing systems. It is in this latter area that information from studies of human grasping and processing of object information may make the greatest contribution.

Single-subject methodology: an alternative approach

The purpose of this paper is to identify and discuss an alternative experimental methodology, single-subject (SS) design. The primary premise for SS analyses forms the basis for many research questions in areas such as movement/motor control, individual performance patterns/strategies, and injury mechanisms. A brief historical perspective elucidating the evolution of modern-day group statistical techniques and the relationship to the individual is presented. Rationale for the SS design within this context is also discussed. Specific statistical applications include mean comparison tests (ANOVA, Model Statistics), correlation, and multiple regression. Validation of the underlying statistical assumptions of independence and normality relative to the applications are briefly discussed. Finally, several examples are included.

Coordination of index finger movements

The purpose of this investigation was to describe the patterns of coordination among the joint motions of the index finger, and among the EMGs of index finger muscles. Index finger movements involving all three joints were varied in speed and direction. Joint motions were recorded along with fine-wire EMG from all the muscles that insert into the index finger. We observed nearly linear relationships for angular position between the two interphalangeal (IP) joints, and between the metacarpophalangeal (MP) and proximal IP (PIP) joints regardless of movement, speed and direction. The activities of the extrinsic flexors were of similar magnitude and were highly correlated when they acted as agonists but were poorly correlated when they acted as antagonists to the movement. Extrinsic extensor muscles behaved in this way also. The activation patterns of the intrinsic musculature correlated weakly except for extension movements voluntarily limited to the IP joints. We conclude that the highly coordinated action of the extrinsic flexors during flexion contribute importantly to the linked motions of the IP joints in part because these muscles span two or all the three index finger joints. Hence, interjoint movement patterns appear not to arise solely from restraints imposed by passive tissues, especially for fast flexion movements. The weakly correlated intrinsic muscle activity does not uncouple the flexion motions at the PIP and DIP joints because these muscles exert extensor torques at both IP joints. However, the actions of the intrinsic muscles are necessary for stabilizing the MP joint in flexion postures during IP motion and in producing motions voluntarily limited to the MP joint.

Determining Movement Onsets from Temporal Series

With the advent of recent measurement techniques, kinematic and kinetic measures commonly are used to describe events over time. Often, the central and peripheral nature of the control processes involved are derived from these temporal series. For example, movement onset often arbitrarily defines the end of the central and the beginning of the peripheral processes. Because of its critical temporal location, we examined whether response dynamics (average movement velocity) affects the determination of movement onset. Interactive graphics and numerical methods of determining movement onsets from temporal series were evaluated on various kinematic signals. Variations in the initial rate of change in a given signal significantly affected the determination of movement onset. Consequently, measurements of component latency must be regarded with caution. A cursory description of related problems elucidated in previous research is discussed, and procedures that can minimize these artifacts are suggested.

Principles for learning single-joint movements. I. Enhanced performance by practice

This study investigated changes in myoelectric and mechanical variables for movements made "as fast as possible" as a function of practice in the context of the dual-strategy hypothesis of motor control (Gottlieb et al. 1989b). Five male subjects made 1400 rapid elbow flexion movements in ten blocks of 20 trials over seven experimental sessions. Improved performance was defined as increased peak movement velocity, decreased peak velocity variability, increased acceleration and deceleration, a proportionately greater increase in peak deceleration than peak acceleration, and greater consistency in terminal location. The changes observed over experimental sessions were very similar to (but larger and more consistent than) those seen for the first experimental session, with the partial exception of the timing of the antagonist electromyogram (EMG). In general, the increases in the values of the measured mechanical variables covary with myoelectric measures in the same way as when subjects are asked to intentionally change speed in accordance with the rules of the speed-sensitive strategy (Corcos et al. 1989). However, there are differences between subjects in the extent to which speed changes can be attributable to the agonist muscle, the antagonist muscle, or in the timing between the muscles. In one of the five subjects, the latency of the antagonist EMG decreased over blocks on the 1st day but increased over experimental sessions and was consequently activated proportionately later in the movement. This suggests that extended practice can give at least some subjects flexibility in modifying the motor programs that underlie movement.

Transformations between visual and kinesthetic coordinate systems in reaches to remembered object locations and orientations

The abilities of human subjects to perform reach and grasp movements to remembered locations/orientations of a cylindrical object were studied under four conditions: (1) visual presentation of the object-reach with vision allowed; (2) visual presentation-reach while blindfolded; (3) kinesthetic presentation of the object-reach while blindfolded and (4) kinesthetic presentation-reach with vision. The results showed that subjects were very accurate in locating the object in the purely kinesthetic condition and that directional errors were low in all four conditions; but, predictable errors in reach distance occurred in conditions 1,2, and 4. The pattern of these distance errors was similar to that identified in previous research using a pointing task to a small target (i.e., overshoots of close targets, undershoots of far targets). The observation that the pattern of distance errors in condition 4 was similar to that of conditions 1 and 2 suggests that subjects transform kinesthetically defined hand locations into a visual coordinate system when vision is available during upper limb motion to a remembered kinesthetic target. The differences in orientation of the upper limb between target and reach positions in condition 3 were similar in magnitude to the errors associated with kinesthetic perceptions of arm and hand orientations in three-dimensional space reported in previous studies. However, fingertip location was specified with greater accuracy than the orientation of upper limb segments. This was apparently accomplished by compensation of variations in shoulder (arm) angles with oppositely directed variations in elbow joint angles. Subjects were also able to transform visually perceived object orientation into an appropriate hand orientation for grasp, as indicated by the relation between hand roll angle and object orientation (elevation angle). The implications of these results for control of upper limb motion to external targets are discussed.

Sensory-motor coordination during grasping and manipulative actions

Goal-directed grasping and manipulation of objects are human skills that depend on automatic sensory control in which predictive feed-forward mechanisms integrate somatosensory and visual signals with sensory-motor memory systems. Memory representations of physical and task-relevant properties of the object play a pivotal role. Anticipatory strategies are crucial when purposeful actions arise from learned relationships between afferent patterns and efferent commands. The development of even elementary precision grip skills is a protracted process not concluded until early adolescence. Not surprisingly, the neural control of manual actions engages most central nervous system areas known to be involved in motor control.