Systematic changes in motor cortex cell activity with arm posture during directional isometric force generation

We report here the activity of 96 cells in primate primary motor cortex (MI) during exertion of isometric forces at the hand in constant spatial directions, while the hand was at five to nine different spatial locations on a plane. The discharge of nearly all cells varied significantly with both hand location and the direction of isometric force before and during force-ramp generation as well as during static force-hold. In addition, nearly all cells displayed changes in the variation of their activity with force direction at different hand locations. This change in relationship was often expressed in part as a change in the cell's directional tuning at different hand locations. Cell directional tuning tended to shift systematically with hand location even though the direction of static force output at the hand remained constant. These directional effects were less pronounced before the onset of force output than after force onset. Cells also often showed planar modulations of discharge level with hand location. Sixteen proximal arm muscles showed similar effects, reflecting how hand location-dependent biomechanical factors altered their task-related activity. These findings indicate that MI single-cell activity does not covary exclusively with the level and direction of net force output at the hand and provides further evidence that MI contributes to the transformation between extrinsic and intrinsic representations of motor output during isometric force production.     

General coordination of shoulder,elbow and wrist dynamics during multijoint arm movements

Studies of multijoint arm movements have demonstrated that the nervous system anticipates and plans for the mechanical effects that arise from motion of the linked limb segments. The general rules by which the nervous system selects appropriate muscle activities and torques to best deal with these intersegmental effects are largely unknown. In order to reveal possible rules, this study examined the relationship of muscle and interaction torques to joint acceleration at the shoulder, elbow and wrist during point-to-point arm movements to a range of targets in the horizontal plane. Results showed that, in general, dynamics differed between the joints. For most movements, shoulder muscle torque primarily determined net torque and joint acceleration, while interaction torque was minimal. In contrast, elbow and wrist net torque were determined by a combination of muscle and interaction torque that varied systematically with target direction and joint excursion. This "shoulder-centered pattern" occurred whether subjects reached targets using straight or curved finger paths. The prevalence of a shoulder-centered pattern extends findings from a range of arm movement studies including movement of healthy adults, neurological patients, and simulations with altered interaction effects. The shoulder-centered pattern occurred for most but not all movements. The majority of the remaining movements displayed an "elbow-centered pattern," in which muscle torque determined initial acceleration at the elbow and not at the shoulder. This occurred for movements when shoulder excursion was <50% of elbow excursion. Thus, both shoulder- and elbow-centered movements displayed a difference between joints but with reversed dynamics. Overall, these findings suggest that a difference in dynamics between joints is a general feature of horizontal plane arm movements, and this difference is most commonly reflected in a shoulder-centered pattern. This feature fits well with other general shoulder-elbow differences suggested in the literature on arm movements, namely that: (a) agonist muscle activity appears more closely related to certain joint kinematics at the shoulder than at the elbow, (b) adults with neurological damage display less disruption of shoulder motion than elbow motion, and (c) infants display adult-like motion first in the shoulder and last at the wrist.     

Effects of visual information on perceived posture of an experimental phantom foot

Our previous studies showed that a fully extended finger, wrist, and elbow became a flexed phantom hand and arm with ischemic anesthesia, and vice versa (Inui et al. in J Physiol 589:5775–5784, 2011, Exp Brain Res 221:369–375, 2012a, Exp Brain Res 218:487–494, 2012b). It was anticipated that if the ankle and knee were fixed in full extension or flexion before and during ischemic anesthesia, the perceived positions would move in the opposite direction. The present study examined what happened when participants looked at their fixed foot and leg at the end of the anesthesia. Using the left ankle and knee, ten healthy participants demonstrated the perceived postures of the right joints during an ischemic block of the right thigh (40 min) and after they looked at the right joints at the end of the block. When the right ankle and knee were fully extended before and during the block, the final joints were perceived as flexed by all participants, and vice versa. Although there was no significant difference between joints for the magnitude of the perceived changes in flexion, the magnitude in the knee was larger than that in the ankle in extension. At the end of the experiment, when participants were allowed to see their foot, its perceived position reverted to that indicated by them earlier, during the first 25 min of cuff inflation. This new finding suggests that the position of limbs is coded by visual input more dominantly than by proprioceptive input in the brain.     

