Marc Klimstra

A sigmoid function is the best fit for the ascending limb of the Hoffmann reflex recruitment curve

The Hoffmann (H)-reflex has been studied extensively as a measure of spinal excitability. Often, researchers compare the H-reflex between experimental conditions with values determined from a recruitment curve (RC). An RC is obtained experimentally by varying the stimulus intensity to a nerve and recording the peak-to-peak amplitudes of the evoked H-reflex and direct motor (M)-wave. The values taken from an RC may provide different information with respect to a change in reflex excitability. Therefore, it is important to obtain a number of RC parameters for comparison. RCs can be obtained with a measure of current (HCRC) or without current (HMRC). The ascending limb of the RC is then fit with a mathematical analysis technique in order to determine parameters of interest such as the threshold of activation and the slope of the function. The purpose of this study was to determine an unbiased estimate of the specific parameters of interest in an RC through mathematical analysis. We hypothesized that a standardized analysis technique could be used to ascertain important points on an RC, regardless of data presentation methodology (HCRC or HMRC). For both HCRC and HMRC produced using 40 randomly delivered stimuli, six different methods of mathematical analysis [linear regression, polynomial, smoothing spline, general least squares model with custom logistic (sigmoid) equation, power, and logarithmic] were compared using goodness of fit statistics (r-square, RMSE). Behaviour and robustness of selected curve fits were examined in various applications including RCs generated during movement and somatosensory conditioning from published data. Results show that a sigmoid function is the most reliable estimate of the ascending limb of an H-reflex recruitment curve for both HCRC and HMRC. Further, the parameters of interest change differentially with respect to the presentation methodology and the analysis technique. In conclusion, the sigmoid function is a reliable analysis technique which mimics the physiologically based prediction of the input/output relation of the ascending limb of the recruitment curve. Therefore, the sigmoid function should be considered an acceptable and preferable analytical tool for H-reflex recruitment curves obtained with reference to stimulation current or M-wave amplitude.

Rhythmic leg cycling modulates forearm muscle H-reflex amplitude and corticospinal tract excitability.

Rhythmic arm cycling leads to suppression of H-reflexes in both leg and arm muscles, and a reduction in the excitability of corticospinal projections to the forearm flexors. It is unknown, however, whether leg cycling modulates excitability in neural projections to the arms. Here we studied the extent to which rhythmic movement of the legs alters reflex (Experiment 1) and corticospinal (Experiment 2) transmission to arm muscles. In experiment 1, flexor carpi radialis (FCR) H-reflex recruitment curves were recorded with the legs static, and during rhythmic leg movement, while the FCR was both contracted and relaxed. The results indicate that rhythmic leg movement suppresses reflex transmission, both when FCR is at rest and during tonic contraction, but that the effect is not phase-dependent. In experiment 2, we used transcranial magnetic stimulation (TMS) to elicit motor-evoked potentials in the contracted and relaxed FCR during static leg, and leg cycling conditions. Sub-threshold TMS was also used to condition H-reflexes in order to provide specific information about cortical excitability during leg cycling. Both resting and tonically contracting arm muscles showed a greater corticospinal excitability during leg cycling than during the static leg condition. The magnitude of TMS facilitation of H-reflexes was similar during leg cycling and rest, suggesting a considerable sub-cortical component to the increased corticospinal excitability. The results suggest a differential regulation of afferent and descending projections to the arms during leg cycling, and are consistent with the idea that there is a loose, but significant, neural coupling between the arms and legs during rhythmic movement.

