Postural adjustments for online corrections of arm movements in standing humans

The aim of this study was to investigate how humans correct ongoing arm movements while standing. Specifically, we sought to understand whether the postural adjustments in the legs required for online corrections of arm movements are predictive or rely on feedback from the moving limb. To answer this question we measured online corrections in arm and leg muscles during pointing movements while standing. Nine healthy right-handed subjects reached with their dominant arm to a visual target in front of them and aligned with their midline. In some trials, the position of the target would switch from the central target to one of the other targets located 15°, 30°, or 45° to the right of the central (midline) target. For each target correction, we measured the time at which arm kinematics, ground reaction forces, and arm and leg muscle electromyogram significantly changed in response to the target displacement. Results show that postural adjustments in the left leg preceded kinematic corrections in the limb. The corrective postural muscle activity in the left leg consistently preceded the corrective reaching muscle activity in the right arm. Our results demonstrate that corrections of arm movements in response to target displacement during stance are preceded by postural adjustments in the leg contralateral to the direction of target shift. Furthermore, postural adjustments preceded both the hand trajectory correction and the arm-muscle activity responsible for it, which suggests that the central nervous system does not depend on feedback from the moving arm to modify body posture during voluntary movement. Instead, postural adjustments lead the online correction in the arm the same way they lead the initiation of voluntary arm movements. This suggests that forward models for voluntary movements executed during stance incorporate commands for posture that are produced on the basis of the required task demands.     

Colour vision can contribute to fast corrections of arm movements

Can chromatic information be used for the fast on-line control of action? In order to find out we asked subjects to tap a red square as quickly as possible. In half of the trials the red square’s position changed as soon as the subject’s hand started to move. We examined how quickly subjects could adjust their movements to this change. In half of the trials there was also a green square of the same luminance as the red one. If there was a green square, and the red square’s position changed, the change consisted of the two squares exchanging places, so that all that really changed was the squares’ colours. In such cases subjects could not have adjusted their movements without having analysed the colour. Nevertheless, subjects could respond adequately within as little as 120 ms. This was even so if the squares’ luminances changed considerably at the moment that the subject’s hand started to move. Thus, chromatic information can be used for the fast on-line control of action     

Role of agonist and antagonist muscles in fast arm movements in man

Summary Fast goal-directed voluntary movements of the human upper extremity are known to be associated with three distinct bursts of EMG activity in antagonistic muscles. The role of each burst (AG1, ANT, AG2) in controlling motion is not fully understood, largely because overall limb response is a complex function of the entire sequence of bursts recorded during experimental trials. In order to investigate the role of each burst of muscle activity in controlling motion, we studied fast voluntary arm movements and also developed two simulation techniques, one employing a mathematical model of the limb and the other using electrical stimulation of human arm muscles. These techniques show that two important movement parameters (peak displacement, time to reach peak displacement) are non-linear functions of the magnitude of the antagonist input (torque and stimulation voltage, respectively, in our two simulations). In the fastest movements, the agonist muscle is primarily responsible for the distance moved, while the antagonist muscle provides an effective means of reducing movement time. The third component of the triphasic pattern moderates the antagonist braking forces and redirects the movement back to the target.     

Triggering of balance corrections and compensatory strategies in a patient with total leg proprioceptive loss