Reprogramming of muscle activation patterns at the wrist in compensation for elbow reaction torques during planar two-joint arm movements

The relationship between wrist kinematics, dynamics and the pattern of muscle activation were examined during a two-joint planar movement in which the two joints moved in opposite directions, i.e. elbow flexion/wrist extension and elbow extension/wrist flexion. Elbow movements (ranging from 10 to 70 deg) and wrist movements (ranging from 10 to 50 deg) were performed during a visual, step-tracking task in which subjects were required to attend to the initial and final angles at each joint. As the elbow amplitude increased, wrist movement duration increased and the wrist movement trajectories became quite variable. Analysis of the torques acting at the wrist joint showed that elbow movements produced reaction torques acting in the same direction as the intended wrist movement. Distinct patterns of muscle activation were observed at the wrist joint that were dependent on the relative magnitude of the elbow reaction torque in relation to the net wrist torque. When the magnitude of the elbow reaction torque was quite small, the wrist agonist was activated first. As the magnitude of the elbow reaction torque increased, activity in the wrist agonist decreased significantly. In conditions where the elbow reaction torque was much larger than the net wrist torque, the wrist muscle torque reversed direction to oppose the intended movement. This reversal of wrist muscle torque was directly associated with a change in the pattern of muscle activation where the wrist antagonist was activated prior to the wrist agonist. Our findings indicate that motion of the elbow joint is an important consideration in planning wrist movement. Specifically, the selection of muscle activation patterns at the wrist is dependent on the relative magnitude and direction of the elbow reaction torque in relation to the direction of wrist motion.     

Human arm stiffness characteristics during the maintenance of posture

Summary When the hand is displaced from an equilibrium position, the muscles generate elastic forces to restore the original posture. In a previous study, Mussa-Ivaldi et al. (1985) have measured and characterized the field of elastic forces associated with hand posture in the horizontal plane. Hand stiffness which describes the relation between force and displacement vectors in the vicinity of equilibrium position was measured and graphically represented by an ellipse, characterized by its size, shape and orientation. The results indicated that the shape and orientation of the stiffness ellipse are strongly dependent on arm configuration. At any given hand position, however, the values of these parameters were found to remain invariant among subjects and over time. In this study we investigate the underlying causes for the observed spatial pattern of variation of the hand stiffness ellipse. Mathematically analyzing the relation between hand and joint stiffness matrices, we found that in order to produce the observed spatial variations of the stiffness ellipse, the shoulder stiffness must covary in the workspace with the stiffness component provided by the two-joint muscles. This condition was found to be satisfied by the measured joint stiffness components. Using anatomical data and considering the effects that muscle cross-sections and changes in muscle moment arms have on the joint stiffness matrix, we found that these anatomical factors are not sufficient to account for the observed pattern of variation of joint stiffness in the workspace. To examine whether the coupling between shoulder and two-joint stiffnesses results from the coactivation of muscles contributing to these stiffnesses, EMG signals were recorded from shoulder, elbow and two-joint muscles. Our results indicated that, while some muscle coactivation may indeed exist, it can be found for only some of the muscles and in only part of the workspace.     

Intramuscular pressure of the infra- and supraspinatus muscles in relation to hand load and arm posture

In work engaging the upper extremities, the musculoskeletal system of the shoulder is sometimes exposed to prolonged excessive load, leading to musculoskeletal disorders of the shoulder. One way of reducing work-related shoulder disorders is to establish guidelines for working postures. The purpose of this study was to identify harmful working positions, by performing a comprehensive survey of the intramuscular pressure (IMP) in the infra- and supraspinatus muscles in relation to different arm positions and external loads. Ten healthy males participated, and the IMP in the infra- and supraspinatus muscles was studied in a total of 112 combinations of arm positions and hand loads at levels that occur frequently in industrial work. High-precision spatial recordings were accomplished with a three-dimesional motion-analysis system, and the IMP was measured using the microcapillary infusion technique. The mean IMP of the infraspinatus muscle as well as that of the supraspinatus muscle increased continuously from a resting pressure at 0° of upper arm elevation to a maximal pressure at 90° of upper arm elevation, for all elevation planes. The mean IMP of the supraspinatus muscle appeared to be more dependent upon the elevation plane and less dependent upon the hand load, compared to the infraspinatus muscle. Even during only moderate arm elevation, the mean IMP of the infra- and supraspinatus muscles, presented here in polar diagrams, had already exceeded the levels of reduced recovery from local muscle fatigue and blood flow impairment. The elevation angle and the hand load primarily influence the development of IMP in the infra- and supraspinatus muscles. Accepted: 6 June 2000     

