Reduced effects of tendon vibration with increased task demand during active, cyclical ankle movements

Tendon vibration can alter proprioceptive feedback, one source of sensory information which humans can use to produce accurate movements. However, the effects of tendon vibration during functional movement vary depending on the task. For example, ankle tendon vibration has considerably smaller effects during walking than standing posture. The purpose of this study was to test whether the effects of ankle tendon vibration are predictably influenced by the mechanical demands of a task, as quantified by peak velocity. Twelve participants performed symmetric, cyclical ankle plantar flexion/dorsiflexion movements while lying prone with their ankle motion unconstrained. The prescribed movement period (1, 3 s) and peak-to-peak amplitude (10°, 15°, 20°) were varied across trials; shorter movement periods or larger amplitudes increased the peak velocity. In some trials, vibration was continuously and simultaneously applied to the right ankle plantar flexor and dorsiflexor tendons, while the left ankle tendons were never vibrated. The vibration frequency (40, 80, 120, 160 Hz) was varied across trials. During trials without vibration, participants accurately matched the movement of their ankles. The application of 80 Hz vibration to the right ankle tendons significantly reduced the amplitude of right ankle movement. However, the effect of vibration was smaller during more mechanically demanding (i.e., higher peak velocity) movements. Higher vibration frequencies had larger effects on movement accuracy, possibly due to parallel increases in vibration amplitude. These results demonstrate that the effects of ankle tendon vibration are dependent on the mechanical demand of the task being performed, but cannot definitively identify the underlying physiological mechanism.     

Unimanual and bimanual voluntary movement in Huntington's disease

Unimanual and bimanual cyclical forearm movements were studied in 15 Huntington's disease (HD) patients and 15 healthy, gender- and age-matched controls. Whereas the unimanual task was only performed at maximal speed, the bimanual movements were performed according to the in-phase and anti-phase mode at different cycling frequencies. The HD patients also performed the tasks after 12 months of follow-up. Findings revealed that maximal cycling frequency during unimanual movement was significantly lower in HD patients as compared with controls. In addition, measures of relative phasing established that bimanual cyclical movements were performed with lower accuracy and higher variability in HD patients. The differential variability between both groups was magnified by increasing the cycling frequency and coordinative complexity whereas only coordinative complexity differentially affected the accuracy of relative phasing. The obtained performance measures were found to be significantly correlated with disease duration (unimanual) and with the score on the total motor scale, the Mini-Mental State Examination and the Stroop Interference Test (uni- and bimanual). After 12 months, maximal cycling frequency of unimanual elbow flexion–extension was significantly decreased in HD patients whereas the quality of the in-phase and anti-phase movement patterns remained stable. Electronic Publication     

Modulation of human cutaneous reflexes during rhythmic cyclical arm movement

The organization and pattern of cutaneous reflex modulation is unknown during rhythmic cyclical movements of the human upper limbs. On the assumption that these cyclic arm movements are central pattern generator (CPG) driven as has been suggested for leg movements such as walking, we hypothesized that cutaneous reflex amplitude would be independent of electromyographic (EMG) muscle activation level during rhythmic arm movement (phase-dependent modulation, as is often the case in the lower limb during locomotion). EMG was recorded from eight muscles crossing the human shoulder, elbow, and wrist joints while whole arm rhythmic cyclical movements were performed. Cutaneous reflexes were evoked with trains of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating paresthesia) to the superficial radial nerve innervating the lateral portion of the back of the hand. Phasic bursts of rhythmic muscle activity occurred throughout the movement cycle. Rhythmic EMG and kinematic patterns were similar to what has been seen in the human lower limb during locomotor activities such as cycling or walking: there were extensive periods of reciprocal activation of antagonist muscles. For most muscles, cutaneous reflexes were modulated with the movement cycle and were strongly correlated with the movement-related background EMG amplitude. It is concluded that cutaneous reflexes are primarily modulated by the background muscle activity during rhythmic human upper limb movements, with only some muscles showing phase-dependent modulation.     

Neural control of rhythmic human arm movement: phase dependence and task modulation of hoffmann reflexes in forearm muscles

Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180 degrees out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% (P < 0.005), 65.3% (P < 0.001), and 52.6% (P < 0.001) for bilateral, active ipsilateral, and passive ipsilateral movements, respectively. In contrast, movement of the contralateral limb did not significantly alter H-reflex amplitude. H-reflexes were also modulated by limb position (P < 0.005). Thus task- and phase-dependent modulation were observed in the arm as previously demonstrated in the leg. The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.     