Neuromechanical considerations for incorporating rhythmic arm movement in the rehabilitation of walking

We have extensively used arm cycling to study the neural control of rhythmic movements such as arm swing during walking. Recently rhythmic movement of the arms has also been shown to enhance and shape muscle activity in the legs. However, restricted information is available concerning the conditions necessary to maximally alter lumbar spinal cord excitability. Knowledge on the neuromechanics of a task can assist in the determination of the type, level, and timing of neural signals, yet arm swing during walking and arm cycling have not received a detailed neuromechanical comparison. The purpose of this research was to provide a combined neural and mechanical measurement approach that could be used to assist in the determination of the necessary and sufficient conditions for arm movement to assist in lower limb rehabilitation after stroke and spinal cord injury. Subjects performed three rhythmic arm movement tasks: (1) cycling (cycle); (2) swinging while standing (swing); and (3) swinging while treadmill walking (walk). We hypothesized that any difference in neural control between tasks (i.e., pattern of muscle activity) would reflect changes in the mechanical constraints unique to each task. Three-dimensional kinematics were collected simultaneously with force measurement at the hand and electromyography from the arms and trunk. All data were appropriately segmented to allow a comparison between and across conditions and were normalized and averaged to 100% movement cycle based on shoulder excursion. Separate mathematical principal components analysis of kinematic and neural variables was performed to determine common task features and muscle synergies. The results highlight important neural and mechanical features that distinguish differences between tasks. For example, there are considerable differences in the anatomical positions of the arms during each task, which relate to the moments experienced about the elbow and shoulder. Also, there are differences between tasks in elbow flexion/extension kinematics alongside differential muscle activation profiles. As well, mechanical assistance and constraints during all tasks could affect muscle recruitment and the functional role of muscles. Overall, despite neural and mechanical differences, the results are consistent with conserved common central motor control mechanisms operational for cycle, walk, and swing but appropriately sculpted to demands unique to each task. However, changing the mechanical parameters could affect the role of afferent feedback altering neural control and the coupling to the lower limbs.

Neuromechanical interactions between the limbs during human locomotion: an evolutionary perspective with translation to rehabilitation.

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Walking Phase Modulates H-Reflex Amplitude in Flexor Carpi Radialis

ABSTRACT It is well established that remote whole-limb rhythmic movement (e.g., cycling or stepping) induces suppression of the Hoffman (H-) reflex evoked in stationary limbs. However, the dependence of reflex amplitude on the phase of the movement cycle (i.e., phase-dependence) has not been consistent across this previous research. The authors investigated the phase-dependence of flexor carpi radialis (FCR) H-reflex amplitudes during active walking and in kinematically matched static postures across the gait cycle. FCR H-reflexes were elicited in the stationary forearm with electrical stimulation to the median nerve. Significant phase-dependent modulation occurred during walking when the gait cycle was examined with adequate phase resolution. The suppression was greatest during midstance and midswing, suggesting increased ascending communication during these phases. There was no phase-dependent modulation in static standing postures and no correlation between lower limb background electromyography levels and H-reflex amplitude during active walking. This evidence, along with previous research demonstrating no phase modulation during passive walking, suggests that afferent feedback associated with joint position and leg muscle activation levels are not the sole source of the phase modulation seen during active walking. Possible sources of phase modulation include combinations of afferent feedback related to active movement or central motor commands or both.

Biomechanical outcomes and neural correlates of cutaneous reflexes evoked during rhythmic arm cycling

During walking cutaneous stimulation of the foot yields neural and mechanical reflexes that serve a functional purpose to correct or assist the ongoing movement. Concurrently, while cutaneous stimulation of the hand during rhythmic arm movement parallel the neural responses observed in the legs, studies of rhythmic arm movement have only limited mechanical measurements. Therefore it is difficult to determine whether reflex responses in the arms during rhythmic arm movement serve a functional purpose similar to those seen in the lower limbs. The purpose of this study was to explore the mechanical outcomes of stimulating a cutaneous nerve innervating the hand during arm cycling. We hypothesized that there would be measurable mechanical effects to cutaneous stimulation during arm cycling that function to correct or assist the task of arm cycling. Specifically, kinetic responses measured at the handle would be considered assistive if they were tangential to the arm cycling path in the direction of forward progression. Also, limb kinematic responses would be considered corrective if they allowed limb movement that would result in removal of the limb from stimulus while not altering the kinetic profile at the handle necessary for arm cycling progression. Participants performed seated arm cycling while EMG was recorded from the arm and trunk muscles, kinematic data was recorded from the right arm, and kinetic data was recorded from the handle. Cutaneous reflexes were evoked by stimulating the superficial radial nerve. The results show that there are observable mechanical responses to cutaneous stimulation of the hand during arm cycling. Subjects responded to cutaneous stimulation of the hand during arm cycling with significant changes in backward and lateral forces at the handle as well as wrist abduction/adduction and wrist flexion/extension kinematics. These responses, related to the task and phase of movement, are consistent with the anatomical location of the stimulus and are correlated to the neural responses. Therefore, these responses are comparable to functionally relevant responses in the legs during rhythmic movement. However, while there is a single observation of a kinematic corrective strategy, the kinetics measured at the handle are not tangential to the arm cycling path and therefore not considered an assistive response. Therefore, unlike the observations in the lower limbs, the mechanical responses during arm cycling are not clearly related to the functional context of the ongoing task.

