Kinematic synergies for the control of hand shape.
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Biomechanical Phenomena↗
Biomedical subjects
M Santello
Publications and source records attributed to M Santello.
Visual and non-visual control of landing movements in humans.
1. The role of vision in controlling leg muscle activation in landing from a drop was investigated. Subjects (n = 8) performed 10 drops from four heights (0.2, 0.4, 0.6 and 0.8 m) with and without vision. Drop height was maintained constant throughout each block of trials to allow adaptation. The aim of the study was to assess the extent to which proprioceptive and vestibular information could substitute for the lack of vision in adapting landing movements to different heights. 2. At the final stages of the movement, subjects experienced similar peak centre of body mass (CM) displacements and joint rotations, regardless of the availability of vision. This implies that subjects were able to adapt the control of landing to different heights. The amplitude and timing of electromyographic signals from the leg muscles scaled to drop height in a similar fashion with and without vision. 3. However, variables measured throughout the execution of the movement indicated important differences. Without vision, landings were characterised by 10 % larger ground reaction forces, 10 % smaller knee joint rotations, different time lags between peak joint rotations, and more variable ground reaction forces and times to peak CM displacement. 4. We conclude that non-visual sensory information (a) could not fully compensate for the lack of continuous visual feedback and (b) this non-visual information was used to reorganise the motor output. These results suggest that vision is important for the very accurate timing of muscle activity onset and the kinematics of landing.
Adult↗
Force synergies for multifingered grasping.
Compared with the control of precision grips involving the thumb and one or two fingers, the control of grasping using the entire hand involves a larger number of degrees of freedom that has to be controlled simultaneously, and it introduces indeterminacies in the distribution of grip forces suitable for holding an object. We studied the control of five-digit grasping by measuring contact forces when subjects lifted, held, and replaced a manipulandum. This study focused primarily on the patterns of coordination among the normal forces exerted by each of the digits, assessed by varying the center of mass of the manipulandum. The force patterns during the lift and hold phases were modulated as a function of the location of the center of mass. A frequency domain analysis revealed a consistent temporal synergy by which digits tended to exert normal forces in phase with each other across all experimental conditions. This tendency for in-phase covariations by the normal forces exerted by the digits extended over the entire functional frequency range (up to 10 Hz). When the effect of thumb force was removed, a second synergy was revealed in which forces in two fingers could be modulated 180 degrees out of phase (also prevailing throughout the range of frequencies studied). The first synergy suggests the presence of a "common drive" to all of the extrinsic finger muscles, whereas the second one suggests another input, ultimately resulting in a reciprocally organized pattern of activity of some of these muscles.
Analysis of Variance↗
Postural hand synergies for tool use.
Subjects were asked to shape the right hand as if to grasp and use a large number of familiar objects. The chosen objects typically are held with a variety of grips, including "precision" and "power" grips. Static hand posture was measured by recording the angular position of 15 joint angles of the fingers and of the thumb. Although subjects adopted distinct hand shapes for the various objects, the joint angles of the digits did not vary independently. Principal components analysis showed that the first two components could account for >80% of the variance, implying a substantial reduction from the 15 degrees of freedom that were recorded. However, even though they were small, higher-order (more than three) principal components did not represent random variability but instead provided additional information about the object. These results suggest that the control of hand posture involves a few postural synergies, regulating the general shape of the hand, coupled with a finer control mechanism providing for small, subtle adjustments. Because the postural synergies did not coincide with grip taxonomies, the results suggest that hand posture may be regulated independently from the control of the contact forces that are used to grasp an object.
Adult↗
The control of timing and amplitude of EMG activity in landing movements in humans.
The control of self-initiated falls from different heights was studied. The objective of the study was to investigate in a quantitative manner the modulation of EMG timing (i.e. onset from take-off and duration from onset to touch-down) and amplitude (before and after foot contact) as a function of fall height. The muscles studied were m. soleus and m. tibialis anterior. Kinematic (ankle joint angle) and kinetic (ground reaction force) variables were also measured. Six subjects took part in the experiments that consisted of ten landings from each of five heights (0.2, 0.4, 0.6, 0.8 and 1 m) onto a force platform. We found a consistent pattern of co-contraction before and after touch-down across the fall heights studied. In both muscles, the onset of pre-landing EMG activity occurred at a longer latency following take-off when landing from greater heights. The absolute EMG duration was affected to a lesser extent by increasing fall height. These findings suggest that the onset of muscle activity of the muscles studied prior to foot contact is timed relative to the expected time of foot contact. Pre- and post-landing EMG amplitude tended to increase with height. Despite a doubling in the magnitude of ground reaction force, the amplitude of ankle joint rotation caused by the impact remained constant across heights. These findings suggest that the observed pattern of co-contraction is responsible for increasing ankle joint stiffness as fall height is increased. The attainment of an appropriate level of EMG amplitude seems to be controlled by (a) timing muscle activation at a latency timed from the expected instant of foot contact and (b) varying the rate at which EMG builds up.
Adult↗
Gradual molding of the hand to object contours.
Subjects were asked to reach to and to grasp 15 similarly sized objects with the four fingers opposed to the thumb. The objects' contours differed: some presented a concave surface to the fingers, others a flat one, and yet others a convex surface. Flexion/extension at the metacarpal-phalangeal and proximal interphalangeal joints of the fingers was recorded during the reaching movement. We used discriminant analysis, cluster analysis, and information theory to determine the extent to which the shape of the hand was affected by the objects' shapes along a convexity/concavity gradient. Maximum aperture of the hand was reached about midway in the reaching movement. At that time, the hand's posture was influenced by the shape of the object to be grasped but imperfectly. The information transmitted by hand posture about object shape increased gradually and monotonically as the hand approached the object, reaching a maximum at the time the object was in the grasp of the hand. We also asked subjects to shape the hand so as to grasp the object without moving the arm. Their performance was poorer on this task in the sense that hand shape discriminated among fewer objects and that trial-to-trial variability was greater than when the distal and proximal components of the motion were linked. The results indicate that the hand is molded only gradually to the contours of an object to be grasped. Because other parameters of the motion, such as movement direction, for example, already are specified fully early on in a movement, the results also suggest that the specification of diverse aspects of a movement does not evolve at a uniform rate.
Arm↗
Matching object size by controlling finger span and hand shape.
The ability of human subjects to accurately control finger span (distance between thumb and one finger) was studied. The experiments were performed without visual feedback of the hand and were designed to study the dependence of accuracy on object size, shape, distance, orientation and finger configuration. The effects of finger combination and sensory modality used to perceive object size (vision and haptics) were also studied. Subjects were quite proficient at this task; the small errors tended to be predominantly negative, i.e., finger span < object size. The thumb-little finger combination was less accurate than the other finger combinations, irrespective of the sensory modality used. Subjects made larger under-estimating errors when matching the size of cylinders than when matching cubes and parallelepipeds. No effect of viewing distance, object orientation and finger configuration was found. Accuracy in matching object size was not dependent on the sensory modality used. The question of how the individual degrees of freedom of the fingers and thumb contributed to the control of finger span was also addressed. Principal components analysis showed that two components could characterize the hand postures used, irrespective of object size. The amplitude of the first principal component was constant, and the amplitude of the second scaled linearly with object size. This finding suggests that all of the degrees of freedom of the hand are controlled as a unit. This result is discussed in relation to the 'virtual finger' hypothesis for grasping.
Adult↗