Vladimir M Zatsiorsky

Pennsylvania State University | Penn State · Department of Kinesiology

The use of the wavelet transformation for analyzing ground reaction force (GRF) data is proposed and demonstrated. The method permits: (a) time localization of the wavelet coefficients which represent the contributions of varying frequency, e.g. high frequency oscillations are mainly present immediately after foot contact with the ground; (b) recon...

Recollections on meetings with Dick Nelson in the 1970s, his interactions with Soviet authorities, his impact on data collection at Olympic Games, and his work as the President of the International Society of Biomechanics.

Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at var...

We address the nature of unintentional changes in performance in two papers. This second paper tested hypotheses related to stability of task-specific performance variables estimated using the framework of the uncontrolled manifold (UCM) hypothesis. Our first hypothesis was that selective stability of performance variables would be observed even wh...

We address the nature of unintentional changes in performance in two papers. This first paper tested a hypothesis that unintentional changes in performance variables during continuous tasks without visual feedback are due to two processes. First, there is a drift of the referent coordinate for the salient performance variable toward the actual coor...

Manipulating objects with the hands requires the accurate production of resultant forces including shear forces; effective control of these shear forces also requires the production of internal forces normal to the surface of the object(s) being manipulated. In the present study, we investigated multi-finger synergies stabilizing shear and normal c...

Synergy means “work together” in Greek. Hence, two requirements have to be met in order for this word to be applicable to an action. First, more than one element has to take part in any synergy. Second, the elements have to do something together, act towards a common goal. The concept of “‘synergy”’ is used in many fields of knowledge. We limit our...

Resistance to deformation may depend not only on its magnitude—as it was discussed in Chapter 2 —but also on the rate. In liquids and gels the latter resistance is labeled as viscosity . Viscosity arises due to internal friction. When neighboring particles are moving at different speeds, they experience friction forces. However, in muscles, velocit...

It has been known for centuries that movements with a rhythmic structure of kinetic and/or kinematic variables can be produced in the absence of a similarly structured input from the brain. For example, a chicken with the head chopped off runs for some time and flaps wings. These rhythmic movements are obviously produced by the spinal cord, possibl...

The words redundancy and abundance have many different meanings in various areas outside the fields of motor control and biomechanics. Before trying to define and analyze these terms in specific ways that would make them useful for analysis of biological movements, it makes sense to look at the meanings of these words in other areas and try to summ...

Reactions to unexpected stimuli at time delays (latencies) longer than those of typical reflexes (>40 ms) and shorter than those of the quickest simple reaction time (<100 ms) have been addressed using many names. These include long-loop reflexes, M2-3, functional stretch reflexes, transcortical reflexes, triggered reactions, and preprogrammed reac...

The problem of interaction between perception and action is one of the central ones in fields such as psychology, neurophysiology of perception, and motor control. This problem is bidirectional. Sensory signals play an important role in movements. The most obvious example is muscle reflexes (see Chapter 6) that originate from sensory endings and le...

People interact daily with various objects: workings tools, utensils, glasses with liquids, door handles, a computer mouse, and many others. Hand actions include three sequential parts: (1) selection of a specific grasp pattern—depending on the performance goal and the object shape, people grasp objects differently; (2) a reach-to-grasp movement; a...

Concept of joint torques—or joint moments as many prefer to call them—is one of the fundamental concepts in the biomechanics of human motion and motor control. In classical mechanics however the concept of joint torques (moments) is not defined and is not used. In this chapter the concept is defined and discussed in detail.

Humans and animals spend energy to move. Through diverse metabolic processes, the chemically bound energy is transformed into mechanical work and heat. Examination of the mechanical work and energy in human and animal movements is an important field of research. This research direction benefits from relying on a basic law of nature: the law of cons...

The equilibrium point (EP) hypothesis was introduced by Anatol Feldman about 50 years ago (Feldman, 1966). Its fate is very unusual. It has been the target of waves of criticism and attempts to disprove it, while only a handful of researchers have tried to work with this hypothesis and develop it. Despite this imbalance between the numbers of oppon...