Human Cutaneous Reflexes Evoked with Simultaneous Multiple Nerve Stimulation during Rhythmic Locomotor-Like Arm and Leg Cycling in Stroke Subjects

The neural coupling between arms and legs during rhythmic activity has previously been investigated in healthy volunteers by cutaneous stimulation during concurrent arm and leg cycling. In this study, the coupling between arms and legs was investigated in 15 chronic stroke survivors with hemiplegia. Cutaneous reflexes were evoked by electrical stimulation of nerves innervating the foot of the most affected leg. Participants performed rhythmic cycling with the arms and legs during four trials with different stimulus configurations. Non-painful stimulations were delivered at the ankle level to the superficial peroneal, sural, tibial, and by stimulation of all three nerves simultaneously. EMG was recorded from four ipsilateral muscles (biceps femoris (BF), vastus lateralis (VL), medial gastrocnemius (MG), and tibialis anterior (TA) in the more affected leg. Phase-dependency was tested by recording reflexes at four different equally spaced phases of the locomotor cycle. Large excitatory VL reflexes were seen in the relaxation phase of the stimulated leg. For BF, an inhibitory reflex was observed in the power phase while an excitatory reflex was observed in the relaxation phase of the cycle. In the TA muscle, significantly larger reflexes were observed when all three nerves were stimulated simultaneously compared to stimulation of the three nerves individually (ANOVA P<0.05, post-hoc P<0.05). No differences were found between reflexes evoked by the three nerves in any of the muscles. No ANOVA interaction between phases and nerves was observed in any of the ANOVA analysis. Central non-linear summation of afferent non-nociceptive input occurs within the spinal reflex circuits after stroke. Sensory integration in reflex pathways could be exploited in gait rehabilitation after stroke by supporting ankle dorsi-flexion and thereby facilitating the relearning of gait in the subacute phase.

Comparing executive function, evoked hemodynamic response, and gait as predictors of variations in mobility for older adults

ABSTRACT Objective: Falls represent a major concern for older adults and may serve as clinically salient index events for those presenting in the prodromal stages of mild cognitive impairment. Declines in executive function performance and in gait consistency have shown promise in predicting fall risk; however, associated neurophysiological underpinnings have received less attention. In this study, we used a multimodal approach to assess fall risk in a group of older adults with and without a previous fall history. Method: Processing speed, inductive reasoning, verbal fluency, crystallized ability, episodic memory, and executive functioning were assessed using standardized neuropsychological tests. Cognitive interference was assessed using the Multi-Source Interference Task. Spatiotemporal gait parameters were assessed with and without cognitive load using a 6.4-m instrumented walkway. Hemodynamic responses were measured using functional near-infrared spectroscopy. Results: Whereas no group differences were observed in cognitive behavioral performance, during a cognitive interference task fallers displayed more oxygenated hemoglobin across the prefrontal cortex than nonfallers, suggesting that engaging in the cognitive task was more effortful for them overall, therefore eliciting greater cortical activation. Between-group differences in spatial as well as temporal gait parameters were also observed. Conclusions: These results are in keeping with assertions that diminished executive control is related to fall risk. Notably, the group differences observed in prefrontal cortical activation and in gait parameters may ultimately precede those observed in cognitive behavioral performance, with implications for measurement sensitivity and early identification.

