Facilitation of soleus H-reflex amplitude evoked by cutaneous nerve stimulation at the wrist is not suppressed by rhythmic arm movement

Neural connections between the cervical and lumbosacral spinal cord may assist in arm and leg coordination during locomotion. Currently the extent to which arm activity can modulate reflex excitability of leg muscles is not fully understood. We showed recently that rhythmic arm movement significantly suppresses soleus H-reflex amplitude probably via modification of presynaptic inhibition of the IA afferent pathway. Further, during walking reflexes evoked in leg muscles by stimulation of a cutaneous nerve at the wrist (superficial radial nerve; SR) are phase and task dependent. However, during walking both the arms and legs are rhythmically active thus it is difficult to identify the locus of such modulation. Here we examined the influence of SR nerve stimulation on transmission through the soleus H-reflex pathway in the leg during static contractions and during rhythmic arm movements. Nerve stimulation was delivered with the right shoulder in flexion or extension. H-reflexes were evoked alone (unconditioned) or with cutaneous conditioning via stimulation of the SR nerve (also delivered alone without H-reflex in separate trials). SR nerve stimulation significantly facilitated H-reflex amplitude during static contractions with the arm extended and countered the suppression of reflex amplitude induced by arm cycling. The results demonstrate that cutaneous feedback from the hand on to the soleus H-reflex pathway in the legs is not suppressed during rhythmic arm movement. This contrasts with the observation that rhythmic arm movement suppresses facilitation of soleus H-reflex when cutaneous nerves innervating the leg are stimulated. In conjunction with other data taken during walking, this suggests that the modulation of transmission through pathways from the SR nerve to the lumbosacral spinal cord is partly determined by rhythmic activity of both the arms and legs.     

Contralateral cerebellar damage impairs imperative planning but not updating of aimed arm movements in humans

The specific motor control processes supported by the cerebellum and impaired with cerebellar damage remain unclear. The cerebellum has been implicated in both planning and updating of accurate movements. Previously, we used a statistical model to parcel aiming performance that was constrained by a timed-response paradigm into contributions attributed to a specified plan and feedforward updating. Here, we apply this procedure to determine the putative role of the cerebellum in planning and updating goal-directed aiming by comparing the performance of subjects with unilateral cerebellar stroke to controls. Subjects rapidly moved to targets in predictable or unpredictable conditions and cerebellar subjects used the contralesional limb to control for ipsilesional motor execution deficits. Displacement-derived movement velocity was used in the statistical model to determine the effect of planning and updating on accuracy. Compared to controls, the cerebellar group demonstrated errors in final position that were primarily determined by planning deficits. This finding is manifest in four ways: Cerebellar subjects (1) were less accurate than controls in both predictable and unpredictable conditions; (2) they showed minimal benefit from increased preparation time for target amplitude specification; (3) with ample time to plan direction, wrong direction response frequency was greater; and (4) final position was minimally determined by the plan. Because these deficits were found contralesional to the moving limb, the cerebellum’s role in planning is not lateralized to one hemisphere but rather our findings suggest that cerebellar output affects motor planning for both upper limbs. Indeed, a lesion analysis showed that the dentate nucleus, an area implicated in planning motor strategies and the primary cerebellar output nucleus, was the only common region affected by our patient group with contralateral cerebellar strokes.     

6 -Hydroxydopamine decreases benzodiazepine but not GABA receptor binding in rat cerebellum

The noradrenergic innervation to the cerebellum was lesioned by intracerebroventricular 6-hydroxydopamine and the effects on GABA and benzodiazepine receptors followed by radiolabelling experiments. This lesion produced a 20% decrease in benzodiazepine receptor labelling in the cerebellum with no change in the labelling of GABA receptors. This provides further evidence for a lack of identity between the GABA and benzodiazepine receptors.     

Control of the wrist in three-joint arm movements to multiple directions in the horizontal plane

In a reaching movement, the wrist joint is subject to inertial effects from proximal joint motion. However, precise control of the wrist is important for reaching accuracy. Studies of three-joint arm movements report that the wrist joint moves little during point-to-point reaches, but muscle activities and kinetics have not yet been described across a range of movement directions. We hypothesized that to minimize wrist motion, muscle torques at the wrist must perfectly counteract inertial effects arising from proximal joint motion. Subjects were given no instructions regarding joint movement and were observed to keep the wrist nearly motionless during center-out reaches to directions throughout the horizontal plane. Consistent with this, wrist muscle torques exactly mirrored interaction torques, in contrast to muscle torques at proximal joints. These findings suggest that in this reaching task the nervous system chooses to minimize wrist motion by anticipating dynamic inertial effects. The wrist muscle torques were associated with a direction-dependent choice of muscles, also characterized by initial reciprocal activation rather than initial coactivation to stiffen the wrist joint. In a second experiment, the same pattern of muscle activities persisted even after many trials reaching with the wrist joint immobilized. These results, combined with similar features at the three joints, such as cosine-like tuning of muscle torques and of muscle onsets across direction, suggest that the nervous system uses similar rules for muscles at each joint, as part of one plan for the arm during a point-to-point reach.     