Modulation of cutaneous reflexes in arm muscles during walking: further evidence of similar control mechanisms for rhythmic human arm and leg movements

Stimulation of cutaneous nerves innervating the hand evokes prominent reflexes in many arm muscles during arm cycling. We hypothesized that the mechanisms controlling reflex modulation during the rhythmic arm swing of walking would be similar to that documented during arm cycling. Thus, we expected cutaneous reflexes to be modulated by position in the walking cycle (phase dependence) and be different when walking compared to contraction while standing (task dependence). Subjects performed static postures similar to those occurring during walking and also walked on a treadmill while the superficial radial nerve was electrically stimulated pseudorandomly throughout the step cycle. EMG was recorded bilaterally from upper limb muscles and kinematic recordings were obtained from the elbow and shoulder joints. Step cycle information was obtained from force-sensing insoles. Analysis was conducted after averaging contingent upon the occurrence of stimulation in the step cycle. Phase-dependent modulation of cutaneous reflexes at early (~50–80 ms) and middle (~80–120 ms) latencies was observed. Coordinated bilateral reflexes were seen in posterior deltoid and triceps brachii muscles. Task dependency was seen in that reflex amplitude was only correlated with background EMG during static contraction (75% of comparisons for both early and middle latency reflexes). During walking, no significant relationship between reflex amplitude and background EMG level was found. The results show that cutaneous reflex modulation during rhythmic upper limb movement is similar to that seen during arm cycling and to that observed in leg muscles during locomotion. These results add to the evidence that, during cyclical movements of the arms and legs, similar neural mechanisms observed only during movement (e.g. central pattern generators) control reflex output. Electronic Publication     

Proprioceptive control of multijoint movement: bimanual circle drawing

Proprioception is used by the central nervous system (CNS) in the control of the spatial and temporal characteristics of single joint and multiple joint movement. The present study addressed the role of proprioception in the control of bilateral cyclical movements of the limbs. Normal blindfolded human subjects drew circles simultaneously and symmetrically with the two arms (16 cm diameter, 1 /s) upon two digitizing tablets. In selected trials, vibration (60–70 Hz) was applied to the tendon of the biceps and/or anterior deltoid muscles of the dominant arm to distort the proprioceptive information from muscle spindle afferents. One goal of this study was to identify whether tendon vibration influenced the spatial characteristics of circles drawn by the vibrated, dominant arm and the non-vibrated, non-dominant arm. A second goal was to determine the effect of vibration on the temporal coupling between the two arms during circle drawing. The results revealed that tendon vibration affected the spatial characteristics of circles drawn by the vibrated arm in a manner similar to that previously found for unilateral circle drawing. During bimanual circle drawing, vibration had only a minimal effect on the spatial characteristics of the non-vibrated, non-dominant arm. Temporal interlimb coupling was quantified by the relative phasing between the arms. Without tendon vibration, the dominant arm led the non-dominant arm. Vibration of the dominant arm increased the average phase lead. In a first control experiment, vibration of the non-dominant arm decreased the phase lead of the dominant arm, or even reversed it to a non-dominant arm phase lead. In a second control experiment, the subjects performed the bimanual circle-drawing task with vision of only the vibrated arm, in which case there was no spatial distortion of the circles drawn by the vibrated arm, but the phase relation between the two arms was still shifted as if vision were completely unavailable. It was concluded that, in bimanual movements such as these, the spatial and temporal characteristics of movement are controlled independently. Whereas the spatial characteristics of hand movement seem to be controlled unilaterally, the temporal characteristics of interlimb coupling appear to be controlled by proprioceptive information from both limbs, possibly by a proprioceptive triggering mechanism. Received: 29 November 1997 / Accepted: 16 February 1999     

Suppression of soleus H-reflex amplitude is graded with frequency of rhythmic arm cycling

In humans, rhythmic arm cycling has been shown to significantly suppress the soleus H-reflex amplitude in stationary legs. The specific nature of the relationship between frequency of arm cycling and H-reflex modulation in the legs has not been explored. We speculated that the effect of arm cycling on reflexes in leg muscles is related to the neural control of arm movement; therefore, we hypothesized that a graded increase in arm cycling frequency would produce a graded suppression of the soleus H-reflex amplitude. We also hypothesized that a threshold frequency of arm cycling would be identified at which the H-reflex amplitude significantly differed from static control trials (i.e., the arms were stationary). Soleus H-reflexes were evoked in stationary legs with tibial nerve stimulation during both control and rhythmic arm cycling (0.03–2.0 Hz) trials. The results show a significant inverse linear relation between arm cycling frequency and soleus H-reflex amplitude (P < 0.05). Soleus H-reflex amplitude significantly differed from control at an average threshold cycling frequency of 0.8 Hz. The results demonstrate that increased frequency of upper limb movement increases the intensity of interlimb influences on the neural activity in stationary legs. Further, a minimum threshold frequency of arm cycling is required to produce a significant effect. This suggests that achieving a threshold frequency of rhythmic arm movement may be important to incorporate in rehabilitation strategies to engage the appropriate interlimb neural pathways.     

