The effect of muscle vibration on human position sense during movements controlled by lengthening muscle contraction

Summary Muscle vibration studies suggest that during voluntary movement limb position is coded by muscle spindle information derived from the lengthening, antagonist muscle. However, these investigations have been limited to movements controlled by shortening contractions. This study further examined this property of kinesthesia during movements controlled by lengthening contraction. Subjects performed a horizontal flexion of the right forearm to a mechanical stop randomly positioned at 30, 50 and 70° from the starting position. The movement was performed against a flexor load (1 kg) requiring contraction of the triceps muscle. Vision was occluded and movements were performed under three conditions: no vibration, vibration of the right biceps and vibration of the right triceps. The perceived position of the right forearm was assessed by instructing subjects to simultaneously match the right limb position with the left limb. Vibration of the shortening biceps muscle had no effect on limb matching accuracy. However, triceps vibration resulted in significant overestimation of the vibrated limb position (10–13°). The variability in movement distance was uninfluenced by muscle vibration. During movements controlled by lengthening contraction, there is a concurrent gamma dynamic fusimotor input that would enhance primary afferent discharge. Despite this additional regulating input to the muscle spindle, it appears that muscle spindle information from the lengthening muscle is important for the accurate perception of limb movement and/or position.     

Vibration-induced changes in movement-related EMG activity in humans

The effect of muscle tendon vibration during voluntary arm movement was studied in normal humans. Subjects made alternating step flexion and extension movements about the elbow. A small vibrator was mounted over either the biceps or the triceps muscle and vibration was applied during flexion or extension movements. The vibrator was turned off between movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the muscle antagonist to the movement being performed produced an undershoot of the required end-movement position. The undershoot was 20-30% of the total movement amplitude. In contrast, vibration of the muscle agonist to the movement resulted in no change in movement end position. The vibration-induced undershoot was associated with an increase in the EMG activity of the vibrated (antagonist) muscle and a resultant increase in the ratio of the antagonist to agonist EMG activity. The increase in antagonist EMG produced by the vibration occurred with a latency of approximately 60 ms from vibration onset. The observed results are consistent with vibration-induced activation of muscle spindle receptors in the lengthening muscle during movement. It is suggested that, during movement, the sensitivity of the spindle receptors in the shortening muscle is decreased and the information concerning limb position during movement comes primarily from the lengthening muscle.     

The effects of muscle vibration on the attainment of intended final position during voluntary human arm movements

Summary Muscle tendon vibration was applied during voluntary step-tracking arm target-movements performed by normal human subjects. Vibration (freq. = 120 Hz) was applied over either the biceps or triceps tendons. During non-visually guided (eyes closed) trials, vibration of the muscle antagonistic to the movement being performed resulted in an undershoot of the required target. Thus, biceps vibration produced an undershoot of the extension target and triceps vibration an undershoot of the flexion target. The same effect occurred if the vibration was applied continuously over several movements or only during the course of individual movements. In contrast, vibration of the muscle acting as the prime mover had no effect on the correct attainment of the required target. It is suggested that the central nervous system may monitor muscle afferent activity of the lengthening (antagonist) muscle during simple, step movements.Supported by the Medical Research Council of Canada, Grant MA-6699     

Changes in movement final position associated with agonist and antagonist muscle fatigue

We have tested the hypothesis that agonist and antagonist muscle fatigue could affect the final position of rapid, discrete movements. Six subjects performed consecutive elbow flexion and extension movements between two targets, with their eyes closed prior to, and after fatiguing the elbow extensor muscles. The results demonstrate that elbow extension movements performed in the post-test period systematically undershot the final position as compared to pre-test movements. However, attainment of the aimed final position in elbow flexion movements was unaffected by fatiguing of the extensor muscles. Undershoot of the final position obtained in extension movements was associated with agonist muscle fatigue, a result that was expected from the point of view of current motor control theories, and that could be explained by a reduced ability of the shortening muscle to exert force. On the other hand, the absence of the expected overshoot of the final position when the antagonist is fatigued, indicates the involvement of various reflex and/or central mechanisms operating around the stretched muscle that could contribute to returning the limb to the standard final position after a brief prominent overshoot.     