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.     

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.     

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.     

EMG responses to an unexpected load in fast movements are delayed with an increase in the expected movement time

When moving an object, the motor system estimates the dynamic properties of the object and then controls the movement using a combination of predictive feedforward control and proprioceptive feedback. In this study, we examined how the feedforward and proprioceptive feedback processes depend on the expected movement task. Subjects made fast elbow flexion movements from an initial position to a target. The experimental protocol included movements made over a short and a long distance against an expected light or heavy inertial load. In each task in a few randomly chosen trials, a motor applied an unexpected viscous load that produced a velocity error, defined as the difference between the expected and unexpected velocities, and electromyographic (EMG) responses. The EMG responses appeared not earlier than 170-250 ms from the agonist EMG onset. Our main finding is that the onset of the EMG responses was correlated with the expected time of peak velocity, which increased for longer distances and larger loads. An analysis of the latency of the EMG responses with respect to the velocity error suggested that the EMG responses were due to segmental reflexes. We conclude that segmental reflex gains are centrally modulated with the time course dependent on the expected movement task. According to this view, the control of fast point-to-point movement is feedforward from the agonist EMG onset until the expected time of peak velocity after which the segmental reflex feedback is briefly facilitated.     

Multijoint arm movements in cerebellar ataxia: Abnormal control of movement dynamics

In cerebellar ataxia, kinematic aberrations of multijoint movements are thought to originate from deficiencies in generating muscular torques that are adequate to control the mechanical consequences of dynamic interaction forces. At this point the exact mechanisms that lead to an abnormal control of interaction torques are not known. In principle, the generation of inadequate muscular torques may result from an impairment in generating sufficient levels of torques or from an inaccurate assessment and prediction of the mechanical consequences of movements of one limb segment on adjacent joints. We sought to differentiate the relative contribution of these two mechanisms and, therefore, analyzed intersegmental dynamics of multijoint pointing movements in healthy subjects and in patients with cerebellar degeneration. Unrestrained vertical arm movements were performed at three different target movement velocities and recorded using an optoelectronic tracking system. An inverse dynamics approach was employed to compute net joint torques, muscular torques, dynamic interaction torques and gravitational torques acting at the elbow and shoulder joint. In both groups, peak dynamic interaction forces and peak muscular forces were largest during fast movements. In contrast to normal subjects, patients produced hypermetric movements when executing fast movements. Hypermetric movements were associated with smaller peak muscular torques and smaller rates of torque change at elbow and shoulder joints. The patients’ deficit in generating appropriate levels of muscular force were prominent during two different phases of the pointing movement. Peak muscular forces at the elbow were reduced during the initial phase of the movement when simultaneous shoulder joint flexion generated an extensor influence upon the elbow joint. When attempting to terminate the movement, gravitational and dynamic interaction forces caused overshooting extension at the elbow joint. In normal subjects, muscular torque patterns at shoulder and elbow joint were synchronized in that peak flexor and extensor muscular torques occurred simultaneously at both joints. This temporal pattern of muscular torque generation at shoulder and elbow joint was preserved in patients. Our data suggest that an impairment in generating sufficient levels of phasic muscular torques significantly contributes to the patients’ difficulties in controlling the mechanical consequences of dynamic interaction forces during multijoint movements. Received: 28 October 1996 / Accepted: 30 September 1997     

Comparison of external load compensation during rhythmic arm movements and rhythmic jaw movements in humans.

Experiments were performed on human elbow flexor and extensor muscles and jaw-opening and -closing muscles to observe the effect on rhythmic movements of sudden loading. The load was provided by an electromagnetic device, which simulated the appearance of a smoothly increasing spring-like load. The responses to this loading were compared in jaw and elbow movements and between expected and unexpected disturbances. All muscles showed electromyographic responses to unexpected perturbations, with latencies of approximately 65 ms in the arm muscles and 25 ms in the jaw. When loading was predictable, anticipatory responses started in arm muscles approximately 200 ms before and in jaw muscles 100 ms before the onset of loading. The reflex responses relative to the anticipatory responses were smaller for the arm muscles than for the jaw muscles. The reflex responses in the arm muscles were the same with unexpected and expected perturbations, whereas anticipation increased the reflex responses in the jaw muscles. Biceps brachii and triceps brachii showed similar sensory-induced responses and similar anticipatory responses. Jaw muscles differed, however, in that the reflex response was stronger in masseter than in digastric. It was concluded that reflex responses in the arm muscles cannot overcome the loading of the arm adequately, which is compensated by a large centrally programmed response when loading is predictable. The jaw muscles, particularly the jaw-closing muscles, tend to respond mainly through reflex loops, even when loading of the jaw is anticipated. The differences between the responses of the arm and the jaw muscles may be related to physical differences. For example, the jaw was decelerated more strongly by the load than the heavier arm. The jaw was decelerated strongly but briefly, <30 ms during jaw closing, indicating that muscle force increased before the onset of reflex activity. Apparently, the force-velocity properties of the jaw muscles have a stabilizing effect on the jaw and have this effect before sensory induced responses occur. The symmetrical responses in biceps and triceps indicate similar motor control of both arm muscles. The differences in reflex activity between masseter and digastric muscle indicate fundamental differences in sensory feedback to the jaw-closing muscle and jaw-opening muscle.     