Haptic stabilization of posture: changes in arm proprioception and cutaneous feedback for different arm orientations

Postural sway during quiet stance is attenuated by actively maintained contact of the index finger with a stationary surface, even if the level of applied force (<1 N) cannot provide mechanical stabilization. In this situation, changes in force level at the fingertip lead changes in center of foot pressure by approximately 250 ms. These and related findings indicate that stimulation of the fingertip combined with proprioceptive information about the hand and arm can serve as an active sensor of body position relative to the point of contact. A geometric analysis of the relationship between hand and torso displacement during body sway led to the prediction that arm and hand proprioceptive and finger somatosensory information about body sway would be maximized with finger contact in the plane of body sway. Therefore, the most postural stabilization should be possible with such contact. To test this analysis, subjects touched a laterally versus anteriorly placed surface while in each of two stances: the heel-to-toe tandem Romberg stance that reduces medial-lateral stability and the heel-to-heel, toes-outward, knees-bent, "duck stance" that reduces fore-aft stability. Postural sway was always least with finger contact in the unstable plane: for the tandem stance, lateral fingertip contact was significantly more effective than frontal contact, and, for the duck stance, frontal contact was more effective than lateral fingertip contact. Force changes at the fingertip led changes in center of pressure of the feet by approximately 250 ms for both fingertip contact locations for both test stances. These results support the geometric analysis, which showed that 1) arm joint angles change by the largest amount when fingertip contact is maintained in the plane of greatest sway, and 2) the somatosensory cues at the fingertip provide both direction and amplitude information about sway when the finger is contacting a surface in the unstable plane.     

Upper arm posture during human embryonic and fetal development

The upper extremity posture is characteristic of each Carnegie stage (CS), particularly between CS18 and CS23. Morphogenesis of the shoulder joint complex largely contributes to posture, although the exact position of the shoulder joints has not been described. In the present study, the position of the upper arm was first quantitatively measured, and the contribution of the position of the shoulder girdle, including the scapula and glenohumeral (GH) joint, was then evaluated. Twenty-nine human fetal specimens from the Kyoto Collection were used in this study. The morphogenesis and three-dimensional position of the shoulder girdle and humerus were analyzed using phase-contrast X-ray computed tomography and magnetic resonance imaging. Both abduction and flexion of the upper arm displayed a local maximum at CS20. Abduction gradually decreased until the middle fetal period, which was a prominent feature. Flexion was less than 90° at the local maximum, which was discrepant between appearance and measurement value in our study. The scapular body exhibited a unique position, being oriented internally and in the upward direction, with the glenoid cavity oriented cranially and ventrally. However, this unique scapular position had little effect on the upper arm posture because the angle of the scapula on the thorax was canceled as the angle of the GH joint had changed to a mirror image of that angle. Our present study suggested that measuring the angle of the scapula on the thorax and that of the GH joint using sonography leads to improved staging of the human embryo.     

Nondominant arm advantages in load compensation during rapid elbow joint movements

This study was designed to examine interlimb asymmetries in responding to unpredictable changes in inertial loads, which have implications for our understanding of the neural mechanisms underlying handedness. Subjects made repetitive single joint speed constrained 20 degrees elbow flexion movements, while the arm was supported on a horizontal, frictionless, air-jet system. On random trials, a 2-kg mass was attached to the arm splint prior to the "go" signal. Subjects were not given explicit information about the mass prior to movement nor were they able to view their limb or the mass. Accordingly, muscle activity, recorded prior to peak tangential finger acceleration, was the same for loaded and baseline trials. After this point, substantial changes in muscle activity occurred. In both limbs, the load compensation response was associated with a reduction in extensor muscle activity, resulting in a prolonged flexion phase of motion. For the nondominant arm, this resulted in effective load compensation, such that no differences in final position accuracy occurred between loaded and baseline trials. However, the dominant arm response also included a considerable increase in flexor muscle activity. This substantially prolonged the flexor acceleration phase of motion, relative to that of the nondominant arm. As a result, the dominant arm overcompensated the effects of the load, producing a large and systematic overshoot of final position. These results indicate more effective load compensation responses for the nondominant arm; supporting a specialized role of the nondominant arm/hemisphere system in sensory feedback mediated error correction mechanisms. The results also suggest that specialization of the dominant arm system for controlling limb and task dynamics is specifically related to feedforward control mechanisms.     