Separate control of arm position and velocity demonstrated by vibration of muscle tendon in man

Summary The effect of muscle tendon vibration on the performance of some simple motor tasks and on kinesthesia was studied in normal humans. Subjects performed non-visually-guided slow arm movements to match either the position or the velocity of a visual target. In the experiments designed to study kinesthesia subjects indicated the perceived position or velocity of their passively moved arm. Vibration was applied over either the biceps or the triceps tendon. Position and velocity matching were found to be disturbed by vibration in essentially different ways, as were the perception of imposed position and the perception of imposed velocity. However, the vibration induced disturbance of position matching was congruent with the distortion of position perception. The effect of vibration on velocity matching was in accordance with the effect of vibration on the perception of velocity. It is concluded that the afferent information pathways that give rise to the perception of position and velocity respectively can be used separately in the control of slow movements under different conditions.     

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.     

Effect of muscle fatigue on the sense of limb position and movement

We have recently shown that in an unsupported forearm-matching task blindfolded human subjects are able to achieve an accuracy of 2–3°. If one arm was exercised to produce significant fatigue and the matching task was repeated, it led subjects to make position-matching errors. Here that result is confirmed using fatigue from a simple weight-lifting exercise. A 30% drop in maximum voluntary force after the exercise was accompanied by a significant matching error of 1.7° in the direction of extension when the reference arm had been fatigued, and 1.9° in the direction of flexion when the indicator arm had been fatigued. We also tested the effect of fatigue on a simple movement tracking task where the reference forearm was moved into extension at a range of speeds from 10 to 50°s⁻¹. Fatigue was found not to significantly reduce the movement-tracking accuracy. In a second experiment, movement tracking was measured while one arm was vibrated. When it was the reference arm, the subject perceived the movement to be significantly faster (3.7°s⁻¹) than it actually was. When it was the indicator, it was perceived to be slower (4.6°s⁻¹). The data supports the view that muscle spindles are responsible for the sense of movement, and that this sense is not prone to the disturbance from fatigue. By contrast, the sense of position can be disturbed by muscle fatigue. It is postulated, that the sense of effort experienced by holding the arm against the force of gravity is able to provide information about the position in space of the limb and that the increased effort from fatigue produces positional errors.     

Kinaesthetic role of muscle afferents in man,studied by tendon vibration and microneurography

Summary The characteristics of vibration-induced illusory joint movements were studied in healthy human subjects. Unseen by the subject, constant frequency vibration trains applied to the distal tendon of the Triceps or Biceps induced an almost constant velocity illusory movement of the elbow whose direction corresponded to that of a joint rotation stretching the vibrated muscle. Vibration trains of the same duration and frequency applied alternatively to the Biceps and Triceps evoked alternating flexion-extension illusory movements.During successive application of vibration trains at frequencies from 10 to 120 Hz, the perceived velocity of the illusory movements increased progressively from 10 to 70–80 Hz, then decreased from 80 to 120 Hz. The maximal perceived velocity was three times higher during alternating vibration of the Biceps and Triceps than during single muscle stimulation.Unit activity from 15 muscle spindle primary endings and five secondary endings located in Tibialis anterior and Extensor digitorum longus muscles were recorded using microneurography in order to study their responses to tendon vibration and passive and active movements of the ankle.Primary endings were all activated by low amplitude tendon vibration (0.2–0.5 mm) previously used to induce illusory movements of the elbow. The discharge of some was phase-locked with the vibration cycle up to 120 Hz, while others responded one-to-one to the vibration cycle up to 30–50 Hz, then fired in a sub-harmonic manner at higher frequencies. Secondary endings were much less sensitive to low amplitude tendon vibration.Primary and secondary ending responses to ramp and sinusoïdal movements of the ankle joint were compared. During the movement, the primary ending discharge frequency was almost constant, while the secondary ending activity progressively increased. During ankle movements the primary ending discharge appeared mainly related to velocity, while some secondary activities seemed related to both movement velocity and joint angle position.Muscle spindle sensory ending responses to active and passive ankle movements stretching the receptor-bearing muscle (plantar flexion) were qualitatively and quantitatively similar. During passive reverse movements (dorsiflexion) most of the sensory endings stopped firing when their muscle shortened. Active muscle shortening (isotonic contraction) modulated differently the muscle spindle sensory ending discharge, which could stop completely, decrease or some times increase during active ankle dorsiflexion. During isometric contraction most of the muscle spindle sensory endings were activated.The characteristics of the vibration-induced illusory movements and the muscle spindle responses to tendon vibration and to active and passive joint movements strengthened the possibility of the contribution of primary endings to kinaesthesia, as suggested by several previous works. Moreover, the present results led us to attribute to proprioception in the muscle stretched during joint movement a predominant, but not exclusive, role in this kind of perception.     

