John E. Misiaszek

The H-reflex as a tool in neurophysiology: its limitations and uses in understanding nervous system function.

The Hoffmann reflex (H‐reflex) is extensively used as both a research and clinical tool. The ease with which this reflex can be elicited in several muscles throughout the body makes it an attractive tool. This review discusses some of the important limitations in using the H‐reflex. In particular, the inaccurate but widely held assumptions that the H‐reflex (1) represents the monosynaptic reflex of the Ia afferent onto homonymous motoneurons, and (2) can be used to measure motoneuronal excitability are addressed. The second part of this review explores the utility of the H‐reflex as a neural probe in neurophysiology and motor control research. Applications ranging from the investigation of the functional organization of neural circuitry to the study of adaptive plasticity in spinal structures in health and disease suggest that the H‐reflex will continue to be an extensively used tool in motor control neurophysiology. Muscle Nerve 28: 144–160, 2003

Sensori-sensory afferent conditioning with leg movement: gain control in spinal reflex and ascending paths.

Studies are reviewed, predominantly involving healthy humans, on gain changes in spinal reflexes and supraspinal ascending paths during passive and active leg movement. The passive movement research shows that the pathways of H reflexes of the leg and foot are down-regulated as a consequence of movement-elicited discharge from somatosensory receptors, likely muscle spindle primary endings, both ipsi- and contralaterally. Discharge from the conditioning receptors in extensor muscles of the knee and hip appears to lead to presynaptic inhibition evoked over a spinal path, and to long-lasting attenuation when movement stops. The ipsilateral modulation is similar in phase to that seen with active movement. The contralateral conditioning does not phase modulate with passive movement and modulates to the phase of active ipsilateral movement. There are also centrifugal effects onto these pathways during movement. The pathways of the cutaneous reflexes of the human leg also are gain-modulated during active movement. The review summarizes the effects across muscles, across nociceptive and non-nociceptive stimuli and over time elapsed after the stimulus. Some of the gain changes in such reflexes have been associated with central pattern generators. However, the centripetal effect of movement-induced proprioceptive drive awaits exploration in these pathways. Scalp-recorded evoked potentials from rapidly conducting pathways that ascend to the human somatosensory cortex from stimulation sites in the leg also are gain-attenuated in relation to passive movement-elicited discharge of the extensor muscle spindle primary endings. Centrifugal influences due to a requirement for accurate active movement can partially lift the attenuation on the ascending path, both during and before movement. We suggest that a significant role for muscle spindle discharge is to control the gain in Ia pathways from the legs, consequent or prior to their movement. This control can reduce the strength of synaptic input onto target neurons from these kinesthetic receptors, which are powerfully activated by the movement, perhaps to retain the opportunity for target neuron modulation from other control sources.

Enhancement and Resetting of Locomotor Activity by Muscle Afferentsa

Abstract: The generation of the normal motor pattern for walking in mammals requires feedback from muscle proprioceptors. Two characteristics of the motor pattern particularly dependent on proprioceptive signals are (1) the magnitude of activity in knee and ankle extensor muscles and (2) the duration of extensor bursts during stance. Sensory regulation of these characteristics ensures that the level of activity in extensor muscles during stance is appropriate for the load carried by the leg and that the swing phase is not initiated when a leg is loaded. Many different groups of afferents from flexor and extensor muscles can influence the locomotor pattern. Most attention has focused on the action of group I afferents from ankle extensors. Electrical stimulation of these afferents during extension increases the duration and the magnitude of extensor activity. The prolongation of extensor activity depends in part on excitation of the extensor half‐center by group Ib afferents from Golgi tendon organs. The enhancement of the magnitude of extensor bursts is produced primarily via disynaptic and polysynaptic pathways opened only during locomotion. The involvement of the proprioceptive signals in the generation of locomotor activity means that the gains in reflex pathways must be constantly calibrated according to the biomechanical properties of the locomotor system. Alteration of these properties by weakening ankle extensor muscles has recently been found to produce compensatory changes in proprioceptive influences on the locomotor pattern.