Age-related erosion of obstacle avoidance reflexes evoked with electrical stimulation of tibial nerve during walking

In young healthy adults, characteristic obstacle avoidance reflexes have been demonstrated in response to electrical stimulation of cutaneous afferents of the foot during walking. It is unknown whether there is an age-related erosion of this obstacle avoidance reflex evoked with stimulation to the tibial nerve innervating the sole of the foot. The purpose of this study was to identify age-dependent differences in obstacle avoidance reflexes evoked with electrical stimulation of the tibial nerve at the ankle during walking in healthy young and older (70 yr and older) adults with no history of falls. Toe clearance, ankle and knee joint displacement and angular velocity, and electromyograms (EMG) of the tibialis anterior, medial gastrocnemius, biceps femoris, and vastus lateralis were measured. A significant erosion of kinematic and EMG obstacle avoidance reflexes was seen in the older adults compared with the young. Specifically, during swing phase, there was reduced toe clearance, ankle dorsiflexion, and knee flexion angular displacement in older adults compared with the young as well as changes in muscle activation. These degraded reflexes were superimposed on altered kinematics seen during unperturbed walking in the older adults including reduced toe clearance and knee flexion and increased ankle dorsiflexion compared with the young. Notably, during mid-swing the toe clearance was reduced in the older adults compared with the young by 2 cm overall, resulting from a combination of 1-cm reduced reflex response in the older adults superimposed on 1-cm less toe clearance during unperturbed walking. Together, these age-related differences could represent the prodromal phase of fall risk. NEW & NOTEWORTHY This study demonstrated age-dependent erosion of obstacle avoidance reflexes evoked with electrical stimulation of the tibial nerve at the ankle during walking. There was significant reduction in toe clearance, ankle dorsiflexion, and knee flexion reflexes as well as changes in muscle activation during swing phase in older adults with no history of falls compared with the young. These degraded reflexes, superimposed on altered kinematics seen during unperturbed walking, likely represent the prodromal phase of fall risk.

Difference scores between single-task and dual-task gait measures are better than clinical measures for detection of fall-risk in community-dwelling older adults

BACKGROUND As the proportion of older adults in the population increases, so does the associated prevalence of falls, making falls the leading cause of fatal and nonfatal injuries among adults aged ≥65 years. In response, researchers and clinicians seek to develop a clinical tool that accurately predicts fall risk. These Investigations have included measures of clinical mobility and balance tests, strength, physiologically based tests, postural sway, and mean and variability of gait measures. To date, no study has concurrently explored all these measures to determine which measures, alone or in combination, emerge as the most predictive of fall risk. While there is evidence that dual-task gait conditions are sensitive indicators of fall risk, difference scores between dual-task and single-task gait conditions (DS) have not been explored. RESEARCH QUESTION This study included outcome measures representing diverse domains (clinical mobility and balance, strength, physiological, postural sway, and mean and variability of difference scores between dual- and single-task gait conditions) to determine the combination of measures that were the most sensitive for retrospectively classifying fallers from non-fallers. METHODS Forty-two (mean: 75.8 yrs ± 3.3) community-dwelling older adults completed a comprehensive battery of 76 measures and classified into two groups based on self-report of having one or more falls in the previous year. RESULTS Results suggest that 11 measures captured the salient characteristics of the total cohort (fallers (N = 27) and non-fallers (N = 15) and that five gait measures were sufficient for correctly classifying fallers and non-fallers with 92.3% sensitivity and 66.7% specificity with a total model classification of 82.9%. SIGNIFICANCE The five variables comprise mean DS of stride timing, stride width, and stride length and DS in variability for stride width and stride velocity demonstrating that difference in performance between dual-task and single-task gait trials was essential for discriminating fallers and superior to other measures.