Automatic control of limb movement and posture

Body orientation and control of coordinated movements in microgravity

The present paper focuses on the organization of posture and movement under normal and microgravity conditions. Two reference values subserving the control of erect posture and the performance of movements are analyzed. The first is 'geometrical' in nature and corresponds to the orientation of a body segment with respect to the external world. The second reference value, which involves the mass and inertia of the body segments, is the position of the centre of mass with respect to the foot support area. The reorganization of these parameters which occurs under microgravity is discussed in the framework of a hierarchical model of posture. Suggestions are made for training procedures which could be used to prevent loss of balance from occurring in astronauts on landing after long space flights.

Mechanisms for force adjustments to unpredictable frictional changes at individual digits during two-fingered manipulation

Previous studies on adaptation of fingertip forces to local friction at individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory control based on the friction in previous trials. Here we instead analyze mechanisms underlying this adaptation after unpredictable changes in local friction between consecutive trials. With the tips of the right index and middle fingers or the right and left index fingers, subjects restrained a manipulandum whose horizontal contact surfaces were located side by side. At unpredictable moments a tangential force was applied to the contact surfaces in the distal direction at 16 N/s to a plateau at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential forces had to counterbalance the imposed load. The contact surface of the right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force responses at both fingers. When a finger contacted rayon, subjects allowed slips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning of the load force between the digits that resulted in an adequate adjustment of the normal:tangential force ratios to the local friction at each digit. This mechanism required a fine control of the normal forces. Although the normal force at the more slippery surface had to be comparatively low to allow slippage, the normal forces applied by the nonslipping digit at the same time had to be high enough to prevent loss of the manipulandum. The frictional changes influenced the normal forces applied before the load ramp as well as the size of the triggered normal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces increased at both fingers. Thus to independently adapt fingertip forces to the local friction the normal forces were controlled at an interdigital level by using sensory information from both engaged digits. Furthermore, subjects used both short- and long-term anticipatory mechanisms in a manner consistent with the notion that the central nervous system (CNS) entertains internal models of relevant object and task properties during manipulation.

Postural control systems in developmental perspective

How can the adult postural organisation be elucidated using an ontogenetic approach, and what questions can be raised about ontogenesis starting from the organisation of adult posture? These questions will be addressed taking three aspects of postural organisation. The first is the internal representation of erect posture, including the role played by the various sensory inputs in this representation. The second aspect relates to the variables which are controlled during erect posture: is it the body orientation with respect to the vertical or the localisation of the centre of gravity with respect to the feet which is controlled? The third aspect concerns the coordination between posture, equilibrium and movement, focusing on the role played by an internal representation of the external world and its interactions with the body segments in organising the anticipatory postural adjustments. The central organisation of coordinated control will also be considered. Each of these aspects will be discussed in relation to ontogenetic considerations.

Kinematic coordination in human gait: relation to mechanical energy cost

Twenty-four subjects walked at different, freely chosen speeds (V) ranging from 0.4 to 2.6 m s-1, while the motion and the ground reaction forces were recorded in three-dimensional space. We considered the time course of the changes of the angles of elevation of the trunk, pelvis, thigh, shank, and foot in the sagittal plane. These angles specify the orientation of each segment with respect to the vertical and to the direction of forward progression. The changes of the trunk and pelvis angles are of limited amplitude and reflect the dynamics of both right and left lower limbs. The changes of the thigh, shank, and foot elevation are ample, and they are coupled tightly among each other. When these angles are plotted one versus the others, they describe regular loops constrained on a plane. The plane of angular covariation rotates, slightly but systematically, along the long axis of the gait loop with increasing V. The rotation, quantified by the change of the direction cosine of the normal to the plane with the thigh axis (u3t), is related to a progressive phase shift between the foot elevation and the shank elevation with increasing V. As a next step in the analysis, we computed the mass-specific mean absolute power (Pu) to obtain a global estimate of the rate at which mechanical work is performed during the gait cycle. When plotted on logarithmic coordinates, Pu increases linearly with V. The slope of this relationship varies considerably across subjects, spanning a threefold range. We found that, at any given V > 1 m s-1, the value of the plane orientation (u3t) is correlated with the corresponding value of the net mechanical power (Pu). On the average, the progressive rotation of the plane with increasing V is associated with a reduction of the increment of Pu that would occur if u3t remained constant at the value characteristic of low V. The specific orientation of the plane at any given speed is not the same in all subjects, but there is an orderly shift of the plane orientation that correlates with the net power expended by each subject. In general, smaller values of u3t tend to be associated with smaller values of Pu and vice versa. We conclude that the parametric tuning of the plane of angular covariation is a reliable predictor of the mechanical energy expenditure of each subject and could be used by the nervous system for limiting the overall energy expenditure.

