Michael J Grey

University of Birmingham, Birmingham, England, United Kingdom

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Publications (38)104.74 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Transcranial magnetic stimulation is frequently used to construct stimulus response (SR) curves in studies of motor learning and rehabilitation. A drawback of the established method is the time required for data acquisition, which is frequently greater than a participant's ability to maintain attention. The technique is therefore difficult to use in the clinical setting. To reduce the time of curve acquisition by determining the minimum acquisition time and number of stimuli required to acquire an SR curve. SR curves were acquired from first dorsal interosseous (FDI) and abductor digiti minimi (ADM) at 6 interstimulus intervals (ISI) between 1.4 and 4 s in 12 participants. To determine if low-frequency rTMS might affect the SR curve, MEP amplitudes were monitored before and after 3 min of 1 Hz rTMS delivered at 120% of resting motor threshold in 12 participants. Finally, SR curves were acquired from FDI, ADM and Biceps Brachii (BB) in 12 participants, and the minimum number of stimuli was calculated using a sequential MEP elimination process. There were no significant differences between curves acquired with 1.4 s ISI and any other ISI. Low frequency rTMS did not significantly depress MEP amplitude (P = 0.87). On average, 61 ± 18 (FDI), 60 ± 16 (ADM) and 59 ± 16 (BB) MEPs were needed to construct a representative SR curve. This study demonstrates that reliable SR curves may be acquired in less than 2 min. At this rate, SR curves become a clinically feasible method for assessing corticospinal excitability in research and rehabilitation settings.
    Brain Stimulation 09/2013; · 4.54 Impact Factor
  • Mark Schram Christensen, Michael James Grey
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    ABSTRACT: Functional electrical stimulation (FES) is sometimes used as a therapeutic modality in motor rehabilitation to augment voluntary motor drive to effect movement that would otherwise not be possible through voluntary activation alone. Effective motor rehabilitation should require that the central nervous system integrate efferent commands and appropriate afferent information to update the internal models of acquired skills. Here, we investigate whether FES-evoked (FES-ev) and FES-assisted (FES-as) movement are associated with the normal integration of motor commands and sensory feedback in a group of healthy participants during functional magnetic resonance imaging (fMRI). Sensory feedback was removed with a peripheral ischaemic nerve block while the participants performed voluntary (VOL), FES-ev or FES-as movement during fMRI. Before the peripheral nerve block, secondary somatosensory area (S2) activation was greater for the FES-ev and FES-as conditions than for the VOL condition. During the ischaemic nerve block, S2 activation was reduced for the FES-ev condition but not for FES-as and VOL conditions. The nerve block also reduced activation during FES in the primary somatosensory cortex and other motor areas including primary motor cortex, dorsal premotor cortex and supplementary motor area. In contrast, superior parietal lobule (area 7A) and precuneus activation was reduced as a consequence of the ischaemic nerve block in the VOL condition. These data suggest FES-related S2 activation is mainly a sensory phenomenon and does not reflect integration of sensory signals with motor commands.
    European Journal of Neuroscience 03/2013; · 3.75 Impact Factor
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    ABSTRACT: Purpose: Neural tension technique (NTT) is a therapy believed to reduce spasticity and to increase range of motion (ROM). This study compared the ability of NTT and random passive movements (RPMs) to reduce spasticity in the knee flexors in 10 spastic patients with brain injury. Methods: An RCT study with crossover design evaluated muscle tone measured by: 1) hand-held dynamometer; 2) Modified Ashworth Scale (MAS); 3) and ROM by; 4) angles of resistance onset "catch" (R1) compensatory movement (R2); and 5) 'subjectively perceived reduction in muscle tone'. Outcome measures were recorded by three raters before and after a single treatment session. Results: Objective stiffness measured with the hand-held device showed no significant changes for the NTT or RPM (p ≥ 0.09-0.79). The subjective measures showed significant changes after the NTT for the non-blinded rater (MAS: p < 0.05: R1: p < 0.05; R2: p < 0.05), but for the blinded rater a significant reduction was found only for R1 (p < 0.05) and R2 (p < 0.05). For the non-blinded rater intervention effects were found for R1 (p < 0.01), R2 (p < 0.01) and subjectively perceived tone reduction (p < 0.01). For the blinded rater no intervention effect was found. Conclusions: An objective evaluation of NTT demonstrates that it does not reduce spasticity. However, it does increase ROM with the same effect as RPM. [Box: see text].
