Reproducibility and variability of the cost functions reconstructed from experimental recordings in multifinger prehension

The study examines whether the cost functions reconstructed from experimental recordings are reproducible over time. Participants repeated the trials on three days. By following Analytical Inverse Optimization procedures, the cost functions of finger forces were reconstructed for each day. The cost functions were found to be reproducible over time: application of a cost function C(i) to the data of Day j (i≠j) resulted in smaller deviations from the experimental observations than using other commonly used cost functions. Other findings are: (a) the 2nd order coefficients of the cost function showed negative linear relations with finger force magnitudes; (b) the finger forces were distributed on a 2-dimensional plane in the 4-dimensional finger force space for all subjects and all testing sessions; (c) the data agreed well with the principle of superposition, i.e. the action of object prehension can be decoupled into the control of rotational equilibrium and slipping prevention.     

Directional variability of the isometric force vector produced by the human hand in multijoint planar tasks

Numerous studies have examined control of force magnitude, but relatively little research has considered force direction control. The subjects applied isometric forces to a handle and the authors compared within-trial variability when force is produced in different directions. The standard deviation of the force parallel to the prescribed direction of force production increased linearly with the targeted force level, as did the standard deviation of the force perpendicular to the instructed direction. In contrast, the standard deviation of the angle of force production decreased with increased force level. In the 4 (of 8) instructed force directions where the endpoint force was generated due to a joint torque in only 1 joint (either the shoulder or elbow) the principal component axes in force space were well aligned with the prescribed direction of force production. In the other directions, the variance was approximately equal along the 2 force axes. The variance explained by the first principal component was significantly larger in torque space compared to the force space, and mostly corresponded to positive correlation between the joint torques. Such coordinated changes suggest that the torque variability was mainly due to the variability of the common drive to the muscles serving 2 joints, although this statement needs to be supported by direct studies of muscle activation in the future.     

Finger inter-dependence: linking the kinetic and kinematic variables

We studied the dependence between voluntary motion of a finger and pressing forces produced by the tips of other fingers of the hand. Participants moved one of the fingers (task finger) of the right hand trying to follow a cyclic, ramp-like flexion-extension template at different frequencies. The other fingers (slave fingers) were restricted from moving; their flexion forces were recorded and analyzed. Index finger motion caused the smallest force production by the slave fingers. Larger forces were produced by the neighbors of the task finger; these forces showed strong modulation over the range of motion of the task finger. The enslaved forces were higher during the flexion phase of the movement cycle as compared to the extension phase. The index of enslaving expressed in N/rad was higher when the task finger moved through the more flexed postures. The dependence of enslaving on both range and direction of task finger motion poses problems for methods of analysis of finger coordination based on an assumption of universal matrices of finger interdependence.     

Effects of joint immobilization on standing balance

We investigated the effect of joint immobilization on the postural sway during quiet standing. We hypothesized that the center of pressure (COP), rambling, and trembling trajectories would be affected by joint immobilization. Ten young adults stood on a force plate during 60 s without and with immobilized joints (only knees constrained, CK; knees and hips, CH; and knees, hips, and trunk, CT), with their eyes open (OE) or closed (CE). The root mean square deviation (RMS, the standard deviation from the mean) and mean speed of COP, rambling, and trembling trajectories in the anterior–posterior and medial–lateral directions were analyzed. Similar effects of vision were observed for both directions: larger amplitudes for all variables were observed in the CE condition. In the anterior–posterior direction, postural sway increased only when the knees, hips, and trunk were immobilized. For the medial–lateral direction, the RMS and the mean speed of the COP, rambling, and trembling displacements decreased after immobilization of knees and hips and knees, hips, and trunk. These findings indicate that the single inverted pendulum model is unable to completely explain the processes involved in the control of the quiet upright stance in the anterior–posterior and medial–lateral directions.     

Interaction of afferent and efferent signals underlying joint position sense: empirical and theoretical approaches

A model of joint position sense is considered based on the concept that central motor commands are adequately expressed in terms of shifts of the so-called invariant length-tension characteristics for agonist and antagonist muscles. The main points of this concept are discussed to clarify them and to prevent misunderstanding. The basic idea of the model is that the efferent copy of the central motor commands plays the role of a reference frame for evaluation of afferent proprioceptive discharges. An experiment with reproduction of constrained movements was performed in order to investigate the influence of voluntary flexor or extensor muscle tension on position sense in the elbow joint. The results demonstrate adequate perception of joint position on the background of voluntary muscle tension and thus are quite consistent with the model. The role of afferent and efferent signals in position sense is discussed with special reference to views expressed recently by different authors. The interrelation between position sense and sense of effort is considered, based on a concept of senso-motor space.     