Triggering of balance corrections may depend on both leg and trunk proprioceptive inputs. To study this issue and to determine how a total proprioceptive loss in the legs (ToLPL) would affect postural reactions in different directions, we investigated the postural control of a patient with a long-standing dorsal root ganglionopathy. This patient had absent stretch reflexes at the ankle and knee joints, delayed reflexes at the hips, but normal muscle strength. Postural control was probed with support-surface movements driven by two different experimental protocols. The first protocol concentrated on leg muscle responses by varying ankle inputs during pitch plane perturbations. The second protocol focussed on the directional sensitivity of upper body responses using combined roll and pitch tilt perturbations. For both protocols, identical techniques were used to record ankle torques, angular velocities of the upper legs and trunk, and surface EMG from leg, hip and trunk muscles. For the first protocol, pitch plane stance perturbations with three different ankle inputs were imposed by a movable support surface. A simultaneous 4-cm rearward translation and 4-deg toe-up rotation produced an 80-deg/s "enhanced ankle input", a simple toe-up rotation gave a 40-deg/s "normal" ankle input and a simultaneous 4-cm rearward translation and 4-deg "toe-down" rotation yielding a 0-deg/s "nulled ankle input". Responses in the ToLPL patient were compared to those of healthy controls and those of patients with lower-leg proprioceptive loss (LLPL). Following normal and enhanced ankle input perturbations, stretch reflexes were absent in ankle and knee joint muscles of the ToLPL patient. Balance correcting responses in the lower legs were diminished and delayed by some 45 ms. In quadriceps, balance-correcting responses were larger than normal, peaked earlier and were not delayed. During the nulled ankle input condition, the ankle muscle responses in the ToLPL patient were again diminished and delayed by 40 ms with respect to both normal subjects and LLPL patients. However, the ToLPL patient again generated an earlier, larger, balance correcting response in quadriceps. For the second protocol, combinations of roll and pitch perturbations were also delivered by a moving support surface. The amplitude was 7.5 deg at 50 deg/s. Eight different directions were applied randomly (pure "toes down", pure "toes up" and directions at 45-deg intervals of roll). As with the first protocol pre-stimulus background muscle activity was excessive in all trunk and most leg muscles. Responses to roll tilt produced several striking changes from normal in the ToLPL patient. First reflexes in gluteus medius were delayed. Second, the trunk roll which commences around 30 ms in normals was in the opposite direction. This roll was accompanied by oppositely directed stretch reflexes in paraspinal muscles. Third, directional sensitivity of balance corrections was far more roll oriented in leg and trunk muscles. Fourth, some tilt directions caused a deactivation response of background activity. This "deactivation strategy" strongly contrasted with the strategy of controls who had low pre-stimulus background activity and activated responses around 100 ms to correct postural instability. These findings provide new insights into the generation of pitch and roll plane directed balance corrections based on the interaction of proprioceptive trigger signals from the ankles, knees and hips. Without proprioceptive input from the ankle and knee, ankle muscle responses are delayed but not absent. Upper leg and trunk responses are not delayed. This suggests that most, if not all, lower leg balance correcting responses are triggered by hip and, possibly, trunk proprioceptive inputs. When leg proprioceptive input is absent, balance correcting responses lose pitch plane sensitivity. The solution used by the patient to overcome these deficits was to markedly raise background muscle activity levels, presumably to provide a stiffer body structure. The lack of trunk flexibility and lateral instability this produced for roll tilts was offset by the ability to compensate by using a hitherto not described "deactivation response" strategy. The patient had a clinical picture usually described as "deafferented"; yet our roll tilt perturbations revealed delayed reflex responses in hip muscles. With vestibulospinal and neck-proprioceptive inputs, these responses may have helped with the development of compensation processes for the total leg proprioceptive deficit.     

The role of vision,speed, and attention in overcoming directional biases during arm movements

Previous research has revealed directional biases (preferences to select movements in specific directions) during horizontal arm movements with the use of a free-stroke drawing task. The biases were interpreted as a result of a tendency to generate motion at either the shoulder or elbow (leading joint) and move the other (subordinate) joint predominantly passively to avoid neural effort for control of interaction torque. Here, we examined influence of vision, movement speed, and attention on the directional biases. Participants performed the free-stroke drawing task, producing center-out strokes in randomly selected directions. Movements were performed with and without vision and at comfortable and fast pace. A secondary, cognitive task was used to distract attention. Preferred directions remained the same in all conditions. Bias strength mildly increased without vision, especially during fast movements. Striking increases in bias strength were caused by the secondary task, pointing to additional cognitive load associated with selection of movements in the non-preferred directions. Further analyses demonstrated that the tendency to minimize active interference with interaction torque at the subordinate joint matched directional biases in all conditions. This match supports the explanation of directional biases as a result of a tendency to minimize neural effort for interaction torque control. The cognitive load may enhance this tendency in two ways, directly, by reducing neural capacity for interaction torque control, and indirectly, by decreasing capacity of working memory that stores visited directions. The obtained results suggest strong directional biases during daily activities because natural arm movements usually subserve cognitive tasks.     