    Disability and Rehabilitation 03/2012; 34(23):1978-85. · 1.54 Impact Factor
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    ABSTRACT: Force field adaptation of locomotor muscle activity is one way of studying the ability of the motor control networks in the brain and spinal cord to adapt in a flexible way to changes in the environment. Here, we investigate whether the corticospinal tract is involved in this adaptation. We measured changes in motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in the tibialis anterior (TA) muscle before, during, and after subjects adapted to a force field applied to the ankle joint during treadmill walking. When the force field assisted dorsiflexion during the swing phase of the step cycle, subjects adapted by decreasing TA EMG activity. In contrast, when the force field resisted dorsiflexion, they increased TA EMG activity. After the force field was removed, normal EMG activity gradually returned over the next 5 min of walking. TA MEPs elicited in the early swing phase of the step cycle were smaller during adaptation to the assistive force field and larger during adaptation to the resistive force field. When elicited 5 min after the force field was removed, MEPs returned to their original values. The changes in TA MEPs were larger than what could be explained by changes in background TA EMG activity. These effects seemed specific to walking, as similar changes in TA MEP were not seen when seated subjects were tested during static dorsiflexion. These observations suggest that the corticospinal tract contributes to the adaptation of walking to an external force field.
    Experimental Brain Research 03/2012; 217(1):99-115. · 2.22 Impact Factor
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    ABSTRACT: It has recently been demonstrated that soleus motor-evoked potentials (MEPs) are facilitated prior to the onset of dorsiflexion. The purpose of this study was to examine if this could be explained by removal of spinal inhibition of the descending command to soleus motoneurons. To test this, we investigated how afferent inputs from the tibialis anterior muscle modulate the corticospinal activation of soleus spinal motoneurons at rest, during static contraction and prior to movement. MEPs activated by transcranial magnetic stimulation (TMS) and Hoffmann reflexes (H-reflexes), activated by electrical stimulation of the posterior tibial nerve (PTN), were conditioned by prior stimulation of the common peroneal nerve (CPN) at a variety of conditioning-test (CT) intervals. MEPs in the precontracted soleus muscle were inhibited when the TMS pulse was preceded by CPN stimulation with a CT interval of 35 ms, and they were facilitated for CT intervals of 50-55 ms. A similar inhibition of the soleus H-reflex was not observed. To investigate which descending pathways might be responsible for the afferent-evoked inhibition and facilitation, we examined the effect of CPN stimulation on short-latency facilitation (SLF) and long-latency facilitation (LLF) of the soleus H-reflex induced by a subthreshold TMS pulse at different CT intervals. SLF is known to reflect the excitability of the fastest conducting, corticomotoneuronal cells whereas LLF is believed to be caused by more indirect descending pathways. At CT intervals of 40-45 ms, the LLF was significantly more inhibited compared to the SLF when taking the effect on the H-reflex into account. Finally, we investigated how the CPN-induced inhibition and facilitation of the soleus MEP were modulated prior to dorsiflexion. Whereas the late facilitation (CT interval: 55 ms) was similar prior to dorsiflexion and at rest, no inhibition could be evoked at the earlier latency (CT interval: 35 ms) prior to onset of dorsiflexion. The observation that the CPN-induced inhibition of soleus MEPs disappears prior to onset of dorsiflexion may explain why soleus MEPs are facilitated prior to onset of dorsiflexion contraction. A possible mechanism involves the removal of inhibition of the descending command to the motoneurons at a spinal interneuronal level because the inhibition was seen in LLF and not in SLF, and the MEP inhibition was not observed in the H-reflex. The data illustrate that spinal interneuronal pathways modify descending commands to human spinal motoneurons and influence the size of MEPs elicited by TMS.