Training of Walking Skills Overground and on the Treadmill: Case Series on Individuals With Incomplete Spinal Cord Injury

Background and Purpose: Walking in the home and community is an important goal for individuals with incomplete spinal cord injury (iSCI). Walking in the community requires various skills, such as negotiating curbs, doors, and uneven terrain. This case report describes the use of a method to retrain walking overground that is intensive, variable, and relevant to daily walking (skill training). The aims of this case series were to determine the effectiveness of skill training in a small group of people with iSCI and to compare skill training with body-weight–supported treadmill training (BWSTT) in the same individuals. Case Description: Four individuals who were a median of 2.7 years (interquartile range [IQR]=12.8) after iSCI participated in alternating phases of intervention, each 3 months long. All patients started with BWSTT. Two patients subsequently engaged in skill training while the other 2 patients engaged in BWSTT, after which a third phase of intervention (opposite to the second) was repeated. Outcomes: The Modified Emory Functional Ambulation Profile, the 10-Meter Walk Test, the 6-Minute Walk Test, the Berg Balance Scale, and the Activities-specific Balance Confidence Scale were administered before training, monthly throughout training, and 3 months after training. Discussion: Overall improvements in walking speed met or exceeded the minimal clinically important difference for individuals with iSCI (≥0.05 m/s), particularly during the skill training phase (skill training: median=0.09 m/s, IQR=0.13; BWSTT: median=0.01 m/s, IQR=0.07). Walking endurance, obstacle clearance, and stair climbing also improved with both types of intervention. Three of the 4 patients had retained their gains at follow-up (retention of walking speed: median=92%, IQR=63%). Thus, the findings suggest that skill training was effective in this small group of individuals.

Early activation of arm and leg muscles following pulls to the waist during walking

Many studies have investigated the compensatory reactions in humans elicited during walking when the support surface is perturbed. This has led to the description of characteristic responses generated in the muscles of the legs and torso, and recently the arms. The present study aimed to investigate the compensatory reactions elicited when balance was challenged by a perturbation applied to the waist, to determine to what extent balance corrective responses are generalized across perturbation modalities. A second aim was to characterize the arm responses elicited by the perturbations applied to the waist. We measured muscle activity of the left arm and leg following application of backward pulls of the waist while the subjects walked on a motorized treadmill. This resulted in robust activation of tibialis anterior and vastus lateralis, with co-activation of soleus and biceps femoris also evident when perturbations were applied at heel strike. These early responses occurred with a distal to proximal temporal organization. The responses in the leg muscles displayed a phase-dependent modulation in amplitude, decreasing in amplitude later in the stance phase. Leg muscle responses were not evident during the swing phase, except for the end of swing, just prior to heel strike. Arm muscle responses were observed in all subjects; however, the pattern of the arm responses varied considerably between subjects. Generally, shoulder muscles were more likely to respond than elbow muscles, at latencies consistent with the leg responses. Two important conclusions are drawn from the present study. First, the responses evoked in the legs with a pull to the waist are very similar to what has been reported for perturbations of the support surface, despite the very different locus of the perturbation. This suggests that balance control during walking may be achieved by preprogrammed reactions or synergies, which are triggered by multiple sensory cues. Second, rapid arm actions are integrated with these leg responses. However, the arm responses are more flexible, likely reflecting the fewer constraints imposed upon the actions of the arms, compared to the legs, during normal locomotion.

Postural uncertainty leads to dynamic control of cutaneous reflexes from the foot during human walking

Cutaneous reflexes evoked by stimulation of nerves innervating the foot are modulated in a phase-dependent manner during locomotion. The pattern of modulation of these reflexes has been suggested to indicate a functional role of cutaneous reflexes in assisting to maintain stability during walking. We hypothesized that if cutaneous reflexes assist in maintaining stability during gait, then these reflexes should be modulated in a context-dependent manner when subjects are asked to walk in an environment in which stability is challenged. To do this, we asked subjects to walk on a treadmill under five conditions: (1) normally, (2) with the arms crossed, (3) while receiving unpredictable anterior-posterior (AP) perturbations, (4) with the arms crossed while receiving unpredictable AP perturbations, and (5) with the hands holding onto fixed handles. Cutaneous reflexes arising from electrical stimulation of the superficial peroneal (SP; relevant to stumbling) or distal tibial (TIB; relevant to ground contact sensation) nerves were recorded bilaterally, at four points in the step cycle. Reflexes evoked with SP nerve stimulation showed marked facilitation during the most unstable walking condition in 4 of the 7 muscles tested. SP nerve-evoked reflexes in the muscles of the contralateral leg also showed suppression during the most stable walking condition. Reflexes evoked with TIB nerve stimulation were less affected by changes in the walking task. We argue that the specific adaptation of cutaneous reflexes observed with SP nerve stimulation supports the hypothesis that cutaneous reflexes from the foot contribute to the maintenance of stability during walking.