Control of hand orientation and arm movement during reach and grasp

We studied the coordination of arm and wrist motion in a task requiring fine control of hand orientation. Subjects were instructed to reach and grasp one of two targets positioned in the frontal plane at various orientations. The task was performed under three target conditions: fixed orientation, predictably perturbed orientation, and randomly perturbed orientation. For fixed target orientations, the hand began to rotate to the required orientation from the beginning of the reach. Hand peak supination angles scaled linearly with target orientations. The trajectories of hand/arm joint angles also had a one-to-one relationship with different target orientations. These demonstrate that target orientation is a constraint on the hand/arm final orientation, a control variable to be specified in advance by the central nervous system (CNS). Under perturbation conditions, subjects were still able to complete the task smoothly. In the early trials of the predictable perturbation, the hand rotated first to the original target orientation and then corrected for the final target orientation. Initial corrections occurred about 200 ms after the onset of perturbation. This latency decreased as the subjects adapted to the perturbation, and the hand orientation trajectory shifted to match the unperturbed trajectory for the final orientation. By contrast, we observed no clear changes in orientation trajectory under the randomly perturbed conditions. These suggest that feedback control is important to the execution of the movement, but that the CNS tends to optimize feedforward planning rather than feedback correction when the disturbance information is predictable.     

Neural and electromyographic correlates of wrist posture control

In identical experiments in and out of a MR scanner, we recorded functional magnetic resonance imaging and electromyographic correlates of wrist stabilization against constant and time-varying mechanical perturbations. Positioning errors were greatest while stabilizing random torques. Wrist muscle activity lagged changes in joint angular velocity at latencies suggesting trans-cortical reflex action. Drift in stabilized hand positions gave rise to frequent, accurately directed, corrective movements, suggesting that the brain maintains separate representations of desired wrist angle for feedback control of posture and the generation of discrete corrections. Two patterns of neural activity were evident in the blood-oxygenation-level-dependent (BOLD) time series obtained during stabilization. A cerebello-thalamo-cortical network showed significant activity whenever position errors were present. Here, changes in activation correlated with moment-by-moment changes in position errors (not force), implicating this network in the feedback control of hand position. A second network, showing elevated activity during stabilization whether errors were present or not, included prefrontal cortex, rostral dorsal premotor and supplementary motor area cortices, and inferior aspects of parietal cortex. BOLD activation in some of these regions correlated with positioning errors integrated over a longer time-frame consistent with optimization of feedback performance via adjustment of the behavioral goal (feedback setpoint) and the planning and execution of internally generated motor actions. The finding that nonoverlapping networks demonstrate differential sensitivity to kinematic performance errors over different time scales supports the hypothesis that in stabilizing the hand, the brain recruits distinct neural systems for feedback control of limb position and for evaluation/adjustment of controller parameters in response to persistent errors.     

Selection of wrist posture in conditions of motor ambiguity

In our everyday motor interactions with objects, we often encounter situations where the features of an object are determinate (i.e., not perceptually ambiguous), but the mapping between those features and appropriate movement patterns is indeterminate, resulting in a lack of any clear preference for one posture over another. We call this indeterminacy in stimulus-response mapping 'motor ambiguity'. Here, we use a grasping task to investigate the decision mechanisms that mediate the basic behavior of selecting one wrist posture over another in conditions of motor ambiguity. Using one of two possible wrist postures, participants grasped a dowel that was presented at various orientations. At most orientations, there was a clear preference for one wrist posture over the other. Within a small range of orientations, however, participants were variable in their posture selection due to the fact that the dowel was ambiguous with respect to the hand posture it afforded. We observed longer reaction times (RT) during 'ambiguous' trials than during the 'unambiguous' trials. In two subsequent experiments, we explored the effects of foreknowledge and trial history on the selection of wrist posture. We found that foreknowledge led to shorter RT unless the previous trial involved selecting a posture in the ambiguous region, in which case foreknowledge gave no RT advantage. These results are discussed within the context of existing models of sensorimotor decision making.     