Perceptual and motor effects of agonist-antagonist muscle vibration in man

Summary Perceptual and motor effects of vibration applied simultaneously to the distal tendons of the Biceps and Triceps muscles, in isometric conditions and without sight of the stimulated arm, have been studied in human volunteers. Motor effects, measured by surface EMG, are inexistent when the flexor and extensor muscles are simultaneously vibrated at the same frequency. However, EMG activity appears in the muscle being vibrated at the lower frequency when simultaneous vibration is applied at different frequencies. The sensations felt by the subjects were reproduced by the nonvibrated arm and recorded by a goniometer. The studies show that the velocity and the amplitude of the ilusory movement is related to the difference in vibration frequency applied to the two muscles. The direction of movement felt (flexion or extension) is that produced by shortening of the muscle being vibrated at the lower frequency. When the two vibration frequencies are the same, there is either no sensation of movement, or a sensation of very slow movement. These results support the notion that the sensation of movement at a joint may be derived from a central processing of the proprioceptive inflow data obtained from flexor and extensor muscles. This interpretation may also be valid for the results obtained earlier by vibration of a single muscle. Furthermore, it is coherent with data on spindle afferent fibres obtained by microneurography in man during passive or active movements.This work was supported by grants from the Ministère de l'Industrie et de la Recherche     

How does action resist visual illusion? Uncorrected oculomotor information does not account for accurate pointing in peripersonal space

The amplitudes and signs of cutaneous reflexes are modulated during rhythmic movements of the arms and legs (during walking and arm or leg cycling for instance). This reflex modulation is frequently independent of the background muscle activity and may involve central pattern generator (CPG) circuits. The purpose of the present study was to investigate the nature and degree of coupling between the upper limbs during arm cycling, with regard to the regulation of cutaneous reflexes. Responses to electrical stimulations of the right, superficial radial nerve (five 1 ms pulses, 300 Hz) were recorded bilaterally in six arm muscles of eight participants during arm cycling involving only the limb ipsilateral to the stimulation, only the limb contralateral to the stimulation, and bilateral movement when the limbs were both in-phase and 180° out of phase. The pattern of cutaneous reflex modulation throughout the arm cycle was independent of the functional state of the limb contralateral to the recording site, irrespective of whether recordings were made ipsilateral or contralateral to the stimulation. Furthermore, cutaneous reflexes were significantly (p<0.05) modulated with arm position in only 8% of cases in which the limb containing the responding muscle was either stationary or being moved passively by the experimenter. The results show that there is relatively weak coupling between the arms with regard to the regulation of cutaneous reflexes during rhythmic, cyclical arm movements. This suggests a loose connection between the CPGs for each arm that regulate muscle activity and reflex amplitude during rhythmic movement.     

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.     

Hierarchical control of different elbow-wrist coordination patterns

The present paper focused on the role of mechanical factors arising from the multijoint structure of the musculoskeletal system and their use in the control of different patterns of cyclical elbow-wrist movements. Across five levels of cycling frequency (from 0.45 Hz up to 3.05 Hz), three movement patterns were analyzed: (1) unidirectional, including rotations at the elbow and wrist in the same direction; (2) bidirectional, with rotation at the joints in opposite directions, and (3) free-wrist pattern, which is characterized by alternating flexions and extensions at the elbow with the wrist relaxed. Angular position of both joints and electromyographic activity of biceps, triceps, the wrist flexor, and the wrist extensor were recorded. It was demonstrated that control at the elbow was principally different from control at the wrist. Elbow control in all three patterns was similar to that typically observed during single-joint movements: elbow accelerations-decelerations resulted from alternating activity of the elbow flexor and extensor and were largely independent of wrist motion at all frequency plateaus. The elbow muscles were responsible not only for the elbow movement, but also for the generation of interactive torques that played an important role in wrist control. There were two types of interactive torques exerted at the wrist: inertial torque arising from elbow motion and restraining torque arising from physical limits imposed on wrist rotation. These interactive torques were the primary source of wrist motion, whereas the main function of wrist-muscle activity was to intervene with the interactive effects and to adjust the wrist movement to comply with the required coordination pattern. The unidirectional pattern was more in agreement with interactive effects than the bidirectional pattern, thus causing their differential difficulty at moderate cycle frequencies. When cycling frequency was further increased, both the unidirectional and bidirectional movements lost their individual features and acquired features of the free-wrist pattern. The deterioration of the controlled patterns at high cycling frequencies suggests a crucial role for proprioceptive information in wrist control. These results are suppportive of a hierachical organization of control with respect to elbow-wrist coordination, during which the functions of control at the elbow and wrist are principally different: the elbow muscles generate movement of the whole linkage and the wrist muscles produce corrections of the movement necessary to fulfill the task. Received: 5 August 1997 / Accepted: 29 January 1998     