Coordination of arm movements in three-dimensional space. Sensorimotor mapping during drawing movement

In this paper data are presented concerning the motion of limb segments during drawing movements executed in different planes in free space. The technique used allows the determination of the wrist and elbow positions in space as well as the measurement of the elbow angle of extension. Other kinematic variables are determined trigonometrically. Elbow and shoulder torque is also calculated. For circles and ellipses, it was found that the motion at the wrist is sinusoidal in two orthogonal directions in the plane of motion. Angular motion, when described by a set of angles previously identified psychophysically as constituting an appropriate coordinate system, is also sinusoidal. Although the number of degrees of freedom of the arm affords many possible ways of performing the task, there is a fixed phase relation between the angles of elevation of the upper arm and forearm for naturally executed movements in all planes of space. Also, the phase of the yaw angles of the upper arm and forearm relative to the angles of elevation are related to the plane of motion and to the slant of ellipses in a fixed manner. There is a simple mapping between angular motion and intended wrist trajectory. Because this mapping is not valid for all planes of space, the actual trajectory can deviate from the intended one. However, the subject has no cognizance of the distortion. The calculated torque deviates substantially from sinusoidal and does change significantly when the same movement is executed in different planes. Results of simulations and mathematical analysis indicate that the fixed phase relationship between angles of elevation leads to a minimal distortion from sinusoidal motion at the wrist in an average sense and that the characteristic distortions observed in the sagittal plane result inevitably from this constraint on the phase relations. The results support the assumption that the topology of the sensorimotor map used for the production of the movement and for its perception is the same. The problem of invariant relationships between kinematic parameters is discussed and the suggestion is made that they represent a general constraint, leading through learning and practice to an optimal solution in an average sense.     

Naturalistic arm movements during obstacle avoidance in 3D and the identification of movement primitives

By studying human movement in the laboratory, a number of regularities and invariants such as planarity and the principle of isochrony have been discovered. The theoretical idea has gained traction that movement may be generated from a limited set of movement primitives that would encode these invariants. In this study, we ask if invariants and movement primitives capture naturalistic human movement. Participants moved objects to target locations while avoiding obstacles using unconstrained arm movements in three dimensions. Two experiments manipulated the spatial layout of targets, obstacles, and the locations in the transport movement where an obstacle was encountered. We found that all movement trajectories were planar, with the inclination of the movement plane reflecting the obstacle constraint. The timing of the movement was consistent with both global isochrony (same movement time for variable path lengths) and local isochrony (same movement time for two components of the obstacle avoidance movement). The identified movement primitives of transport (movement from start to target position) and lift (movement perpendicular to transport within the movement plane) varied independently with obstacle conditions. Their scaling accounted for the observed double peak structure of movement speed. Overall, the observed naturalistic movement was astoundingly regular. Its decomposition into primitives suggests simple mechanisms for movement generation.     

Activity patterns of upper arm muscles in relation to direction of rapid wrist movement in man

Summary Adjustment of arm posture associated with rapid wrist movements was studied by EMG analysis. Seven healthy adults, seated and holding their right arm with the shoulder in a neutral position with the elbow in 90° flexion and the wrist position neutral, were instructted to flex or extend the wrist as fast as possible. To examine whether the activity patterns of the upper arm muscles were related to the prime mover or the direction of the movement in space, the forearm was in two postures, supinate and pronate. The surface EMGs of biceps brachii, brachialis, triceps brachii and the prime movers were recorded along with the angular displacement of the wrist. The sequences of the upper arm muscle activities changed in relation to the direction of the movement. The earliest activities of the upper arm muscles were considered to counteract the dynamic perturbation induced by the rapid wrist movement. The onsets of the earliest activity of the upper arm muscles preceded the movement onset by 50–60 ms. These results revealed that the activity patterns of the arm muscles associated with the rapid wrist movements were functionally compatible with the anticipatory postural adjustment and were controlled according to the direction of the movement in space.     