Armed against falls: the contribution of arm movements to balance recovery after tripping

Arm movements after perturbations like tripping over an obstacle have been suggested to be aspecific startle responses, serve a protective function or contribute to balance recovery. This study aimed at determining if and how arm movements play a functional role in balance recovery after a perturbation. We tripped young subjects using an obstacle that suddenly appeared from the floor at exactly mid-swing. We measured arm muscle EMG, quantified body rotations after tripping, and established the effects of arm movements by calculating how the body would have rotated without arms. Strong asymmetric shoulder muscle responses were observed within 100 ms after trip initiation. Significantly faster and larger responses were found in the contralateral arm abductors on the non-tripped (right) side. Mean amplitudes were larger in the ipsilateral retroflexors and contralateral anteflexors. The resulting asymmetric arm movements had a small effect on body rotation in the sagittal and frontal planes, but substantially affected the body orientation in the transverse plane. With the enlargement of the ongoing arm swing, the arms contributed to balance recovery by postponing the transfer of arm angular momentum to the trunk. This resulted in an axial rotation of the lower segments of the body towards the non-tripped side, which increases the length of the recovery step in the sagittal plane, and therefore facilitates braking the impending fall.     

Directional sensitivity of stretch reflexes and balance corrections for normal subjects in the roll and pitch planes

A large body of evidence has been collected which describes the response parameters associated with automatic balance corrections in man to perturbations in the pitch plane. However, perturbations to human stance can be expected from multiple directions. The purpose of the present study was to describe the directional sensitivities of muscle responses re-establishing disturbed stance equilibrium in normal subjects. The contributions of stretch reflex and automatic balance-correcting responses to balance control, and concomitant biomechanical reactions, were examined for combinations of pitch and roll perturbations of the support surface. More specifically, muscle responses, initial head accelerations and trunk velocities were analyzed with the intention of identifying possible origins of directionally specific triggering signals and to examine how sensory information is used to modulate triggered balance corrections with respect to direction. Fourteen healthy adults were required to stand on a dual-axis rotating platform capable of delivering rotational perturbations with constant amplitude (7.5°) and velocity (50°/s) through multiple directions in the pitch and roll planes. Each subject was randomly presented with 44 support surface rotations through 16 different directions separated by 22.5° first under eyes-open, and then, for a second identical set of rotations, under eyes-closed conditions. Bilateral muscle activities from tibialis anterior, soleus, lateral quadriceps and paraspinals were recorded, averaged across direction, and areas calculated over intervals with significant bursts of activity. Trunk angular velocity and ankle torque data were averaged over intervals corresponding to significant biomechanical events. Stretch reflex (intervals of 40–100, 80–120 ms) and automatic balance-correcting responses (120–220, 240–340 ms) in the same muscle were sensitive to distinctly different directions. The directions of the maximum amplitude of balance-correcting activity in leg muscles were oriented along the pitch plane, approximately 180° from the maximum amplitude of stretch responses. Ankle torques for almost all perturbation directions were also aligned along the pitch plane. Stretch reflexes in paraspinal muscles were tuned along the 45° plane but at 90° to automatic balance corrections and 180° to unloading responses in the same muscle. Stretch reflex onsets in paraspinal muscles were observed at 60 ms, as early as those of soleus muscles. In contrast, unloading reflexes in released paraspinal muscles were observed at 40 ms for perturbations which caused roll of the trunk towards the recorded muscle. Onsets of trunk roll velocities were earlier and more rapid than those observed for pitch velocities. Trunk pitch occurred for pure roll directions but not vice versa. When considered together, early stretch and unloading of paraspinals, and concomitant roll and pitch velocities of the trunk requiring a roll-and-pitch-based hip torque strategy, bring into question previous hypotheses of an ankle-based trigger signal or ankle-based movement strategies for postural balance reactions. These findings are compatible with the hypothesis that stretch-, force- and joint-related proprioceptive receptors at the level of the trunk provide a directionally sensitive triggering mechanism underlying a, minimally, two-stage (pitch-based leg and pitch-and-roll-based trunk) balance-correcting strategy. Accelerometer recordings from the head identified large vertical linear accelerations only for pitch movements and angular roll accelerations only during roll perturbations with latencies as early as 15 ms. Thus, it appears that balance corrections in leg and trunk muscles may receive strong, receptor-dependent (otolith or vertical canal) and directionally sensitive amplitude-modulating input from vestibulospinal signals. Received: 4 September 1998 / Accepted: 30 April 1999     