Stability control of grasping objects with different locations of center of mass and rotational inertia

The objective of this study was to observe how the digits of the hand adjust to varying location of the center of mass (CoM) above or below the grasp and rotational inertia (RI) of a handheld object. Such manipulations do not immediately affect the equilibrium equations while stability control is affected. Participants were instructed to hold a handle, instrumented with 5 force-torque transducers and a 3-D rotational tilt sensor, while either the location of the CoM or the RI values were adjusted. On the whole, people use 2 mechanisms to adjust to the changed stability requirements; they increase the grip force and redistribute the total moment between the normal and tangential forces offsetting internal torques. The increase in grip force, an internal force, and offsetting internal torques allows for increases in joint and hand rotational apparent stiffness while not creating external forces-torques that would unbalance the equations of equilibrium.     

A New Book by N. A. Bernstein: “On Dexterity and Its Development”

A book, ‘On Dexterity and its Development,’ written by N. A. Bernstein in the 1940s has recently been published in Moscow. We describe briefly the amazing fate of this miraculously saved book and of its famous author. The book is unique in its reader-friendly handling of the most complicated issues of motor control, motor development, motor skill acquisition, and more general aspects of brain activity. It presents the basis for what is now called motor control and for some of the more fundamental issues of theoretical neuroscience. Bernstein described his views on multilevel control of movements, the role of sensory feedback, preprogramming, feedback and feedforward components in motor control, the role of exercise, and other subjects. Despite the fact that the book was written nearly half a century ago, its deep philosophical content remains beyond any competition.     

Organization of drinking: postural characteristics of arm-head coordination

Postural characteristics of components of a natural movement-taking a sip from a mug filled with water-were studied. The authors hypothesized that important postural characteristics of each of the components involved in natural drinking can be described with a single or a few significant parameters. Seated participants (N = 7) took a small sip from a mug (selected from a set of 4) placed in front of them on the table. Movement components selected for analysis were transporting the mug, tilting the mug, moving and tilting the head, and opening the mouth. The ratio of mug diameter to the distance from the level of water to the rim of the mug was introduced as an index of difficulty (ID). ID showed significant logarithmic relationships to a number of parameters that described different components of the movement, including movement time (MT). Significant logarithmic relationships between ID and MT were observed during movements performed under visual control and with eyes closed. The authors tentatively suggest that those relationships are consequences of Fitts' law and that Fitts' law be expanded to tasks that are characterized by permissible error margin, not necessarily related to the size of a spatial target, and that are not constrained by the typical instruction "as fast and accurate as possible". A possible interpretation of the present observations is that the task constraints imposed on a number of movement parameters during the task of drinking can be described with a single parameter.     

Stabilization of the total force in multi-finger pressing tasks studied with the 'inverse piano' technique

When one finger changes its force, other fingers of the hand can show unintended force changes in the same direction (enslaving) and in the opposite direction (error compensation). We tested a hypothesis that externally imposed changes in finger force predominantly lead to error compensation effects in other fingers thus stabilizing the total force. A novel device, the “inverse piano”, was used to impose controlled displacements to one of the fingers over different magnitudes and at different rates. Subjects (n = 10) pressed with four fingers at a constant force level and then one of the fingers was unexpectedly raised. The subjects were instructed not to interfere with possible changes in the finger forces. Raising a finger caused an increase in its force and a drop in the force of the other three fingers. Overall, total force showed a small increase. Larger force drops were seen in neighbors of the raised finger (proximity effect). The results showed that multi-finger force stabilizing synergies dominate during involuntary reactions to externally imposed finger force changes. Within the referent configuration hypothesis, the data suggest that the instruction “not to interfere” leads to adjustments of the referent coordinates of all the individual fingers.     

A new book by N. A. Bernstein: "On dexterity and its development"

A book, "On Dexterity and Its Development," written by N. A. Bernstein in the 1940s has recently been published in Moscow. We describe briefly the amazing fate of this miraculously saved book and of its famous author. The book is unique in its reader-friendly handling of the most complicated issues of motor control, motor development, motor skill acquisition, and more general aspects of brain activity. It presents the basis for what is now called motor control and for some of the more fundamental issues of theoretical neuroscience. Bernstein described his views on multilevel control of movements, the role of sensory feedback, preprogramming, feedback and feedforward components in motor control, the role of exercise, and other subjects. Despite the fact that the book was written nearly half a century ago, its deep philosophical content remains beyond any competition.