The central nervous system does not minimize energy cost in arm movements

It has been widely suggested that the many degrees of freedom of the musculoskeletal system may be exploited by the CNS to minimize energy cost. We tested this idea by having subjects making point-to-point movements while grasping a robotic manipulandum. The robot created a force field chosen such that the minimal energy hand path for reaching movements differed substantially from those observed in a null field. The results show that after extended exposure to the force field, subjects continued to move exactly as they did in the null field and thus used substantially more energy than needed. Even after practicing to move along the minimal energy path, subjects did not adapt their freely chosen hand paths to reduce energy expenditure. The results of this study indicate that for point-to-point arm movements minimization of energy cost is not a dominant factor that influences how the CNS arrives at kinematics and associated muscle activation patterns.     

Misdirections in slow,goal-directed arm movements are not primarily visually based

In a previous study we found that the initial direction of slow, goal-directed arm movements deviates consistently from the direction of the actual straight line between the starting position and the target position. We now investigate whether these deviations are caused by imperfections or peculiarities in the processing of vision-related spatial information, such as retinal information, and eye- and head-position information. This could lead to incorrect localization of the target relative to the starting position. Subjects were seated in front of a horizontal surface and had to move their arm slowly and accurately in the direction of target positions. We varied the amount of vision-related spatial information. In experiment 1, subjects were presented with visual targets and could see their moving arm. In experiment 2, the subjects were again presented with visual targets, but now they could not see their moving arm. In experiment 3, the subjects were blindfolded and had to move their arm towards tactile targets. In all three experiments we found comparable consistent deviations in the initial movement direction. We also instructed congenitally and early-blind subjects to move their arm towards tactile targets. Their performance showed deviations congruous with those found in the sighted subjects, and possibly somewhat larger. We conclude that the deviations in the initial movement direction of slow, goal-directed arm movements are not primarily visually based. The deviations are generated after all spatial information has been integrated.     

Mefenamic acid decreases inflammation but not joint lesions in experimental osteoarthritis

Mefenamic acid is a non‐steroidal anti‐inflammatory drug able to control the symptoms of osteoarthritis (OA), but its effects on protection of cartilage and bone are still unclear. This study aimed to investigate whether the control of inflammation by mefenamic acid translates into decreased joint lesions in experimental OA in rats. OA was induced by injecting 1 mg of monosodium iodoacetate (MIA) into the joints of rats. The animals were treated with mefenamic acid (50 mg/kg, daily, oral gavage) either pre‐MIA injection (preventive) or post‐MIA injection (therapeutic). Joint swelling and hyperalgesia were evaluated at baseline and 1, 3, 14 and 28 days after induction of OA. Intra‐articular lavage and kinetics of cell migration into the synovium were measured 3 and 28 days after OA induction. Histopathological analysis, Osteoarthritis Research Society International (OARSI) score, total synovium cells count, cartilage area and levels of proteoglycans in joints were also evaluated. Mefenamic acid prevented joint oedema and hyperalgesia induced by MIA in the acute phase (3 days) of the disease. In the chronic phase (28 days), preventive and therapeutic regimens decreased the number of mononuclear cells in the joint cavity. In contrast, thickening of the synovium, bone resorption, loss of cartilage and levels of proteoglycans were unaffected by mefenamic acid when it was administered either preventively or therapeutically. Thus, mefenamic acid had anti‐inflammatory effects but did not reduce the progression of OA lesions, thereby indicating that it is only effective for symptomatic control of OA.     

Uterine contractility decreases at the time of blastocyst transfers

High-frequency uterine contractions at the time of non-cavitating embryo transfer influence adversely IVF-embryo transfer outcome. This prompted us to quantify prospectively the possible decline in uterine contraction frequency occurring during later stages of the luteal phase of ovarian stimulation, up to the time of blastocyst transfers, in 43 IVF-embryo transfer candidates. Contractility was assessed on the day of human chorionic gonadotrophin (HCG) administration, 4 days after HCG (non-cavitating embryo transfer; HCG + 4), and 7 days after HCG (blastocyst transfers; HCG + 7). For this, 2 min sagittal uterine scans were obtained by ultrasound and digitized with a computerized system for the assessment of uterine contraction frequency. Our results indicated that a slight, yet significant, decrease in uterine contraction frequency, observed from the day of HCG (4.4 +/- 0.2 contractions/min) to HCG + 4 (3.5 + 0.2 contractions/min), was followed by a more pronounced, additional decrease between HCG + 4 and HCG + 7 (1.5 +/- 0.2 contractions/min; P < 0.001). In conclusion, during the luteal phase of ovarian stimulation, uterine contractility decreases progressively, and reaches a nearly quiescent status 7 days after HCG administration, at the time of blastocyst transfers. It is possible that such a uterine relaxation assists blastocyst implantation.