Representation of virtual arm movements in precuneus

Arm movements can easily be adapted to different biomechanical constraints. However, the cortical representation of the processing of visual input and its transformation into motor commands remains poorly understood. In a visuo-motor dissociation paradigm, subjects were presented with a 3-D computer-graphical representation of a human arm, presenting movements of the subjects' right arm either as right or left arm. In order to isolate possible effects of coordinate transformations, coordinate mirroring at the body midline was implemented independently. In each of the resulting four conditions, 10 normal, right-handed subjects performed three runs of circular movements, while being scanned with O(15)-Butanol-PET. Kinematic analysis included orientation and accuracy of a fitted ellipsoid trajectory. Imaging analysis was performed with SPM 99 with activations threshold at P < 0.0001 (not corrected). The shape of the trajectory was dependent on the laterality of the arm, irrespective of movement mirroring, and accompanied by a robust activation difference in the contralateral precuneus. Movement mirroring decreased movement accuracy, which was related to increased activation in the left insula. Those two movement conditions that cannot be observed in reality were related to an activation focus at the left middle temporal gyrus, but showed no influence on movement kinematics. These findings demonstrate the prominent role of the precuneus for mediating visuo-motor transformations and have implications for the use of mirror therapy and virtual reality techniques, especially avatars, such as Nintendo Wii in neurorehabilitation.     

Kinematics of arm movements in elderly humans

Changes in the kinematics of arm movements with age were studied. "Young" (aged 21-23) and "elderly" (aged 68-95) subjects performed a visually guided, step tracking task. Each subject performed movements of eight different amplitudes (10-80 deg) at two volitionally determined speeds ("own speed," "fast and accurate"). Movement durations and maximum velocities were similar for both groups and increased with movement amplitude under all conditions. The young subjects made movements in which approximately the same length of time was spent in accelerating as in decelerating the movement. In contrast, movements by the elderly subjects were temporally asymmetrical, the deceleration phase being longer than the acceleration phase. The ratio of maximum to average velocities (Vm/Vav) was markedly different between the young and elderly subjects, particularly in the smaller amplitude movements (less than 40 deg). Values of this parameter ranged from 1.5 to 6 in the elderly subjects. Movements made by the elderly subjects were more variable than those of the young subjects, particularly so at smaller amplitudes. Variability was most apparent in the deceleratory phase of movement where the elderly subjects often showed hypermetria. Some elderly subjects also showed movement decomposition, the movements being made as a series of discrete submovements. Thus, the elderly subjects displayed some of the characteristics classically described as arising from cerebellar dysfunction.     

A systematic directional error in 2-D arm movements increases with increasing delay between visual target presentation and movement execution

Forty-seven normal subjects performed two-dimensional arm movements on a digitizer board using a mouse device. The movements were projected on a computer monitor. Subjects were instructed to move the mouse using the whole arm from a center position to a peripheral target so that the projected movement would pass over the target without stopping on the target. A large number of targets (360) were used to cover the entire directional continuum. The direction of the arm movement was the parameter of interest, which was measured at an initial position, at one third of the distance towards the target, and at the vicinity of the target. Four conditions of delay between target presentation and movement execution were used (0, 2, 4, 6 s). A systematic directional error was observed at the initial portion of the trajectory. This error resulted from a clustering of movement directions on an axis that was perpendicular to the axis of the resting forearm before movement onset. This pattern of errors can be explained by the initial inertial anisotropy of the arm. As the trajectory evolved, a different directional error emerged, resulting from a clustering of movement directions in two orthogonal axes. This pattern of directional error increased in amplitude as the delay increased, in contrast to the error at the initial portion of the trajectory which remained invariant with increasing delay. Finally, the information transmitted by the movement direction was shown to increase with the evolution of the trajectory. The increase in delay resulted in a decrease in directional-information transmission. It is proposed that the directional bias towards the end of the movement trajectory might reflect the action of "movement primitives", that is patterns of muscle activation resulting from spinal interneuronal activation. It is further proposed that the directional bias observed at the vicinity of the target might reflect a loss of cortical directional information with increasing delay between target presentation and movement onset.     