Cutaneous receptors contribute to kinesthesia at the index finger, elbow, and knee

The neural mechanisms underlying the sense of joint position and movement remain controversial. While cutaneous receptors are known to contribute to kinesthesia for the fingers, the present experiments test the hypothesis that they contribute at other major joints. Illusory movements were evoked at the interphalangeal (IP) joints of the index finger, the elbow, and the knee by stimulation of populations of cutaneous and muscle spindle receptors, both separately and together. Subjects matched perceived movements with voluntary movements of homologous joints on the contralateral side. Cutaneous receptors were activated by stretch of the skin (using 2 intensities of stretch) and vibration activated muscle spindle receptors. Stimuli were designed to activate receptors that discharge during joint flexion. For the index finger, vibration was applied over the extensor tendons on the dorsum of the hand, to evoke illusory metacarpophalangeal (MCP) joint flexion, and skin stretch was delivered around the IP joints. The strong skin stretch evoked the illusion of flexion of the proximal IP joint in 6/8 subjects (12 +/- 5 degrees, mean +/- SE). For the group, strong skin stretch delivered during vibration increased the perceived flexion of the proximal IP joint by eight times with a concomitant decrease in perceived flexion of the MCP joint compared with vibration alone (P < 0.05). For the elbow, vibration was applied over the distal tendon of triceps brachii and skin stretch over the dorsal forearm. When delivered alone, strong skin stretch evoked illusory elbow flexion in 5/10 subjects (9 +/- 4 degrees). Simultaneous strong skin stretch and vibration increased the illusory elbow flexion for the group by 1.5 times compared with vibration (P < 0.05). For the knee, vibration was applied over the patellar tendon and skin stretch over the thigh. Skin stretch alone evoked illusory knee flexion in 3/10 subjects (8 +/- 4 degrees) and when delivered during vibration, perceived knee flexion increased for the group by 1.4 times compared with vibration (P < 0.05). Hence inputs from cutaneous receptors, muscle receptors, and combined inputs from both receptors likely subserve kinesthesia at joints throughout the body.     

Supraspinal control of spinal centers for antagonist muscles in man

Summary Monosynaptic testing with the H-reflex was used to determine reflex excitability of motoneurons of the gastrocnemius muscle during single voluntary movements: extension or flexion of the ankle. For the last 60 msec of the latent period before the onset of voluntary extension of the foot, reflex excitability of motoneurons of the gastrocnemius (the agonist in extension) gradually increases. With voluntary flexion of the ankle reflex excitability of motoneurons to the gastrocnemius (the antagonist in flexion) is unchanged throughout the latent period until the onset of movement. Simultaneously (accuracy to 10 msec) with the beginning of the myogram of voltary foot flexion, reflex excitability of motoneurons of the gastrocnemius (antagonist) drops sharply. These results provide a basis for discussing an hypothesis concerning supraspinal control of spinal centers for antagonist muscles in man.     

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.     

Differences in control of limb dynamics during dominant and nondominant arm reaching