“Stiffness” (of muscles, joints, body limbs, etc.) is one of the most broadly used terms in human biomechanics and motor control literature. Regrettably, the term is also frequently ill-used, that is, used incorrectly, without a precise understanding of its meaning. The origin of the confusion is in the application of the concept developed for rela...

The notion of “reflex” is arguably one of the oldest, most broadly used, and least precisely defined notions in the field of movement studies. PubMed (the site of the US National Library of Medicine) produced at the end of 2013 over 100,000 entries for “reflex,” and this number has been growing consistently by over 2500 per year. This notion is cen...

It is intuitively obvious that when we learn a novel movement (e.g., riding a bicycle), at first we approximate the desired motor action with suboptimal means, leading frequently to failure. With practice, we develop a way to perform the movement successfully, but the success requires constant attention and correction of the ongoing movement if it...

Studies of posture are very common. A quick search on PubMed yields over 3000 papers on the topic of vertical posture alone. While the notion of posture is intuitively appealing, there is no clear definition for this notion in the movement science literature. Postural control may refer to keeping a configuration of a hand (e.g., sign language is co...

We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely the fingertip refe...

We investigated multidigit synergies stabilizing components of the resultant force vector during joint performance of a static prehension task by two persons as compared to similar tasks performed by a single person using both hands. Subjects transferred the instrumented handle from the right hand to the left hand (one-person condition) or passed t...

The authors explored application of analytical inverse optimization (ANIO) method to the normal finger forces in unimanual and bimanual prehensile tasks with discrete and continuously changing constraints. The subjects held an instrumented handle vertically with one or two hands. The external torque and grip force changed across trials or within a...

We explored the origins of unintentional changes in performance during accurate force production in isometric conditions seen after turning visual feedback off. The idea of control with referent spatial coordinates suggests that these phenomena could result from drifts of the referent coordinate for the effector. Subjects performed accurate force/m...

We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed con...

We use an approach rooted in the recent theory of synergies to analyze possible co-variation between two hypothetical control variables involved in finger force production based in the equilibrium-point hypothesis. These control variables are the referent coordinate (R) and apparent stiffness (C) of the finger. We tested a hypothesis that inter-tri...

Biomechanics and Motor Control: Defining Central Concepts provides a thorough update to the rapidly evolving fields of biomechanics of human motion and motor control with research published in biology, psychology, physics, medicine, physical therapy, robotics, and engineering consistently breaking new ground. This book clarifies the meaning of the...

We used the framework of the referent configuration hypothesis to explore the possibility that accurate performance can result from inter-trial co-variation among control variables that show relatively high variability. In particular, we assumed that force production by a finger in isometric conditions results from controlling the fingertip referen...

Our main goal was to test a hypothesis that transient changes in performance of a steady-state task would result in motor equivalence. We also estimated effects of visual feedback on the amount of reorganization of motor elements. Healthy subjects performed two variations of a four-finger pressing task requiring accurate production of total force (...

We explored the phenomenon of unintentional changes in the apparent stiffness of the human arm produced by transient changes in the external force. The subjects performed a positional task against a constant baseline force and were instructed not to react to changes in the force. A HapticMaster robot produced a smooth force increase (a perturbation...

We investigated the ability of two persons to produce force-stabilizing synergies in accurate multi-finger force production tasks under visual feedback on the total force only. The subjects produced a time profile of total force (the sum of two hand forces in one-person tasks and the sum of two subject forces in two-person tasks) consisting of a ra...

This study used the framework of the referent configuration hypothesis and slow changes in the external conditions during vertical oscillation of a hand-held object to infer the characteristics of hypothetical control variables. The study had two main objectives: (1) to show that hypothetical control variables, namely, referent coordinates and appa...

The authors explored task-specific stability during accurate multifinger force production tasks with different numbers of instructed fingers. Subjects performed steady-state isometric force production tasks and were instructed not to interfere voluntarily with transient lifting-and-lowering perturbations applied to the index finger. The main result...

