Koichi Kitano

Spinal reflex in human lower leg muscles evoked by transcutaneous spinal cord stimulation.

The H-reflex is one of the most common and useful techniques in the field of motor control. However, the H-reflex technique also involves difficulty in data interpretation when stimulus intensity is high enough to stimulate both sensory and motor fibers (antidromic current). On the other hand, transcutaneous stimulation applied on the spinous processes is able to stimulate the dorsal root, resulting in selective stimulation of only sensory fibers without evoking a direct motor response and antidromic current on the motor fibers. The purpose of this study was to examine the maximal reflex response that can be elicited in the lower leg muscles using transcutaneous spinal stimulation. Seven subjects participated in the study. EMG signals were recorded from triceps surae (SOL, MG, LG) in the prone position. Transcutaneous stimulation was applied both to the spinous process (between T11 and T12, spinal stimulation, SS) and to the popliteal fossa (peripheral stimulation, PS). Using SS and PS, H(max) amplitudes of triceps surae muscles were measured and standardized with M(max). H(max) values in MG and LG by SS (31% and 41%) were significantly greater than those by PS (20% and 23%, respectively). Although not significant, H(max) amplitude in SOL by SS (76%) was also greater than that by PS (60%). It is suggested that transcutaneous stimulation is able to evoke H-reflex without a direct motor response. H(max) amplitudes traditionally measured by stimulation applied to a mixed nerve may underestimate the potential connectivity between the sensory and motor systems in humans.

Role of vision and task complexity on soleus H-reflex gain

There exists extensive evidence supporting the presence of reflex modulation in humans during a variety of motor tasks. The soleus H-reflex has been shown to be modulated during static and dynamic balance conditions as well as during various motor tasks. The purpose of this study was to examine the effects of two different stance positions and visual conditions on soleus H-reflex gain in 15 apparently healthy adults (mean age=30.27+/-6.92 yrs). The soleus H-reflexes were examined in two experimental stance conditions: two-legged (stable) and one-leg (unstable), and two visual conditions: eyes open and eyes closed. To assess the reflex gain, subjects performed ten trials under each of the four conditions and a soleus H-reflex was elicited during the performance of each trial. For each condition the peak-to-peak amplitude of the H-reflex and the EMG activity 50 ms prior to the stimulus was recorded. Differences in the peak-to-peak amplitudes of the soleus H-reflex for the experimental conditions were compared with a 2x2 (Stance x Vision) repeated measures ANOVA. The level of significance was p<0.05. Results demonstrated significant differences in reflex gain for both the vision (F(l,15)=4.87, p<0.05) and the stance condition (F(l,15)=14.86, p<0.05). Although both the stance condition and vision significantly affected the H-reflex gain, there was no interaction between these two variables (F(l,15)=0.17). From these results, we conclude that H-reflex gain was decreased both as stance complexity increased and as visual inputs were removed. Consistent with previous reports, it may be speculated that changes in presynaptic inhibition to the soleus Ia fibers regulate these gain changes. We propose that vision and stability of stance affect soleus H-reflex gain, but do so without any interactive effects.

Differential control of H-reflex amplitude in different weight-bearing conditions in young and elderly subjects.

OBJECTIVE This study measured the modulation of conditioned (femoral nerve, paired-stimuli) and unconditioned soleus H-reflexes in young and elderly subjects when changing weight-bearing (WB) requirements and body position. METHODS Conditioned and unconditioned H-reflexes were examined in 14 elderly subjects and 11 young subjects during six different WB conditions: (1) lying supine with no WB, (2) supine position inclined by 30° with 50% WB, (3) standing with 50%, (4) 75%, (5) 100% and (6) 125% WB. RESULTS The elderly subjects had consistently higher background soleus EMG activity across the WB conditions compared to the young. Femoral nerve conditioning caused facilitation of the H-reflex that changed across WB conditions in the young subjects, but not in the elderly subjects. Finally, elderly subjects had less depression with paired-stimulation (PRD) across WB conditions, which was not observed in the young subjects. CONCLUSIONS The elderly may have more direct activation of motoneurons from descending pathways, coupled with less segmental spinal control of inhibitory interneurons, as evidenced by the increased background soleus activity, H/M-max ratios and the lack of modulatory control observed when conditioning the H-reflex. SIGNIFICANCE There was an age-specific response from descending and segmental pathways during conditions that involved either different WB requirements or changes in body position.

Amplification of background EMG activity affects the interpretation of H-reflex gain

In many H-reflex studies, the modulation of the H-reflex is usually compared relative to the normal EMG activity within the muscle. Such comparisons enable the investigators to infer whether the change in the amplitude of the H-reflex was independent of normally occurring muscle activity. This interpretation of the H-reflex is regarded as H-reflex gain, a popular dependent variable in human H-reflex studies. However, in many studies to date, the muscle activity level has been determined from the same EMG signal from which the H-reflex is recorded. This leads to an important methodological consideration: measuring the ongoing normal EMG activity from the same signal might result in an inaccurate measurement, since this EMG signal will need to be minimally amplified to capture the synchronous volley of the H-reflex amplitude. In this study we examined this possibility and found that comparing the EMG activity level from the seated position to standing position yields different results (on average 8.03% in the measurement of the increase of muscle activity). This difference was both dependent on the task and also on the EMG instrumentation used. To solve this problem we suggest the bifurcation of the EMG signal from the recording electrodes with differential amplification of the signal. With this method, both the naturally occurring muscle activity and the H-reflex signal are collected from the same area of the muscle and a more accurate measurement of the H-reflex gain will be yielded.

The effect of blood flow on h-reflex and motor responses in adults with type 2 diabetes.