New quantitative and qualitative measures on functional mobility prediction for stroke patients

The purpose of this study was to explore whether we could provide supportive laboratory evidence for clinical observations that a stroke patient has lost functional mobility/locomotion capability based on dynamic balance responses (centre of pressure, COP sway patterns) and motor control activities (EMG patterns) during the motor task of sit-to-stand. A computerized controlled dynamic postural control assessment system was developed and used in this study. Various dynamic balance indices were introduced and derived from COP sway patterns expressed in four domains (i.e. space, time, force, and frequency). Motor control was assessed by multi-channel surface electromyography of each side of the lower limb during the same motor task. The functional mobility capability was evaluated using a traditional FIM method. Fourteen stroke patients with right hemiplegia and nine healthy elderly were recruited as the experimental and control groups respectively. Muscle activity was recorded for quadriceps, hamstrings, anterior tibialis, and triceps surae muscles and used for analysis. Centre of pressure sway patterns and ground reaction forces were registered. All signals were synchronized at 'seat-off'. Surface electromyographic patterns of activities recorded during sit-to-stand and dynamic balance indices computed from centre of pressure sway patterns were categorized and compared with the functional mobility scores. The results show that both the motor control patterns and dynamic balance indices correlated well to the extent of mobility impairment evaluated using the traditional FIM method. An important conclusion for rehabilitation medicine is that the functional mobility capability of stroke patients may be quantified analytically using dynamic balance indices and visualized graphically through EMG motor patterns.

Comparison of balance responses and motor patterns during sit-to-stand task with functional mobility in stroke patients

This study was undertaken to explore whether we could provide supportive laboratory evidence for the clinical observations that a stroke patient has lost functional mobility/locomotion capability based on dynamic balance responses (center of force sway patterns) and motor control activities (electromyography patterns) during the motor task of sit-to-stand. A computerized controlled dynamic postural control assessment system was developed and used in this study. Various dynamic balance indexes were introduced and derived from center of force sway patterns expressed in four domains (space, time, force, and frequency). Motor control was assessed by multichannel surface electromyography of each side of the lower limb during the same motor task. The functional mobility capability was evaluated using the traditional FIM method. Fourteen stroke patients with right hemiplegia and nine healthy elderly individuals were recruited as the experimental and control groups, respectively. Muscle activity was recorded for quadriceps, hamstrings, anterior tibialis, and triceps surae muscles and was used for analysis. Center of force sway patterns and ground reaction forces were registered. All signals were synchronized at "seat-off." Surface electromyographic patterns of activities recorded during sit-to-stand and dynamic balance indexes computed from center of force sway patterns were categorized and compared with the functional mobility scores. Results show that both the motor control patterns and dynamic balance indexes correlated well to the extent of mobility impairment evaluated using the traditional FIM method. An important conclusion for rehabilitation medicine is that the functional mobility capability of stroke patients may be expressed numerically using dynamic balance indexes and visualized graphically through electromyographic motor patterns.

Nondigital afferent input in reactive control of fingertip forces during precision grip

Sensory inputs from the digits are important in initiating and scaling automatic reactive grip responses that help prevent frictional slips when grasped objects are subjected to destabilizing load forces. In the present study we analyzed the contribution to grip-force control from mechanoreceptors located proximal to the digits when subjects held a small manipulandum between the tips of the thumb and index finger. Loads of various controlled amplitudes and rates were delivered tangential to the grip surfaces at unpredictable times. Grip forces (normal to the grip surfaces) and the position of the manipulandum were recorded. In addition, movements of hand and arm segments were assessed by recording the position of markers placed at critical points. Subjects performed test series during normal digital sensibility and during local anesthesia of the index finger and thumb. To grade the size of movements of tissues proximal to the digits caused by the loadings, three different conditions of arm and hand support were used; (1) in the hand-support condition the subjects used the three ulnar fingers to grasp a vertical dowel support and the forearm was supported in a vacuum cast; (2) in the forearm-support condition only the forearm was supported; finally, (3) in the no-support condition the arm was free. With normal digital sensibility the size of the movements proximal to the digits had small effects on the grip-force control. In contrast, the grip control was markedly influenced by the extent of such movements during digital anesthesia. The poorest control was observed in the hand-support condition, allowing essentially only digital movements. The grip responses were either absent or attenuated, with greatly prolonged onset latencies. In the forearm and no-support conditions, when marked wrist movements took place, both the frequency and the strength of grip-force responses were higher, and the grip response latencies were shorter. However, the performance never approached normal. It is concluded that sensory inputs from the digits are dominant in reactive grip control. However, nondigital sensory input may be used for some grip control during impaired digital sensibility. Furthermore, the quality of the control during impaired sensibility depends on the extent of movements evoked by the load in the distal, unanesthetized parts of the arm. The origin of these useful sensory signals is discussed.

What can we expect from models of motor control?

The lambda model of servocontrol seems superior to the alpha model in terms of dealing with the mechanical complexities of nonlinear and multiarticular muscles. Both, however, can be trivialized by noting that the “control variable” may simply be the sum of descending influences at propriospinal interneurons in the case of the lambda model or in the muscles themselves in the case of the alpha model. The notion that the brain explicitly computes output in terms of any such control variables may be an engineering metaphor, useful for conceptual understanding but not for generating predictive hypotheses about higher motor circuitry.