Efficient control of arm movements in advanced age

The present study addresses the influence of aging on the ability to regulate mechanical effects arising during arm movements due to the multi-joint structure of the arm. Two mechanical factors were considered, interaction torque (IT) and inertial resistance (IR). Regulation of these two factors can be demanding in terms of the timing and magnitude of the required muscle torque (MT), specifically during fast movements. We hypothesized that aging exacerbates the challenge regarding the regulation of these effects with muscular control due to declines in the motor system. This hypothesis was tested by comparing performance of a cyclic line-drawing task in two age groups, young and older adults. Only two joints, the shoulder and elbow, participated in motion. Four orientations of the lines were used to provide variations in the requirements for regulation of IT and IR. Cyclic frequency was manipulated to emphasize the dependence of the mechanical factors on movement speed. Various characteristics of fingertip motion showed that there were no age-related deteriorations in accuracy of line drawing. However, older adults were systematically slower, particularly in the directions of high IR. A detailed analysis of the magnitude of MT and the contribution of this torque to production of net torque at each joint demonstrated that older adults modified joint control and decreased the demands for MT by skillful exploitation of IT in a way specific for each particular line orientation. The results point to a tendency in older adults to decrease the production of muscle force. Nevertheless, older adults also demonstrated an ability to partially compensate for declines in the force production by developing sophisticated strategies of joint control that exploit the multi-joint mechanical structure of the arm. This ability suggests that the internal representation of inter-segmental dynamics and the capability to use it for movement control does not decay with age. The study emphasizes the importance of analysis of joint motion and control characteristics for the investigation of arm movements and for comparison of these movements between different subject populations.     

Velocity curves of human arm and speech movements

Summary The velocity curves of human arm and speech movements were examined as a function of amplitude and rate in both continuous and discrete movement tasks. Evidence for invariance under scalar transformation was assessed and a quantitative measure of the form of the curve was used to provide information on the implicit cost function in the production of voluntary movement. Arm, tongue and jaw movements were studied separately. The velocity curves of tongue and jaw movement were found to differ in form as a function of movement duration but were similar for movements of different amplitude. In contrast, the velocity curves for elbow movements were similar in form over differences in both amplitude and duration. Thus, the curves of arm movement, but not those of tongue or jaw movement, were geometrically equivalent in form. Measurements of the ratio of maximum to average velocity in arm movement were compared with the theoretical values calculated for a number of criterion functions. For continuous movements, the data corresponded best to values computed for the minimum energy criterion; for discrete movement, values were in the range of those predicted for the minimum jerk and best stiffness criteria. The source of a rate dependent asymmetry in the form of the velocity curve of speech movements was assessed in a control study in which subjects produced simple raising and lowering movements of the jaw without talking. The velocity curves of the non-speech control gesture were similar in form to those of jaw movement in speech. These data, in combination with similar findings for human jaw movement in mastication, suggest that the asymmetry is not a direct consequence of the requirements of the task. The biomechanics and neural control of the orofacial system may be possible sources of this effect.     

Control of simple arm movements in elderly humans

Eight elderly subjects (aged 68-95 years) and 6 young adults (aged 21-24 years) performed elbow flexion and extension movements in a visual step-tracking paradigm. Movement amplitudes ranging from 10 degrees-80 degrees were made under two instructions: "move at own speed" and "move fast and accurate." In a second experiment, 5 elderly subjects practiced 30 degrees movements for a total of 180 flexion and 180 extension movements under the instruction to increase movement speed, while maintaining accuracy, during practice. Movement trajectories became more variable as both movement amplitude and speed increased. Trajectory variability was greater in the elderly subjects for both the acceleratory and deceleratory phases of movements. This was due primarily to a greater rate of increase in trajectory variability during the acceleration phase in the elderly. With practice, elderly subjects could substantially reduce trajectory variability with little change in movement speed. The agonist burst initiating movements was qualitatively normal in the elderly subjects. However, there was considerable tonic cocontraction of agonist and antagonist muscles prior to and during movement. Phasic antagonist EMG activity was obviously abnormal in many elderly subjects. There was often no clear antagonist burst associated with deceleration of the movements or, if present, it was timed inappropriately early. With practice, combined agonist-antagonist EMG variability decreased. A clear antagonist burst also developed during practice in most elderly subjects, but its inappropriate timing remained in all but one subject. The results show that movement trajectories are less accurately controlled in the elderly.(ABSTRACT TRUNCATED AT 250 WORDS)     