    The Journal of Physiology 12/2011; 589(Pt 23):5819-31. · 4.38 Impact Factor
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    ABSTRACT: Spasticity is a common complication with neurological diseases and CNS lesions. Instrumented systems to evaluate spasticity often cannot provide an immediate result, thus limiting their clinical usefulness. In this study we investigated the accuracy and reliability of the portable Neurokinetics RA1 Ridgidity Analyzer to measure stiffness of the ankle joint in 46 controls, 14 spinal cord injured (SCI) and 23 multiple sclerosis (MS) participants. Ankle stiffness measures were made twice by two raters, at speeds above and below the expected stretch reflex threshold. Ankle torque was measured with the portable device and a stationary torque motor. Inter- and intra-rater reliability was assessed with the intra-class correlation coefficient (ICC). Stiffness measures with the portable and stationary devices were significantly correlated for controls and MS participants (p < 0.01). Intra-rater reliability for the portable device ranged from 0.60-0.89 (SCI) and 0.63-0.67 (control) and inter-rater reliability ranged from 0.70-0.73 (SCI) and 0.61-0.77 (control). Ankle stiffness measures in SCI and MS participants were significantly larger than in controls for both slow (p < 0.05) and fast movements (p < 0.01), with stiffness being larger for fast compared to slow movements in SCI and MS participants (p < 0.05), but not in controls (p = 0.5). The portable device correlated well with measures obtained by a torque motor in both controls and MS participants, showed high intra- and inter-rater reliability for the SCI participants, and could easily distinguish between stiff and control ankle joints. However, the device, in its current form, may be less accurate during rapid movements when inertia contributes to stiffness and the shape of the air-filled pads did not provide a good interface with the foot. This study demonstrates that a portable device can potentially be a useful diagnostic tool to obtain reliable information of stiffness for the ankle joint.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 11/2011; 123(7):1371-82. · 3.12 Impact Factor
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    ABSTRACT: The therapeutic application of functional electrical stimulation (FES) has shown promising clinical results in the rehabilitation of post-stroke hemiplegia. It appears that the effect is optimal when the patterned electrical stimulation is used in close synchrony with voluntary movement, although the neural mechanisms that underlie the clinical successes reported with therapeutic FES are unknown. One possibility is that therapeutic FES takes advantage of the sensory consequences of an internal model. Here, we investigate fMRI cortical activity when FES is combined with voluntary effort (FESVOL) and we compare this activity to that produced when FES and voluntary activity (VOL) are performed alone. FESVOL revealed greater cerebellar activity compared with FES alone and reduced activity bilaterally in secondary somatosensory areas (SII) compared with VOL alone. Reduced activity was also observed for FESVOL compared with FES alone in the angular gyrus, middle frontal gyrus and inferior frontal gyrus. These findings indicate that during the VOL condition the cerebellum predicts the sensory consequences of the movement and this reduces the subsequent activation in SII. The decreased SII activity may reflect a better match between the internal model and the actual sensory feedback. The greater cerebellar activity coupled with reduced angular gyrus activity in FESVOL compared with FES suggests that the cortex may interpret sensory information during the FES condition as an error-like signal due to the lack of a voluntary component in the movement.
    Human Brain Mapping 05/2011; 33(1):40-9. · 6.88 Impact Factor
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    ABSTRACT: In humans, one of the most common tasks in everyday life is walking, and sensory afferent feedback from peripheral receptors, particularly the muscle spindles and Golgi tendon organs (GTO), makes an important contribution to the motor control of this task. One factor that can complicate the ability of these receptors to act as length, velocity and force transducers is the complex pattern of interaction between muscle and tendinous tissues, as tendon length is often considerably greater than muscle fibre length in the human lower limb. In essence, changes in muscle-tendon mechanics can influence the firing behaviour of afferent receptors, which may in turn affect the motor control. In this review we first summarise research that has incorporated the use of ultrasound-based techniques to study muscle-tendon interaction, predominantly during walking. We then review recent research that has combined this method with an examination of muscle activation to give a broader insight to neuromuscular interaction during walking. Despite the advances in understanding that these techniques have brought, there is clearly still a need for more direct methods to study both neural and mechanical parameters during human walking in order to unravel the vast complexity of this seemingly simple task.
    Journal of electromyography and kinesiology: official journal of the International Society of Electrophysiological Kinesiology 04/2011; 21(2):197-207. · 2.00 Impact Factor
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    ABSTRACT: Given the inherent mechanical complexity of human bipedal locomotion, and that complete spinal cord lesions in human leads to paralysis with no recovery of gait, it is often suggested that the corticospinal tract (CST) has a more predominant role in the control of walking in humans than in other animals. However, what do we actually know about the contribution of the CST to the control of gait? This chapter will provide an overview of this topic based on the premise that a better understanding of the role of the CST in gait will be essential for the design of evidence-based approaches to rehabilitation therapy, which will enhance gait ability and recovery in patients with lesions to the central nervous system (CNS). We review evidence for the involvement of the primary motor cortex and the CST during normal and perturbed walking and during gait adaptation. We will also discuss knowledge on the CST that has been gained from studies involving CNS lesions, with a particular focus on recent data acquired in people with spinal cord injury.