Rhythmic arm cycling differentially modulates stretch and H-reflex amplitudes in soleus muscle

During rhythmic arm cycling, soleus H-reflex amplitudes are reduced by modulation of group Ia presynaptic inhibition. This suppression of reflex amplitude is graded to the frequency of arm cycling with a threshold of 0.8 Hz. Despite the data on modulation of the soleus H-reflex amplitude induced by rhythmic arm cycling, comparatively little is known about the modulation of stretch reflexes due to remote limb movement. Therefore, the present study was intended to explore the effect of arm cycling on stretch and H-reflex amplitudes in the soleus muscle. In so doing, additional information on the mechanism of action during rhythmic arm cycling would be revealed. Although both reflexes share the same afferent pathway, we hypothesized that stretch reflex amplitudes would be less suppressed by arm cycling because they are less inhibited by presynaptic inhibition. Failure to reject this hypothesis would add additional strength to the argument that Ia presynaptic inhibition is the mechanism modulating soleus H-reflex amplitude during rhythmic arm cycling. Participants were seated in a customized chair with feet strapped to footplates. Three motor tasks were performed: static control trials and arm cycling at 1 and 2 Hz. Soleus H-reflexes were evoked using single 1 ms pulses of electrical stimulation delivered to the tibial nerve at the popliteal fossa. A constant M-wave and ~6% MVC activation of soleus were maintained across conditions. Stretch reflexes were evoked using a single sinusoidal pulse at 100 Hz given by a vibratory shaker placed over the triceps surae tendon and controlled by a custom-written LabView program. Results demonstrated that rhythmic arm cycling that was effective for conditioning soleus H-reflexes did not show a suppressive effect on the amplitude of the soleus stretch reflex. We suggest this indicates that stretch reflexes are less sensitive to conditioning by rhythmic arm movement, as compared to H-reflexes, due to the relative insensitivity to Ia presynaptic inhibition.     

Corticospinal excitability is lower during rhythmic arm movement than during tonic contraction

Humans perform rhythmic, locomotor movements with the arms and legs every day. Studies using reflexes to probe the functional role of the CNS suggest that spinal circuits are an important part of the neural control system for rhythmic arm cycling and walking. Here, by studying motor-evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and H-reflexes induced by electrical stimulation of peripheral nerves, we show a reduction in corticospinal excitability during rhythmic arm movement compared with tonic, voluntary contraction. Responses were compared between arm cycling and tonic contraction at four positions, while participants generated similar levels of muscle activity. Both H-reflexes and MEPs were significantly smaller during arm cycling than during tonic contraction at the midpoint of arm flexion (F = 13.51, P = 0.006; F = 11.83, P = 0.009). Subthreshold TMS significantly facilitated the FCR H-reflex during tonic contractions, but did not significantly modulate H-reflex amplitude during arm cycling. The data indicate a reduction in the responsiveness of cells constituting the fast, monosynaptic, corticospinal pathway during arm cycling and suggest that the motor cortex may contribute less to motor drive during rhythmic arm movement than during tonic, voluntary contraction. Our results are consistent with the idea that subcortical regions contribute to the control of rhythmic arm movements despite highly developed corticospinal projections to the human upper limb.     