Adaptations in arm movements for added mass to wrist or ankle during walking

The aim of the present study was to answer the question whether adaptations to local perturbations are restricted to the perturbed limb or whether they induce a reorganization of all co-moving limbs. Specifically, we studied the adaptations in arm movements to mass perturbations in seven healthy adults during walking on a treadmill. Four different perturbation conditions were employed in random order (no perturbation, mass added to both wrists, to the right wrist, and to the right ankle). During each experimental condition ten different belt speeds (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 km/h) were successively offered, while the arm movements and the electromyographic activity of the musculus deltoid posterior and anterior were measured. The results indicated that cadence was not affected by adding mass to the wrist or ankle. However, adding mass to a wrist not only resulted in an increase in muscle activity and a decrease of movement amplitude of the perturbed arm, but also in alterations in the non-perturbed arm. Notably, adding mass to one ankle induced adaptive changes in both arms, in that both muscle activity and arm movement increased. The present results indicate that during walking the loading of one of the limbs induces a general reorganization, involving all participating bodily segments, presumably to maintain balance while providing rhythm constancy.     

Voluntary changes in leg cadence modulate arm cadence during simultaneous arm and leg cycling

Although there is some evidence showing that neural coupling plays an important role in regulating coordination between the upper and lower limbs during walking, it is unclear how tightly the upper and lower limbs are linked during rhythmic movements in humans. The present study was conducted to investigate how coupling of both limbs is coordinated during independent rhythmic movement of the upper and lower limbs. Ten subjects performed simultaneous arm and leg cycling (AL cycling) at their preferred cadences without feedback for 10 s, and then were asked to voluntarily change the cadence (increase, decrease, or stop) of arm or leg cycling. Leg cycling cadence was not affected by voluntary changes in arm cadence. By contrast, arm cycling cadence was significantly altered when leg cycling cadence was changed. These results suggest the existence of a predominant lumbocervical influence of leg cycling on arm cycling during AL cycling.     

Effects of geometric joint constraints on the selection of final arm posture during reaching: a simulation study

 Significant debate exists regarding the neural strategies underlying the positioning and orienting of the hand during voluntary reaching movements of the human upper extremity. Some authors have suggested that positioning and orienting are controlled independently, while others have argued that a strong interdependence exists. In an effort to address this uncertainty, our study employed computer simulations to examine the impact of physiological limitations of joint rotation on the proposed independence of hand position and orientation. Specifically, we analyzed the effects of geometric constraints on final arm postures using a 7 degree-of-freedom model of the human arm. For 20 different hand configurations within the attainable workspace, we computed sets of achievable joint angles by applying inverse kinematics. From each set, we then calculated the locus of possible elbow positions for the particular final hand posture. When the joints were allowed 360° of rotation, the loci formed complete circles; however, when joint ranges were limited to physiological values, the extent of the loci decreased to an average arc angle of 54.6° (±27.9°). Imposition of joint limits also led to practically linear relationships between joint angles within a solution set. These theoretical results suggest a requirement for coordinated interaction between control of the joints associated with hand position and those involved with hand orientation in order to ensure attainable joint trajectories. Furthermore, it is conceivable that some of the correlations observed between joint angles in the course of natural reaching movements result from geometric constraints. Received: 7 December 1998 / Accepted: 14 January 1999     

手腕部外伤的临床分析

手外伤常见的是手及腕部肌腱断裂的锐器伤，如果处理不当常常引起手功能受限。为了提高手腕部锐器伤的治疗疗效，本文收集了15年内收治的有完整资料的手腕部3条以下不同名称的肌腱断裂锐器伤2461例，对其损伤的肌肉、肌腱、神经及病因与诊治进行了临床局解手术学分析。