This study compares the coordination patterns employed for the left and right arms during rapid targeted reaching movements. Six right-handed subjects reached to each of three targets, designed to elicit progressively greater amplitude interaction torques at the elbow joint. All targets required the same elbow excursion (20 degrees ), but different shoulder excursions (5, 10, and 15 degrees, respectively). Movements were restricted to the shoulder and elbow and supported on a horizontal plane by a frictionless air-jet system. Subjects received visual feedback only of the final hand position with respect to the start and target locations. For motivation, points were awarded based on final position accuracy for movements completed within an interval of 400-600 ms. For all subjects, the right and left hands showed a similar time course of improvement in final position accuracy over repeated trials. After task adaptation, final position accuracy was similar for both hands; however, the hand trajectories and joint coordination patterns during the movements were systematically different. Right hand paths showed medial to lateral curvatures that were consistent in magnitude for all target directions, whereas the left hand paths had lateral to medial curvatures that increased in magnitude across the three target directions. Inverse dynamic analysis revealed substantial differences in the coordination of muscle and intersegmental torques for the left and right arms. Although left elbow muscle torque contributed largely to elbow acceleration, right arm coordination was characterized by a proximal control strategy, in which movement of both joints was primarily driven by the effects of shoulder muscles. In addition, right hand path direction changes were independent of elbow interaction torque impulse, indicating skillful coordination of muscle actions with intersegmental dynamics. In contrast, left hand path direction changes varied directly with elbow interaction torque impulse. These findings strongly suggest that distinct neural control mechanisms are employed for dominant and non dominant arm movements. However, whether interlimb differences in neural strategies are a consequence of asymmetric use of the two arms, or vice versa, is not yet understood. The implications for neural organization of voluntary movement control are discussed.     

Cathodal transcranial direct current stimulation of the primary motor cortex improves selective muscle activation in the ipsilateral arm

Proximal upper limb muscles are represented bilaterally in primary motor cortex. Goal-directed upper limb movement requires precise control of proximal and distal agonist and antagonist muscles. Failure to suppress antagonist muscles can lead to abnormal movement patterns, such as those commonly experienced in the proximal upper limb after stroke. We examined whether noninvasive brain stimulation of primary motor cortex could be used to improve selective control of the ipsilateral proximal upper limb. Thirteen healthy participants performed isometric left elbow flexion by contracting biceps brachii (BB; agonist) and left forearm pronation (BB antagonist) before and after 20 min of cathodal transcranial direct current stimulation (c-tDCS) or sham tDCS of left M1. During the tasks, motor evoked potentials (MEPs) in left BB were acquired using single-pulse transcranial magnetic stimulation of right M1 150-270 ms before muscle contraction. As expected, left BB MEPs were facilitated before flexion and suppressed before pronation. After c-tDCS, left BB MEP amplitudes were reduced compared with sham stimulation, before pronation but not flexion, indicating that c-tDCS enhanced selective muscle activation of the ipsilateral BB in a task-specific manner. The potential for c-tDCS to improve BB antagonist control correlated with BB MEP amplitude for pronation relative to flexion, expressed as a selectivity ratio. This is the first demonstration that selective muscle activation in the proximal upper limb can be improved after c-tDCS of ipsilateral M1 and that the benefits of c-tDCS for selective muscle activation may be most effective in cases where activation strategies are already suboptimal. These findings may have relevance for the use of tDCS in rehabilitation after stroke.     

Dual response from human muscle spindles in fast voluntary movements

Single-unit impulses were recorded from the radial nerve of attending human subjects using the microneurography technique. The discharge of muscle spindle afferents from the extensor digitorum muscles was analysed while subjects performed fast lengthening and shortening voluntary movements as well as movements of moderate speed at a single metacarpophalangeal joint. Opposing or assisting loads of moderate size were added in some tests. Fast lengthening movements were, in practically all units, associated with acceleration of spindle discharge. However, the responses were modest and in many primary afferents it was of similar size as their response to small irregularities during slower movements. During shortening movements, most spindle afferents stopped firing altogether, whereas some afferents exhibited a distinct burst of impulses at the onset of active shortening followed by silence during the main part of the movement. This initial shortening responses was sometimes more prominent when the parent muscle worked against an opposing load. It was interpreted as a result of fusimotor drive associated with the building up of force in the contracting muscle. The initial shortening response from the contracting muscle and the stretch response from the antagonist constitute a dual signal, describing accurately the onset of joint movement as seen from the two muscles. It remains to be clarified which role this pattern of afferent responses may have in the design of the current motor output and in the capturing of nature and size of the external load.     