Alteration of proprioceptive messages induced by tendon vibration in man: a microneurographic study

Summary The activities of single proprioceptive fibres were recorded from the lateral peroneal nerve using transcutaneously implanted tungsten microelectrodes. Unitary discharges originating from muscle spindle primary and secondary endings and Golgi tendon organs were identified by means of various physiological tests. The sensitivity of proprioceptors to mechanical vibrations with a constant low amplitude (0.2–0.5 mm) applied at various frequencies to the tendon of the receptor-bearing muscle was studied. Muscle spindle primary endings (Ia fibres) were found to be the most sensitive to this mechanical stimulus. In some cases their discharge could be driven in a one-to-one manner up to 180 Hz. Most of them also fired harmonically with the vibration up to 80 Hz and then discharged in a subharmonic manner (1/2–1/3) with increasing vibration frequencies. Muscle spindle secondary endings (II fibres) and Golgi tendon organs (Ib fibres) were found to be either insensitive or only slightly sensitive to tendon vibration in relaxed muscles. The effects of tendon vibration on muscle spindle sensory endings response to muscle lengthening and shortening induced by imposed constant velocity or sinusoidal movements of the ankle joint were studied. Modulation of the proprioceptive discharge frequency coding the various joint movement parameters was either completely or partly masked by the receptor response to vibration, depending on the vibration frequency. Moreover, vibrations combined with sinusoidal joint movements elicited quantitatively erroneous proprioceptive messages concerning the movement parameters (amplitude, velocity). The sensitivity of the Golgi tendon organs to vibration increased greatly when the receptor-bearing muscle was tonically contracted. These data confirm that vibration is able to preferentially activate the Ia afferent channel, even when the vibration amplitude is low. They define the frequency sensitivity of the muscle spindle primary and secondary endings and the Golgi tendon organs. They also show that the physiological messages triggered by ongoing motor activities undergo a series of changes during the exposure of muscles to vibration.     

Rhythmic movements are larger and faster but with the same frequency on removal of visual feedback

The brain controls rhythmic movement through neural circuits combining visual information with proprioceptive information from the limbs. Although rhythmic movements are fundamental to everyday activities the specific details of the responsible control mechanisms remain elusive. We tested 39 young adults who performed flexion/extension movements of the forearm. We provided them with explicit knowledge of the amplitude and the speed of their movements, whereas frequency information was only implicitly available. In a series of 3 experiments, we demonstrate a tighter control of frequency compared with amplitude or speed. We found that in the absence of visual feedback, movements had larger amplitude and higher peak speed while maintaining the same frequency as when visual feedback was available; this was the case even when participants were aware of performing overly large and fast movements. Finally, when participants were asked to modulate continuously movement frequency, but not amplitude, we found the local coefficient of variability of movement frequency to be lower than that of amplitude. We suggest that a misperception of the generated amplitude in the absence of visual feedback, coupled with a highly accurate perception of generated frequency, leads to the performance of larger and faster movements with the same frequency when visual feedback is not available. Relatively low local coefficient of variability of frequency in a task that calls for continuous change in movement frequency suggests that we tend to operate at a constant frequency at the expense of variation in amplitude and peak speed.     

Velocity curves of human arm and speech movements

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

Muscle activation and cutaneous reflex modulation during rhythmic and discrete arm tasks in orthopaedic shoulder instability

In orthopaedic shoulder instability, muscle activity (EMG) is altered during unconstrained discrete arm movement tasks (e.g. elevation against a load). These findings have been ascribed to deficits in afferent feedback and neural control with glenohumeral instabilities resulting from orthopaedic injury. However, the integrity of neural control during shoulder movements in those with unstable shoulders is unclear. It is not known if there are altered EMG patterns during rhythmic arm movement or during discrete tasks involving no load, as would be experienced in many arm motions performed in daily living. The primary objective of this study was to evaluate neural control of arm movements between those with unstable shoulders and control participants, within a constrained arm movement paradigm involving both rhythmic arm cycling and discrete reaching. To achieve this objective, we determined if the amplitude and timing of EMG related to the movement pattern (background EMG) was significantly different between groups. Cutaneous reflexes were used to simulate a perturbation to the upper limb that would typically evoke a coordinated response. In the elevation phase of the movement path for anterior and posterior deltoid, upper trapezius, infraspinatus and serratus anterior, background EMG during rhythmic arm cycling was significantly (24%, p < 0.05) larger in unstable shoulders than in controls. No differences were found in background EMG between the groups during the discrete task. Significant differences (p < 0.05) were also noted in cutaneous reflexes between groups for both the rhythmic and discrete tasks with the reflex amplitudes being either increased or reduced in unstable shoulders as compared to controls. The differences in the background EMG and the cutaneous reflexes patterns in those with shoulder instabilities suggest that neural control is altered during rhythmic movement.     