    Progress in brain research 01/2011; 192:181-97. · 4.19 Impact Factor
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    ABSTRACT: Spasticity is a common manifestation of lesion of central motor pathways. It is essential for correct anti-spastic treatment that passive and active contributions to increased muscle stiffness are distinguished. Here, we combined biomechanical and electrophysiological evaluation to distinguish the contribution of active reflex mechanisms from passive muscle properties to ankle joint stiffness in 31 healthy, 10 stroke, 30 multiple sclerosis and 16 spinal cord injured participants. The results were compared to routine clinical evaluation of spasticity. A computer-controlled robotic device applied stretches to the ankle plantar flexor muscles at different velocities (8-200deg/s; amplitude 6°). The reflex threshold was determined by soleus EMG. Torque and EMG data were normalized to the maximal torque and EMG evoked by supramaximal stimulation of the tibial nerve. Passive resistance (the torque response to stretches) was confirmed to be a good representation of the passive stiffness also at higher velocities when transmission in the tibial nerve was blocked by ischemia. Passive torque tended to be larger in the neurological than in the healthy participants, but it did not reach statistical significance, except in the stroke group (p<0.05). Following normalization to the maximal stimulus-evoked torque, the passive torque was found to be significantly larger in neurological participants identified with spasticity than in non-spastic participants (p<0.01). There was no significant difference in the reflex threshold between the healthy and the neurological participants. The reflex evoked torque and EMG were significantly larger in all neurological groups than in the healthy group (p<0.001). Twenty three participants with evidence of hypertonia in the plantar flexors (Ashworth score⩾1) showed normal reflex torque without normalization. With normalization this was only the case in 11 participants. Increased reflex mediated stiffness was detected in only 64% participants during clinical examination. The findings confirm that the clinical diagnosis of spasticity includes changes in both active and passive muscle properties and the two can hardly be distinguished based on routine clinical examination. The data suggest that evaluation techniques which are more efficient in distinguishing active and passive contributions to muscle stiffness than routine clinical examination should be considered before anti-spastic treatment is initiated.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 05/2010; 121(11):1939-51. · 3.12 Impact Factor
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    ABSTRACT: Plantar flexor series elasticity can be used to dissociate muscle-fascicle and muscle-tendon behavior and thus afferent feedback during human walking. We used electromyography (EMG) and high-speed ultrasonography concomitantly to monitor muscle activity and muscle fascicle behavior in 19 healthy volunteers as they walked across a platform. On random trials, the platform was dropped (8 cm, 0.9 g acceleration) or held at a small inclination (up to +/-3 degrees in the parasagittal plane) with respect to level ground. Dropping the platform in the mid and late phases of stance produced a depression in the soleus muscle activity with an onset latency of about 50 ms. The reduction in ground reaction force also unloaded the plantar flexor muscles. The soleus muscle fascicles shortened with a minimum delay of 14 ms. Small variations in platform inclination produced significant changes in triceps surae muscle activity; EMG increased when stepping on an inclined surface and decreased when stepping on a declined surface. This sensory modulation of the locomotor output was concomitant with changes in triceps surae muscle fascicle and gastrocnemius tendon length. Assuming that afferent activity correlates to these mechanical changes, our results indicate that within-step sensory feedback from the plantar flexor muscles automatically adjusts muscle activity to compensate for small ground irregularities. The delayed onset of muscle fascicle movement after dropping the platform indicates that at least the initial part of the soleus depression is more likely mediated by a decrease in force feedback than length-sensitive feedback, indicating that force feedback contributes to the locomotor activity in human walking.