Misdirections in slow goal-directed arm movements and pointer-setting tasks

Summary Information about the direction of the virtual line between two positions in space (directional information) is used in many decision-making and motor tasks. We investigated how accurately directional information is processed by the brain. Subjects performed two types of task. In both tasks they sat at a table. In the first task they had to move their hand slowly and accurately from an initial position 40 cm in front of them to visually presented targets at a distance of 30 cm from the initial position (movement task). We analysed the initial movement direction. In the second task subjects had to position pointers in the direction of the targets as accurately as they could (perceptive task). We found that in the movement task the subjects started the movements to most targets in a direction that deviated consistently from the direction of the straight line between initial position and target position. The maximum deviation ranged from 5–10° for the various subjects. The mean standard deviation was 4°. In the perceptive task the subjects positioned the pointer in similarly deviating directions. Furthermore, we found that the maximum deviation in the pointer direction depended on the length of the pointer: the smaller the pointer, the larger the consistent deviations in the pointer direction. The shortest pointer showed deviations comparable to the deviations found in the movement task. These findings suggest that the deviations in the two tasks stem from the same source.     

Shoulder and elbow muscle activity in goal-directed arm movements

 This research examined the electromyographic (EMG) activity of shoulder and elbow muscles during reaching movements of the upper limb. Subjects performed goal-directed arm movements in the horizontal plane. Movements which varied in amplitude, speed, and direction were performed in different sections of the workspace. EMG activity was recorded from the pectoralis major, posterior deltoid, biceps brachii short head, brachioradialis, triceps brachii long head, and triceps brachii lateral head; motion recordings were obtained with an optoelectric system. The analysis focused on the magnitude and timing of opposing muscle groups at the shoulder and elbow joints. For hand movements within any given direction of the workspace direction, kinematic manipulations changed agonist and antagonist EMG magnitude and intermuscle timing in a manner consistent with previous single-joint findings. To produce reaching movements in different directions and areas of the workspace, shoulder and elbow agonist EMG magnitude increased for those hand motions which required higher angular velocities, while the timing between opposing muscle groups at each joint was invariant. Received: 11 January 1996 / Accepted: 24 February 1997     

The curvature and variability of wrist and arm movements

The control of wrist rotations is critical for normal upper limb function, yet has received little attention. In this study, we characterized path shape of wrist rotations in order to better understand the biomechanical and neural factors governing their control. Subjects performed step-tracking wrist rotations in eight directions “at a comfortable speed” and “as fast as possible.” For comparison, we also analyzed subjects’ arm movement paths in a similar task. We found significant differences between wrist and arm movements. Wrist paths were more curved and more variable than arm paths (p < 0.001). The increased curvature and variability can be explained in part by neuromuscular noise (in actuation and sensing) which is known to increase from proximal to distal in the upper limb. The curvature and variability of wrist paths increased with movement speed (p < 0.001), further implicating (signal-dependent) noise. However, noise cannot explain all of our observations. For example, we found that wrist rotations exhibit a systematic pattern: outbound and inbound paths between the same two targets tend to veer to opposite sides of a straight line. We provide evidence indicating that this type of systematic pattern is not likely caused by noise or other neural causes, but may be explained by the unique biomechanics of the wrist.     

Task goals influence online corrections and adaptation of reaching movements

Everyday movements often have multiple solutions. Many of these solutions arise from biomechanical redundancies. Often, however, the goal does not require a unique movement. To examine how people exploit task-related redundancy, we observed as participants produced three-dimensional (3-D) reaching movements, moving to one of two rectangular targets that were diagonally oriented in the frontal (x, y) plane. On most trials, the movement was perturbed by a vertical, velocity-dependent force. Since participants were free to move in 3-D space, online corrections could involve movement along the perturbed, vertical dimension, as well as the nonperturbed, horizontal dimension. If the motor system exploits task redundancies, then corrections along the horizontal dimension should depend on the orientation of the target. Consistent with this prediction, participants modified both the horizontal and vertical coordinates of the trajectory over the course of learning, and the horizontal component was sensitive to the orientation of the target. Furthermore, participants produced online corrections with a horizontal component that brought the hand closer to the target. These results suggest that we not only correct for mismatches between expected and experienced forces but also exploit task-specific redundancies to efficiently improve performance.