Whole-Body Responses: Neural Control and Implications for Rehabilitation and Fall Prevention

Humans are one of the unique species that utilize bipedal gait to ambulate in our environment. Despite this fact, coordination of the arms with the legs and the rest of body is essential for many daily activities. As such, whole-body responses have emerged as the preferred strategy following perturbations to balance during both standing and walking. Complex neural circuitry may allow for this coordination through the use of propriospinal pathways linking lumbar and cervical pattern generators in the spinal cord, with supraspinal centers altering this control depending on the context of the situation. Based on these findings, we argue that whole-body reactions may be exploited for rehabilitation purposes. Preliminary results have indicated training programs designed to elicit whole-body responses are effective in reducing falls and improving functional mobility in older adults with and without neurological impairment.

Task specific adaptations in rat locomotion: Runway versus horizontal ladder

In walking quadrupeds the alternating activity pattern of antagonistic leg muscles and the coordination between legs is orchestrated by central pattern generating networks within the spinal cord. These networks are activated by tonic input from the reticular formation in the brainstem. Under more challenging conditions, such as walking on a horizontal ladder, successful locomotion relies upon additional context dependent input from pathways such as the cortico- and rubro-spinal tracts. In this study we used electromyographic and kinematic approaches to characterize the adaptations in the walking pattern in adult uninjured rats crossing a horizontal ladder. We found that the placement of a hind limb on a rung precisely followed the placement of the ipsilateral fore limb. This is different to normal walking where the hind limb is placed behind the position of the ipsilateral fore limb. The increased reach of the hind limbs is achieved by increased flexion of the hip and rotation of the pelvis during the swing phase. Electromyographic observations showed decreased burst duration in Tibialis anterior an ankle flexor muscle. Further changes in the muscle activity pattern were likely due to the reduced stepping frequency during ladder walking. Following a lesion of the dorsal column, containing major parts of the corticospinal tract, we found an increased number of stepping errors and changes in the stepping strategy. The step length of the fore limbs was reduced and the hind limbs were frequently positioned on rungs other than those occupied by the fore limb.

Restricting arm use enhances compensatory reactions of leg muscles during walking

A number of recent studies have indicated that whole-body coordinated reactions are employed to regain balance following disturbances during walking. However, it is not always the case that all body segments are available to contribute to balance corrective strategies. We hypothesize that balance corrective strategies will adapt to task and environment constraints such that greater responses are generated in the available body segments when other body segments are unable to participate. In this study, we tested the hypothesis that voluntarily restricting the arms during walking would result in an increase in the amplitude of the electromyographic responses evoked in leg muscles when subjects are perturbed at the torso during walking. To do so, subjects were asked to walk on a motorized treadmill while either crossing their arms across their front or back, or with their arms swinging normally. Periodic perturbations, forwards and backwards, were applied at the pelvis randomly throughout the step cycle. This resulted in short latency responses in leg muscles. The amplitude of these responses was increased when subjects walked with their arms crossed, as compared with normal, unrestricted walking. Facilitation of these evoked responses was restricted to the early part of the stance phase, particularly at heel-strike. The pattern of muscle activation and the latency of the responses were not affected by restricting the arms. We suggest that this finding indicates that whole-body balance corrective strategies employed during walking are selected based upon the demands of the general features of the task, but that components of the strategy are scaled according to the specific context-dependent needs of the task.

Weighted Vests, Stereotyped Behaviors and Arousal in Children with Autism

The homeostatic theory of stereotyped behaviors assumes that these behaviors modulate arousal. Weighted vests are used to decrease stereotyped behaviors in persons with autism because the input they provide is thought to serve the same homeostatic function. This small-n, randomized and blinded study measured the effects of wearing a weighted vest on stereotyped behaviors and heart rate for six children with autism in the classroom. Weighted vests did not decrease motoric stereotyped behaviors in any participant. Verbal stereotyped behaviors decreased in one participant. Weighted vests did not decrease heart rate. Heart rate increased in one participant. Based on this protocol, the use of weighted vests to decrease stereotyped behaviors or arousal in children with autism in the classroom was not supported.