    Journal of Neurophysiology 03/2010; 103(3):1262-74. · 3.30 Impact Factor
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    ABSTRACT: Walking requires a constant adaptation of locomotor output from sensory afferent feedback mechanisms to ensure efficient and stable gait. We investigated the nature of the sensory afferent feedback contribution to the soleus motoneuronal drive and to the corrective stretch reflex by manipulating body load and ankle joint angle. The volunteers walked on a treadmill ( approximately 3.6 km/h) connected to a body weight support (BWS) system. To manipulate the load sensitive afferents the level of BWS was switched between 5 and 30% of body weight. The effect of transient changes in BWS on the soleus stretch reflex was measured by presenting dorsiflexion perturbations ( approximately 5 degrees, 360-400 degrees/s) in mid and late stances. Short (SLRs) and medium latency reflexes (MLRs) were quantified in a 15 ms analysis window. The MLR decreased with decreased loading (P = 0.045), but no significant difference was observed for the SLR (P = 0.13). Similarly, the effect of the BWS was measured on the unload response, i.e., the depression in soleus activity following a plantar-flexion perturbation ( approximately 5.6 degrees, 203-247 degrees/s), quantified over a 50 ms analysis window. The unload response decreased with decreased load (P > 0.001), but was not significantly affected (P = 0.45) by tizanidine induced depression of the MLR (P = 0.039, n = 6). Since tizanidine is believed to depress the group II afferent pathway, these results are consistent with the idea that force-related afferent feedback contributes both to the background locomotor activity and to the medium latency stretch reflex. In contrast, length-related afferent feedback may contribute to only the medium latency stretch reflex.
    Journal of Neurophysiology 03/2010; 103(5):2747-56. · 3.30 Impact Factor
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    ABSTRACT: During hopping an early burst can be observed in the EMG from the soleus muscle starting about 45 ms after touch-down. It may be speculated that this early EMG burst is a stretch reflex response superimposed on activity from a supra-spinal origin. We hypothesised that if a stretch reflex indeed contributes to the early EMG burst, then advancing or delaying the touch-down without the subject's knowledge should similarly advance or delay the burst. This was indeed the case when touch-down was advanced or delayed by shifting the height of a programmable platform up or down between two hops and this resulted in a correspondent shift of the early EMG burst. Our second hypothesis was that the motor cortex contributes to the first EMG burst during hopping. If so, inhibition of the motor cortex would reduce the magnitude of the burst. By applying a low-intensity magnetic stimulus it was possible to inhibit the motor cortex and this resulted in a suppression of the early EMG burst. These results suggest that sensory feedback and descending drive from the motor cortex are integrated to drive the motor neuron pool during the early EMG burst in hopping. Thus, simple reflexes work in concert with higher order structures to produce this repetitive movement.
    The Journal of Physiology 03/2010; 588(Pt 5):799-807. · 4.38 Impact Factor
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    ABSTRACT: Human movement sense relies on both somatosensory feedback and on knowledge of the motor commands used to produce the movement. We have induced a movement illusion using repetitive transcranial magnetic stimulation over primary motor cortex and dorsal premotor cortex in the absence of limb movement and its associated somatosensory feedback. Afferent and efferent neural signalling was abolished in the arm with ischemic nerve block, and in the leg with spinal nerve block. Movement sensation was assessed following trains of high-frequency repetitive transcranial magnetic stimulation applied over primary motor cortex, dorsal premotor cortex, and a control area (posterior parietal cortex). Magnetic stimulation over primary motor cortex and dorsal premotor cortex produced a movement sensation that was significantly greater than stimulation over the control region. Movement sensation after dorsal premotor cortex stimulation was less affected by sensory and motor deprivation than was primary motor cortex stimulation. We propose that repetitive transcranial magnetic stimulation over dorsal premotor cortex produces a corollary discharge that is perceived as movement.