Moving models of motion forward: Explication and a new concept

We affirm the dynamical systems approach taken by Feldman and Levin, but argue that a more mathematically rigorous and standard exposition of the model according to dynamical systems theory would greatly increase readability and testability. Such an explication would also have heuristic value, suggesting new variations of the model. We present one such variant, a new solution to the redundancy problem.

Can the λ model benefit from understanding human adaptation in weightlessness(and vice versa)?

Parameters of the lambda model seem tightly linked to certain characteristics of human performance influenced by weightlessness. This commentary suggests that there is a valuable opportunity to probe the lambda model using the changed environment experienced during space flight. The likely benefits are a better model and a better understanding ofthe consequences of weightlessness for human performance.

Twisted pairs: Does the motor system really care about joint configurations?

Extrapersonal frames of reference for aimed movements are representationally convenient. They may, however, carry associated costs when the movement is executed in terms of the complex coordination of multiple joints they require. Studies that have measured both fingertip and joint paths suggest the motor systems may seek a compromise between simplicity of extrapersonal spatial representation and computational simplicity of multi-joint execution.

Is the multi-joint pointing movement model applicable to equilibrium control during upper trunk movements?

Two aspects of the target article, (1) the extension of the equilibrium point theory to multi-joint movements, and (2) the consequence that the EMG pattern is not directly controlled by the central nervous system (CNS), are discussed in light of the experiments on upper trunk bending in humans. The principle component kinematic analysis and the analysis of the EMG data, obtained under microgravity and additional loading conditions, support the application of Feldman and Levin's for multi-joint pointing movement to equilibrium control during upper trunk movement.

The lambda model is only one piece in the motor control puzzle

The lambda model provides a physiologically grounded terminology for describing muscle function and emphasizes the important influence of environmental and reflex-mediated effects on final states. However, lambda itself is only a convenient point on the length-tension curve; its importance should not be overemphasized. Ascribing movement to changes in a lambda-based frame of reference is generally valid, but it leaves unanswered a number of questions concerning mechanisms.

Do control variables exist?

We argue that the concept of a control variable (CV) as described by Feldman and Levin needs to be revised because it does not account for the influence of sensory feedback from the periphery. We provide evidence from the realm of rhythmic movements that sensory feedback can permanently alter the frequency and phase of a centrally generated rhythm.

The λ model: Can it walk?

Generation of swing phase limb trajectory over obstacles during locomotion should be a reasonable test for the λ model proposed by Feldman and Levin. The observed features such as lack of simple amplitude scaling of endpoint (toe) trajectories for different obstacle heights, complex shaped toe velocity profiles, and exploitation of passive intersegmental dynamics to control limb elevation cannot be adequately explained by the λ model.

Position is everything?

Neurophysiological evidence consonant with F&L's lambda model is reviewed and results of additional experiments are presented. The evidence shows that there are neurons in the motor cortex that respond to selective band widths of passive sinusoidal movements; the additional data show how, with movement, directionally sensitive population vectors can be shown to emerge from the data.

Equifinality and phase-resetting: The role of control parameter manipulations

It is argued that the equilibrium point model can lead to new insights regarding transition and stability processes in movement coordination. The role of movement control parameters on equifinality and phase-resetting is discussed; not only control but also external control parameters can affect the global dynamical regime.

The lambda model and a hemispheric motor model of intentional hand movements

The lambda model of Feldman & Levin for intentional hand movement is compared with a hemispheric motor model (IIMM). Both models imply similar conclusions independently. This increases the validity of both models.

Can the λ model be used to interpret the activity of single neurons?

Whereas the λ model provides a useful technique to describe complex movements, the focus on control variables in this model limits its potential for interpreting the activity and function of many cells in motor areas of the CNS.

The case of the missing CVs: Multi-joint primitives

The search for simplifying principles in motor control motivates the target article. One method that the CNS uses to simplify the task of controlling a limb's mechanical properties is absent from the article. Evidence from multi-joint, force-field measurements and from kinematics that points to the existence of multi-joint primitives as control variables is discussed.

Postural control system

The postural control system has two main functions: first, to build up posture against gravity and ensure that balance is maintained; and second, to fix the orientation and position of the segments that serve as a reference frame for perception and action with respect to the external world. This dual function of postural control is based on four components: reference values, such as orientation of body segments and position of the center of gravity (an internal representation of the body or postural body scheme); multisensory inputs regulating orientation and stabilization of body segments; and flexible postural reactions or anticipations for balance recovery after disturbance, or postural stabilization during voluntary movement. The recent data related to the organization of this system will be discussed in normal subjects (during ontogenesis), the elderly and in patients with relevant deficits.

The coactivation command for antagonist muscles involving Ib interneurons in mammalian motor control systems: an electrophysiologically testable model

The hypothesis of a weighted combination of independent reciprocal (R) and co-activation (C) commands to agonist and antagonist motoneurons (MNs) underlying movement is considered. In contrast to the R command, C command does not influence the equilibrium position of the joint. This constraint together with experimental data on descending and segmental afferent pathways to MNs forms the basis of the neuronal model for the C command. In the model, descending systems issue identical signals to agonist and antagonist MNs. To prevent shifts in the equilibrium position, these signals are adjusted by interneurons, in proprioceptive pathways compensating the asymmetry of muscle action.