Disruptions in joint control during drawing arm movements in Parkinson’s disease

Impairments in control of multi-joint arm movements in Parkinsons Disease (PD) were investigated. The PD patients and age-matched elderly participants performed cyclical arm movements, tracking templates of a large circle and four differentially oriented ovals on a horizontal table. The wrist was immobilized and the movements were performed with shoulder and elbow rotations. The task was performed with and without vision at a cycling frequency of 1.5 Hz. Traces of the arm endpoint, joint-motion parameters represented by range of motion and relative phase, and joint-control characteristics represented by amplitude and timing of muscle torque were analyzed. The PD patients provided deformations of the template shapes that were not observed in movements of elderly controls. The deformations were consistent for each shape but differed across the shapes, making quantification of impairments in the endpoint movement difficult. In contrast, the characteristics of joint control and motion demonstrated systematic changes across all shapes in movements of PD patients, although some of these changes were observed only without vision. A specification of the PD influence was observed at the level of joint control and it was not distinguishable in joint and endpoint motion, because of the property of multi-joint movements during which control at each joint influences motion at the other joints. The results suggest that inability of PD patients to provide fine muscle torque regulation coordinated across the joints contributes to the altered endpoint trajectories during multi-joint movements. The study emphasizes the importance of the torque analysis when deficits in multi-joint movements are investigated, because specific impairments that can be detected in joint-control characteristics are difficult to trace in characteristics of joint and endpoint kinematics, because of interactions between joint motions.     

Multi-frequency arm cycling reveals bilateral locomotor coupling to increase movement symmetry

Upright stance has allowed for substantial flexibility in how the upper limbs interact with each other: the arms can be coordinated in alternating, synchronous, or asymmetric patterns. While synchronization is thought to be the default mode of coordination during bimanual movement, there is little evidence for any bilateral coupling during locomotor-like arm cycling movements. Multi-frequency tasks have been used to reveal bilateral coupling during bimanual movements, thus here we used a multi-frequency task to determine whether the arms are coupled during arm cycling. It was hypothesized that bilateral coupling would be revealed as changes in background EMG and cutaneous reflexes when temporal coordination was altered. Twelve subjects performed arm cycling at 1 and 2 Hz with one arm while the contralateral arm was either at rest, cycling at the same frequency, or cycling at a different frequency (i.e., multi-frequency cycling with one arm at 1 Hz and the other at 2 Hz). To evoke reflexes, the superficial radial nerve was stimulated at the wrist. EMG was collected continuously from muscles of both arms. Results showed that background EMG in the lower frequency arm was amplified while reflex amplitudes were unaltered during multi-frequency cycling. We propose that neural coupling between the arms aids in equalizing muscle activity during asymmetric tasks to permit stable movement. Conversely, such interactions between the arms would likely be unnecessary in determining a reflexive response to a perturbation of one arm. Therefore, bilateral coupling was expressed when it was relevant to symmetry.     

Rhythmic arm cycling produces a non-specific signal that suppresses Soleus H-reflex amplitude in stationary legs

Rhythmic arm cycling significantly suppresses Hoffmann (H-) reflex amplitude in Soleus muscles of stationary legs. The specific parameters of arm cycling contributing to this suppression, however, are unknown. Between the arms or legs, movement results in suppression of the H-reflex that is specifically related to the phase of movement and the locus of limb movement. We speculated that the effects of arm movement features on H-reflexes in the leg would be similar and hypothesized that the Soleus H-reflex suppression evoked by arm movement would therefore be specifically related to: (1) phase of the movement; (2) the locus of the movement (i.e., ipsilateral or contralateral arm); (3) range of arm motion; and (4) frequency of arm cycling. Participants performed bilateral arm cycling at 1 and 2 Hz with short and long-crank lengths. Ipsilateral and contralateral arm cycling was also performed at 1 Hz with a long-crank length. Soleus H-reflexes were evoked at four equidistant phases and comparisons were made while maintaining similar evoked motor waves and Soleus activation. Our results show that comparable suppressive effects were seen at all phases of the arm movement: there was no phase-dependence. Further, bilateral or unilateral (whether ipsi- or contralateral arm) cycling yielded equivalent suppression of the H-reflex amplitude. Cycling at 2 Hz resulted in a significantly larger suppression than with 1 Hz cycling. We conclude that a general, rather than a specific, signal related to the command to produce rhythmic arm muscle activity mediates the suppression of Soleus H-reflex during arm cycling.