Effects of tendon vibration on the spatiotemporal characteristics of human locomotion

The present study addressed the involvement of proprioceptive input of the muscle spindles in the spatiotemporal control of human locomotion. Blindfolded subjects walked along a walkway while tendon vibration, a powerful stimulus of Ia afferents, was applied to various muscles of the lower limb. The effects of tendon vibration were measured on joint kinematics and on intralimb and interlimb coordination. Tendon vibration of the tibialis anterior during locomotion led to a decreased plantar flexion at toe-off, whereas vibration of the triceps surae led to a decreased dorsiflexion during swing. Vibration of the quadriceps femoris at the knee led to a decreased knee flexion during swing. These local effects of vibration can be explained in the light of a lengthening illusion of the vibrated muscle in that phase of the gait cycle where the muscle is lengthened. Tendon vibration did not affect the qualitative features of intralimb coordination. With respect to interlimb coordination, only vibration of the biceps femoris showed a significant increase in phase lead of the vibrated limb. The present results suggest the involvement of Ia afferent input in the online control of joint rotations. Additionally it is hypothesized that the proprioceptive input of biceps femoris might be involved in the control of coordination between the limbs, whereas the coordination between the segments of one limb appears to be unaffected by disturbance of muscle spindle input of one muscle.     

The effects of human ankle muscle vibration on posture and balance during adaptive locomotion

This study investigated the contribution of ankle muscle proprioception to the control of dynamic stability and lower limb kinematics during adaptive locomotion, by using mechanical vibration to alter the muscle spindle output of individuals' stance limbs. It was hypothesised that muscle length information from the ankle of the stance limb provides information describing location as well as acceleration of the centre of mass (COM) with respect to the support foot during the swing phase of locomotion. Our prediction, based on this hypothesis was that ankle muscle vibration would cause changes to the position and acceleration of the COM and/or compensatory postural responses. Vibrators were attached to both the stance limb ankle plantarflexors (at the Achilles tendon) and the opposing dorsiflexor muscle group (over tibialis anterior). Participants were required to walk along a 9-m travel path and step over any obstacles placed in their way. There were three task conditions: (1) an obstacle (15 cm in height) was positioned at the midpoint of the walkway prior to the start of the trial, (2) the same obstacle was triggered to appear unexpectedly one step in front of the participant at the walkway midpoint and (3) the subjects' walking path remained clear. The participants' starting position was manipulated so that the first step over the obstacle (when present) was always performed with their right leg. For each obstacle condition participants experienced the following vibration conditions: no vibration, vibration of the left leg calf muscles or vibration of the anterior compartment muscles of the lower left leg. Vibration began one step before the obstacle at left leg heel contact and continued for 1 s. Vibrating the ankle muscles of the stance limb during the step over an obstacle resulted in significant changes to COM behaviour [measured as displacement, acceleration and position with respect to the centre of pressure (COP)] in both the medial/lateral (M/L) and anterior/posterior planes. There were also significant task-specific changes in stepping behaviour associated with COM control (measured as peak M/L acceleration, M/L foot displacement and COP position under the stance foot during the step over the obstacle). The results provide strong evidence that the primary endings of ankle muscle spindles play a significant role in the control of posture and balance during the swing phase of locomotion by providing information describing the movement of the body's COM with respect to the support foot. Our results also provide supporting evidence for the proposal that there are context-dependent changes in muscle spindle sensitivity during human locomotion.     

Proprioceptive illusions created by vibration of one arm are altered by vibrating the other arm

There is some evidence that signals coming from both arms are used to determine the perceived position and movement of one arm. We examined whether the sense of position and movement of one (reference) arm is altered by increases in muscle spindle signals in the other (indicator) arm in blindfolded participants (n = 26). To increase muscle spindle discharge, we applied 70–80 Hz muscle vibration to the elbow flexors of the indicator arm. In a first experiment, proprioceptive illusions in the vibrated reference arm in a forearm position-matching task were compared between conditions in which the indicator arm elbow flexors were vibrated or not vibrated. We found that the vibration illusion of arm extension induced by vibration of reference arm elbow flexors was reduced in the presence of vibration of the indicator elbow flexors. In a second experiment, participants were asked to describe their perception of the illusion of forearm extension movements of the reference arm evoked by vibration of reference arm elbow flexors in response to on/off and off/on transitions of vibration of non-reference arm elbow flexors. When vibration of non-reference arm elbow flexors was turned on, they reported a sensation of slowing down of the illusion of the reference arm. When it was turned off, they reported a sensation of speeding up. To conclude, the present study shows that both the sense of limb position and the sense of limb movement of one arm are dependent to some extent on spindle signals coming from the other arm.     