    PLoS ONE 01/2010; 5(10):e13301. · 3.53 Impact Factor
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    ABSTRACT: During human walking, a sudden trip may elicit a Ia afferent fibre mediated short latency stretch reflex. The aim of this study was to investigate soleus (SOL) muscle mechanical behaviour in response to dorsiflexion perturbations, and to relate this behaviour to short latency stretch reflex responses. Twelve healthy subjects walked on a treadmill with the left leg attached to an actuator capable of rapidly dorsiflexing the ankle joint. Ultrasound was used to measure fascicle lengths in SOL during walking, and surface electromyography (EMG) was used to record muscle activation. Dorsiflexion perturbations of 6 deg were applied during mid-stance at walking speeds of 3, 4 and 5 km h(-1). At each walking speed, perturbations were delivered at three different velocities (slow: approximately 170 deg s(-1), mid: approximately 230 deg s(-1), fast: approximately 280 deg s(-1)). At 5 km h(-1), fascicle stretch amplitude was 34-40% smaller and fascicle stretch velocity 22-28% slower than at 3 km h(-1) in response to a constant amplitude perturbation, whilst stretch reflex amplitudes were unchanged. Changes in fascicle stretch parameters can be attributed to an increase in muscle stiffness at faster walking speeds. As stretch velocity is a potent stimulus to muscle spindles, a decrease in the velocity of fascicle stretch at faster walking speeds would be expected to decrease spindle afferent feedback and thus stretch reflex amplitudes, which did not occur. It is therefore postulated that other mechanisms, such as altered fusimotor drive, reduced pre-synaptic inhibition and/or increased descending excitatory input, acted to maintain motoneurone output as walking speed increased, preventing a decrease in short latency reflex amplitudes.
    The Journal of Physiology 06/2009; 587(Pt 13):3375-82. · 4.38 Impact Factor
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    ABSTRACT: A rapid plantar flexion perturbation in the early stance phase of walking elicits a large stretch reflex in tibialis anterior (TA). In this study we use repetitive transcranial magnetic stimulation (rTMS) to test if this response is mediated through a transcortical pathway. TA stretch reflexes were elicited in the early stance phase of the step cycle during treadmill walking. Twenty minutes of 1 Hz rTMS at 115% resting motor threshold (MT(r)) significantly decreased (P < 0.05) the magnitude of the later component of the reflex at a latency of approximately 100 ms up to 25 min after the rTMS. Control experiments in which stretch reflexes were elicited during sitting showed no effect on the spinally mediated short and medium latency stretch reflexes (SLR and MLR) while the long latency stretch reflex (LLR) and the motor-evoked potential (MEP) showed a significant decrease 10 min after 115% MT(r) rTMS. This study demonstrates that 1 Hz rTMS applied to the leg area of the motor cortex can suppress the long latency TA stretch reflex during sitting and in the stance phase of walking. These results are in line with the hypothesis that the later component of the TA stretch reflex in the stance phase of walking is mediated by a transcortical pathway. An alternative explanation for the observed results is that the reflex is mediated by subcortical structures that are affected by the rTMS. This study also shows that rTMS may be used to study the neural control of walking.
    The Journal of Physiology 03/2009; 587(Pt 8):1669-76. · 4.38 Impact Factor
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    ABSTRACT: Humans maneuver easily over uneven terrain. To maintain smooth and efficient gait the motor system needs to adapt the locomotor output to the walking environment. In the present study we investigate the role of sensory feedback in adjusting the soleus muscle activity during overground walking in 19 healthy volunteers. Subjects walked unrestrained over a hydraulically actuated platform. On random trials the platform was accelerated downward at 0.8 g, unloading the plantar flexor muscles in midstance or late stance. The drop of the platform resulted in a significant depression of the soleus muscle activity of -17.9% (SD 2) and -21.4% (SD 2), with an onset latency of 49 ms (SD 1) and 45 ms (SD 1) in midstance and late stance, respectively. Input to the vestibular apparatus (i.e., the head acceleration) occurred at a latency 10.0 ms (SD 2.4) following the drop and ankle dorsiflexion velocity was decreased starting 22 ms (SD 15) after the drop. To investigate the role of length- and velocity-sensitive afferents on the depression in soleus muscle activity, the ankle rotation was arrested by using an ankle foot orthotic as the platform was dropped. Preventing the ankle movement did not significantly change the soleus depression in late stance [-18.2% (SD 15)], whereas the depression in midstance was removed [+4.9% (SD 13)]. It is concluded that force feedback from ankle extensors increases the locomotor output through positive feedback in late stance. In midstance the effect of force feedback was not observed, suggesting that spindle afferents may have a more significant effect on the output during this phase of the step cycle.