Previous reports show that the forces produced by the fingers of one hand drop exponentially over time in the absence of visual feedback on the forces. We study the force production by the index fingers of both hands with no visual feedback. Subjects produced a specified total force with a specific contribution from each finger by pressing on force...

We used the framework of the referent configuration (RC) hypothesis to reconstruct the hand and grip aperture referent trajectories during vertical oscillation of a hand-held object. Within the RC hypothesis, mechanical variables are consequences of shifts in referent values for salient body coordinates. For the studied task, this translates into t...

ABSTRACT The authors studied effects of practicing a 4-finger accurate force production task on multifinger coordination quantified within the uncontrolled manifold hypothesis. During practice, task instability was modified by changing visual feedback gain based on accuracy of performance. The authors also explored the retention of these effects, a...

We explored stability of multi-finger cyclical accurate force production action by analysis of responses to small perturbations applied to one of the fingers and inter-cycle analysis of variance. Healthy subjects performed two versions of the cyclical task, with and without an explicit target. The “inverse piano” apparatus was used to lift/lower a...

We investigated stability of action by a multi-finger system with three methods: analysis of inter-trial variance, application of transient perturbations, and analysis of the system's motion in different state spaces. The "inverse piano" device was used to apply transient (lifting-and-lowering) perturbations to individual fingers during single- and...

ABSTRACT The authors studied effects of healthy aging on 3 components of the internal force vector during static prehensile tasks. Young and older subjects held an instrumented handle using a 5-digit prismatic grasp under different digit configurations and external torques. Across digit configurations, older subjects showed larger internal normal (...

The extrinsic digit muscles naturally couple wrist action and grip force in prehensile tasks. We explored the effects of wrist position on the steady-state grip force and grip-force change during imposed changes in the grip aperture [apparent stiffness (AS)]. Subjects held an instrumented handle steady using a prismatic five-digit grip. The grip ap...

The effects of muscle fatigue on the stability of precision grasps are not well known. The purpose of the current study was to investigate the effects of exercise-induced fatigue of a digit on prehension synergies in a static precision grasp. One group of participants performed the fatiguing exercise using the thumb (group-thumb) and the second gro...

This study was motivated by the double action of extrinsic hand muscles that produce grip force and also contribute to wrist torque. We explored interactions between grip force and wrist torque in isometric force production tasks. In particular, we tested a hypothesis that an intentional change in one of the two kinetic variables would produce an u...

During single-finger force production, the non-instructed fingers unintentionally produce force (finger enslaving). In this study, enslaving effects were compared between the dominant and non-dominant hands. The test consisted of a series of maximum voluntary contractions with different finger combinations. Enslaving matrices were calculated by mea...

Mechanical properties of human digits may have significant implications for the hand function. We quantified several mechanical characteristics of individual digits in young and older adults. Digit tip friction was measured at several normal force values using a method of induced relative motion between the digit tip and the object surface. A modif...

During power grasp, the number of local force maxima reflects either the central nervous system's preferential use of particular hand regions, or anatomical constraints, or both. Previously, both bimodal and trimodal force maxima have been hypothesized for power grasp of a cylindrical handle. Here we measure the number of local force maxima, with a...

A hypothesis was proposed that the central nervous system controls force production by the fingers through hypothetical neural commands. The neural commands are scaled between values of 0 to 1, indicating no intentional force production or maximal voluntary contraction (MVC) force production, respectively. A matrix of interfinger connections transf...

We explored a hypothesis that transient perturbations applied to a redundant system result in equifinality in the space of task-related performance variables but not in the space of elemental variables. The subjects pressed with four fingers and produced an accurate constant total force level. The "inverse piano" device was used to lift and lower o...

Identical muscles (FDP – flexion, EDC – extension) contribute to both grip force at the fingertips and wrist movements. We investigate the relation between grip force and wrist kinematics to examine: (1) whether grip force scales with grip strength, which was expected to vary with wrist angle; and (2) whether grip force can be predicted from accele...