To examine the effect of blood flow on peripheral nerve function in adults with type 2 diabetes. Ten adults with type 2 diabetes and 10 age- and body mass index-matched controls participated in the study. Soleus H-reflex and motor recruitment curves were determined for subjects at baseline, during 10 minutes of ischemia by femoral artery occlusion, and after a 10-minute bout of leg exercise. At baseline, the H-reflex threshold occurred at 77.44% ± 1.12% and 92.23% ± 0.04% (mean ± SE) of motor threshold at baseline for the diabetes and control groups, respectively. During ischemia, the H-reflex threshold occurred at 72.44% ± 7.19% and 88.79% ± 7.80% of motor threshold at baseline for the diabetic and control groups, respectively. Following exercise, the H-reflex threshold occurred at 64.44% ± 8.47% and 94.93% ± 4.30% of motor threshold at baseline for the diabetes and control groups, respectively, which significantly differed from baseline. Hmax/Mmax ratios were significantly reduced with acute ischemia for the diabetes group (25.6%). Postexercise, the Hmax/Mmax ratio returned to baseline for controls while remaining significantly lower than baseline in the diabetes group. H-reflex and motor responses were hypoexcitable with an accompanying hyperemia for controls. In the diabetes group, the postexercise period resulted in a hyperexcitable H-reflex and motor response. Acute ischemia decreases H-reflex thresholds in adults with diabetes and exercise-induced increases in blood flow further decrease these thresholds. These responses are markedly more pronounced than those of their peers without diabetes and are accompanied by changes in sensory transmission to the motoneuron.

Acute hypercapnia does not alter voluntary drive to the diaphragm in healthy humans

Although systemic hypercapnia is a common outcome of pulmonary disease, the relationship between hypercapnia and voluntary diaphragmatic activation (VAdi) is unclear. To examine whether hypercapnia independent of ventilatory work contributes to reduced central motor drive to the diaphragm in healthy humans, 14 subjects spontaneously breathed room air (NN) or a hypercapnic gas mixture (HH; 7% CO2 with air) while at rest. Thereafter, subjects volitionally hyperventilated room air (NH) matching the minute ventilation recorded during HH while maintained at eucapnic levels. Twitch interpolation with bilateral magnetic stimulation of phrenic nerves at functional residual capacity was used to assess VAdi during the three trials. Although PETCO2 was elevated during HH compared with NN and NH (52 vs 36 mmHg), VAdi was not altered across the trials (HH = 93.3 ± 7.0%, NN = 94.4 ± 5.0%, NH = 94.9 ± 4.6%, p = 0.48). Our findings indicate that the magnitude of hypercapnia acutely imposed may not be effective in inhibiting voluntary neural drives to the diaphragm in normal resting individuals.

Comparison of H-reflex and reciprocal inhibition between running footfall patterns

Bus, S.A., Waaijman, R., Arts, M., de Haart, M., Busch-Westbroek, T., van Baal, J., & Nollet, F. (2013). Effect of custom-made footwear on foot ulcer recurrence in diabetes: A multicenter randomized controlled trial. Diabetes Care, 36, 4109 4116. Chen, W. -M., Park, J., Park, S. -B., Shim, V.P. -W., & Lee, T. (2012). Role of gastrocnemius soleus muscle in forefoot force transmission at heel rise a 3D finite element analysis. Journal of Biomechanics, 45, 1783 1789. Petre, M., Erdemir, A., Panoskaltsis, V.P., Spirka, T.A., & Cavanagh, P.R. (2013). Optimization of nonlinear hyperelastic coefficients for foot tissues using a magnetic resonance imaging deformation experiment. Journal of Biomechanical Engineering, 135, 61001 61012. Spirka, T.A., Erdemir, A., Spaulding, S.E., Yamane, A., Telfer, S., & Cavanagh, P.R. (2014). Simple finite element models for use in the design of therapeutic footwear. Journal of Biomechanics, 47, 2948 2955. Ulbrecht, J.S., Hurley, T., Mauger, D.T., & Cavanagh, P.R. (2014). Prevention of recurrent foot ulcers with plantar pressure-based in-shoe orthoses: The CareFUL Prevention Multicenter randomized controlled trial. Diabetes Care, 37, 1982 1989.

Independent segmental inhibitory modulation of synaptic efficacy of the soleus H-reflex

Synaptic efficacy associated with muscle spindle feedback is partly regulated via depression at the Ia-motorneuron synapse through paired reflex depression (PRD) and presynaptic inhibition (PI). The purpose of this study was to examine PRD and PI of the soleus H-reflex at rest and with a background voluntary muscle contraction. The experiment was conducted on 10 healthy males with no history of neurological deficits. Soleus H-reflex and M-wave curves were elicited in three conditions: unconditioned, PRD (two consecutive H-reflexes with 100 ms interval), and PI (1.2 × MT to tibialis anterior 100 ms prior to soleus H-reflex). Each condition was tested at rest and with a 10% soleus contraction. PRD and PI both produced a pronounced inhibition to the soleus motor pool at rest, with a significant difference observed between threshold values (78.9, 89.3, and 90.4% for unconditioned, PRD, and PI reflexes, respectively). During the voluntary contraction the threshold for both inhibitory mechanisms was significantly reduced, and were not different from the unconditioned H-reflex (74.5, 78.9, and 77.0% for unconditioned, PRD, and PI reflexes, respectively). The slope of PI and the PI Hmax/Mmax ratio were significantly altered during contraction whereas no differences were observed for PRD. The results suggest these inhibitory mechanisms depend on the interaction between background voluntary activation and stimulus intensity. This behavior of these inhibitory mechanisms underscores the specificity of spinal circuitry in the control of motor behaviors.