Afferent input, efference copy, signal noise, and biases in perception of joint angle during active versus passive elbow movements

Psychophysical studies have reported an overestimation of limb position in the direction of movement during the early part of active movements. The main hypothesis tested in this study is that the overestimation results from a process of forward prediction of limb state driven by an efference copy of the outgoing motor command. This hypothesis predicts that position overestimation should decrease or disappear during passive movements, for which there should be no efference copy. Seven subjects were asked to remember and to report the perceived angle of their elbow joint at different times during active and passive movements. They showed a highly velocity-dependent overestimation of the elbow joint angle near the beginning of the movement in both active and passive trials. Toward the end of the movement, subjects showed a relatively velocity-independent underestimation of their elbow angle in all trials. Contrary to the prediction of the efference copy hypothesis, the amplitude and the velocity-dependent slope of the elbow angle overestimation were both greater during the early part of passive movements than active movements. This indicates that psychophysical evidence of early overestimation of arm position on its own is not a sufficient proof of forward prediction based on an efference copy, at least under the conditions of this study. Decreased errors during active movements suggest that an efference copy can improve the accuracy of state estimation during active movements. Error patterns seem to parallel the likely level of sensorimotor noise, suggesting a probabilistic mechanism for position estimation.     

Interlimb transfer of visuomotor rotations depends on handedness

We previously reported that opposite arm adaptation to visuomotor rotations improved the initial direction of right arm movements in right-handers, whereas it only improved the final position accuracy of their left arm movements. We now investigate the pattern of interlimb transfer following adaptation to 30° visuomotor rotations in left-handers to determine whether the direction of transfer depends on handedness. Our results indicate unambiguous transfer across the arms. In terms of final position accuracy, the direction of transfer is opposite to that observed in right-handers, such that transfer only occurred from the left to the right arm movements. Directional accuracy also showed the opposite pattern of transfer to that of right-handers: initial movement direction, calculated at peak tangential acceleration, transferred only from right to left arms. When movement direction was measured later in the movement, at peak tangential velocity, asymmetrical transfer also occurred, such that greater transfer occurred from right to left arms. However, a small, but significant influence of opposite arm adaptation also occurred for the left arm, which might reflect differences in the use of the nondominant arm between left- and right-handers. Overall, our results indicate that left-handers show a mirror-imaged pattern of interlimb transfer in visuomotor adaptation to that previously reported for right-handers. This pattern of transfer is consistent with the hypothesis that asymmetry in interlimb transfer is dependent on differential specialization of the dominant and nondominant hemisphere/limb systems for trajectory and positional control, respectively.     

Reacting while moving: influence of right limb movement on left limb reaction

An experiment was designed to determine whether the activation of a muscle group (flexors or extensors) used to produce an ongoing movement of one limb influenced the reaction time and associated initiation of elbow flexion or extension movements of the contralateral limb. Right-handed participants in the bimanual groups were asked to produce a pattern of flexion/extension movements defined by a sine wave (period = 2 s, amplitude = 16°) with the right limb. While performing the right limb movement, participants were instructed that they were to react as quickly as possible by making a flexion or extension movement with their left limb when the cursor they were using to track the sine wave changed color. Participants in the unimanual groups performed the left limb reaction time task but were not asked to make right limb movements. The reaction time stimulus occurred once in each trial and was presented at one of six locations on one of the six cycles comprising the sinusoidal waveform. Participants performed 7 blocks of 6 test trials. Reaction time was calculated as the time interval between the color change of the cursor and the initiation of the response with the left limb. Movement time was calculated as the interval of time between the initiation of the response and the left limb cursor crossing the upper or lower boundary line. Mean reaction of the left limb was significantly influenced by the concurrent type of movement (flexion/extension) of the right limb. Reaction times were shorter on trials in which both limbs were initiating movement with homologous muscles as compared to trials in which the limbs were initiating movement with non-homologous muscles. No differences were detected when the stimuli were presented during the ballistic phase of the right limb movement, and no differences at any position were detected for the unimanual groups. This result is consistent with the notion that neural crosstalk can influence the time required to react to a stimulus but this influence occurs when contralateral muscles are activated.