    Journal of Neurophysiology 02/2009; 101(4):1705-12. · 3.30 Impact Factor
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    ABSTRACT: To maintain smooth and efficient gait the motor system must adjust for changes in the ground on a step-to-step basis. In the present study we investigated the role of sensory feedback as 19 able-bodied human subjects walked over a platform that mimicked an uneven supporting surface. Triceps surae muscle activation was assessed during stance as the platform was set to different inclinations (±3 deg, ±2 deg and 0 deg rotation in a parasagittal plane about the ankle). Normalized triceps surae muscle activity was significantly increased when the platform was inclined (2 deg: 0.153 ± 0.051; 3 deg: 0.156 ± 0.053) and significantly decreased when the platform was declined (−3 deg: 0.133 ± 0.048; −2 deg: 0.132 ± 0.049) compared with level walking (0.141 ± 0.048) for the able-bodied subjects. A similar experiment was performed with a subject who lacked proprioception and touch sensation from the neck down. In contrast with healthy subjects, no muscle activation changes were observed in the deafferented subject. Our results demonstrate that the ability to compensate for small irregularities in the ground surface relies on automatic within-step sensory feedback regulation rather than conscious predictive control.
    The Journal of Physiology 08/2008; · 4.38 Impact Factor
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    ABSTRACT: Rehabilitation with augmented electrical stimulation can enhance functional recovery after stroke, and cortical plasticity may play a role in this process. The purpose of this study was to compare the effects of three training paradigms on cortical excitability in healthy subjects. Cortical excitability was evaluated by analysing the input-output relationship between transcranial magnetic stimulation intensity and motor evoked potentials (MEPs) from the flexor muscles of the fingers. The study was performed with 25 healthy volunteers who underwent 20-min simulated therapy sessions of: (1) functional electrical stimulation (FES) of the finger flexors and extensors, (2) voluntary movement (VOL) with sensory stimulation, and (3) therapeutic FES (TFES) where the electrical stimulation augmented voluntary activation. TFES training produced a significant increase in MEP magnitude throughout the stimulation range, suggesting an increase in cortical excitability. In contrast, neither the FES nor voluntary movement alone had such an effect. These results suggest that the combination of voluntary effort and FES has greater potential to induce plasticity in the motor cortex and that TFES might be a more effective approach in rehabilitation after stroke than FES or repetitive voluntary training alone.
    Experimental Brain Research 08/2008; 191(1):57-66. · 2.22 Impact Factor
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    ABSTRACT: Reduced depression of transmitter release from Ia afferents following previous activation (post-activation depression) has been suggested to be involved in the pathophysiology of spasticity. However, the effect of this mechanism on the myotatic reflex and its possible contribution to increased reflex excitability in spastic participants has not been tested. To investigate these effects, we examined post-activation depression in Soleus H-reflex responses and in mechanically evoked Soleus stretch reflex responses. Stretch reflex responses were evoked with consecutive dorsiflexion perturbations delivered at different intervals. The magnitude of the stretch reflex and ankle torque response was assessed as a function of the time between perturbations. Soleus stretch reflexes were evoked with constant velocity (175 degrees /s) and amplitude (6 degrees) plantar flexion perturbations. Soleus H-reflexes were evoked by electrical stimulation of the tibial nerve in the popliteal fossa. The stretch reflex and H-reflex responses of 30 spastic participants (with multiple sclerosis or spinal cord injury) were compared with those of 15 healthy participants. In the healthy participants, the magnitude of the soleus stretch reflex and H-reflex decreased as the interval between the stimulus/perturbation was decreased. Similarly, the stretch-evoked torque decreased. In the spastic participants, the post-activation depression of both reflexes and the stretch-evoked torque was significantly smaller than in healthy participants. These findings demonstrate that post-activation depression is an important factor in the evaluation of stretch reflex excitability and muscle stiffness in spasticity, and they strengthen the hypothesis that reduced post-activation depression plays a role in the pathophysiology of spasticity.
    Experimental Brain Research 03/2008; 185(2):189-97. · 2.22 Impact Factor

Publication Stats

593 Citations
104.74 Total Impact Points

Institutions

  • 2011–2013
    • University of Birmingham
      • College of Life and Environmental Sciences
      Birmingham, England, United Kingdom
    • Griffith University
      • School of Rehabilitation Sciences
      Brisbane, Queensland, Australia
  • 2010–2013
    • Copenhagen University Hospital Hvidovre
      Hvidovre, Capital Region, Denmark
  • 2008–2012
    • University of Copenhagen
      • Department of Neuroscience and Pharmacology
      København, Capital Region, Denmark
  • 2001–2011
    • Aalborg University
      • Department of Health Science and Technology
      Aalborg, Region North Jutland, Denmark