ABSTRACT The neural control of movement has been described using different sets of elemental variables. Two possible sets of elemental variables have been suggested for finger pressing tasks: the forces of individual fingers and the finger commands (also called finger modes or central commands). The authors analyzed which of the 2 sets of the eleme...

We investigated the effects of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of the right hand on performance in MVC and accurate cyclic force production tasks in right-handed young (Young group) and strength-matched elderly (Elderly group) participants. We hypothesized that before fatigue the Elderly group would...

The purpose of this study was to investigate the effects of changes in the proprioceptive signals induced by muscle vibration on multi-finger interaction and coordination. We hypothesized that unintended force production by non-instructed fingers (enslaving) would increase with muscle vibration while synergy indices during steady-state force produc...

Extrinsic digit muscles contribute to both fingertip forces and wrist movements (FDP and FPL-flexion, EDC-extension). Hence, it is expected that finger forces depend on the wrist movement and position. We investigated the relation between grip force and wrist kinematics to examine whether and how the force (1) scales with wrist flexion-extension (F...

We studied a mechanism of feed-forward control of a multi-finger action, namely anticipatory synergy adjustments (ASAs), prior to a quick force correction in response to a change in the gain of the visual feedback. Synergies were defined as co-varied across trials adjustments of commands to fingers that stabilized (decreased variance of) the total...

We studied the effects of a single practice session of a variable task with subject-specific adjustments of task difficulty (instability) on indices of multi-finger coordination in young and elderly persons. The main hypothesis was that practicing such a task would lead to contrasting changes in the amounts of two components of variance estimated a...

ABSTRACT The central hypothesis explored in the experiment is that adjustments of fingertip force vectors during object manipulation could result from a simple scaling rule applied to commands to individual digits. The commands have been associated with referent coordinates of the digit tips. The subjects performed quick lifting movements (over 20...

We investigated the effects of exercise-induced fatigue of a digit on the biomechanics of a static prehension task. The participants were divided into two groups. One group performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). We analyzed the prehensile actio...

This study joined two approaches to motor control. The first approach comes from cognitive psychology and is based on the idea that goal postures and movements are chosen to satisfy task-specific constraints. The second approach comes from the principle of motor abundance and is based on the idea that control of apparently redundant systems is asso...

The aim of this study was to test Bernstein’s idea that motor synergies provide solutions to the motor redundancy problem. Forces produced by individual fingers of one hand were recorded in one-, two-, three-, and four-finger tasks. The subjects (n=10) were asked to produce maximal total force (maximal voluntary contraction, MVC) and to match a ram...

During maximal voluntary contraction (MVC) with several fingers, the following three phenomena are observed: (1) the total force produced by all the involved fingers is shared among the fingers in a specific manner (sharing); (2) the force produced by a given finger in a multi-finger task is smaller than the force generated by this finger in a sing...

We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of forcestabilizing synergies i...

This is an exploratory study of the accurate endpoint force vector production by the human arm in isometric conditions. We formulated three common-sense hypotheses and falsified them in the experiment. The subjects (n = 10) exerted static forces on the handle in eight directions in a horizontal plane for 25 s. The forces were of 4 magnitude levels...

ABSTRACT The authors explored effects of practice of a 2-finger accurate force production task on components of finger force variance quantified within the uncontrolled manifold (UCM) hypothesis, V(UCM), that had no effect on total force and V(ORT) that affected total force. A variable task with graded instability was designed to encourage use of v...

We explored digit coordination during the acceleration phase of a quick lifting movement of a hand-held horizontal object. We tested three hypotheses related to: (1) the scaling of mechanical variables produced by the hand with changes in the external load, torque, and moment of inertia; (2) changes in the safety margin for the thumb with both the...

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 han...

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:...

When grasping and manipulating objects, the central controller utilizes the mechanical advantage of the normal forces of the fingers for torque production. Whether the same is valid for tangential forces is unknown. The main purpose of this study was to determine the patterns of finger tangential forces and the use of mechanical advantage as a cont...