Threshold of the soleus muscle H-reflex is less sensitive to the change in excitability of the motoneuron pool during plantarflexion or dorsiflexion in humans

Functional principal component analysis of H-reflex recruitment curves

The primary purpose of this study was to use functional principal component analysis (FPCA) to analyze Hoffman-reflex (H-reflex) recruitment curves. Smoothed and interpolated recruitment curves from 38 participants were used for analysis. Standard methods were used to calculate three discrete variables (i.e., H(max)/M(max) ratio, H(th), H(slp)). FPCA was then used to extract principal component functions (PCFs) from the processed recruitment curves. PCF scores were calculated to determine how much each PCF contributed to an individuals' recruitment curve. The analysis extracted three PCFs, and three sets of PCF scores. Correlation analyses and systematic variation in the PCF scores indicated that the scores for the first PCF were primarily correlated to H-reflex threshold (H(th)) and that the scores for the second and third PCFs were correlated to H-reflex magnitude (H(max)/M(max) ratio) and slope (H(slp)), respectively. In addition, results from the FPCA indicated that the first PCF explained 56.0% of the variance between all H-reflex recruitment curves, whereas the second and third PCFs explained 24.1% and 13.0%, respectively. The high correlations indicate FPCA-derived PCFs capture similar physiological information as the standard discrete variables and suggest that application of FPCA to H-reflex recruitment curves could be used in future studies to complement traditional analyses that investigate excitability of the motoneuron pool.

Intersession reliability of Hoffmann reflex gain and presynaptic inhibition in the human soleus muscle

Hayes BT, Hicks-Little CA, Harter RA, Widrick JJ, Hoffman MA. Intersession reliability of Hoffmann reflex gain and presynaptic inhibition in the human soleus muscle.

OBJECTIVE

To determine the day-to-day reliability of Hoffmann reflex (H-reflex) gain and presynaptic inhibition of spinal reflexes in the human soleus muscle.

DESIGN

Controlled trial.

SETTING

Research laboratory.

PARTICIPANTS

Volunteers (N=30; mean +/- SD age, 23.4+/-3.9y; height, 175.64+/-10.87cm; mass, 84.50+/-24.18kg) with no history of lower extremity pathology and/or injury participated.

INTERVENTIONS

Subjects lay prone with the head, shoulders, arms, and hips supported in a static position by a massage body pillow and the ankle positioned at 90 degrees . Recording electrodes were placed over the soleus and tibialis anterior muscle bellies, and the stimulating electrodes were positioned over the tibial nerve in the popliteal space and the common peroneal nerve near the fibular head.

MAIN OUTCOME MEASURES

The H-reflex and motor wave recruitment curves were then measured and recorded. Presynaptic inhibition was also assessed in the soleus muscle, and a conditioning stimulation of the common peroneal nerve (1 x motor threshold = motor threshold) was used prior to soleus H-reflex measurement. Two testing sessions took place between 2 and 7 days, and each session occurred at the same time of day.

RESULTS

Assessments of H-reflex gain and presynaptic inhibition yielded test-retest reliability of R equal to . 95 and .91, respectively.

CONCLUSIONS

Measures of presynaptic inhibition and H-reflex gain (H slope/M slope) in the human soleus muscle are consistent and reliable day to day.

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.

Standardization of H-reflex analyses

Variability in the H-reflex can make it difficult to identify significant changes using traditional pooled analysis techniques. This study was undertaken to introduce a normalisation approach to calculate both the relative size and the relative stimulus intensity required to elicit the H-reflex response so that comparisons can be made not only with results obtained during different experimental session but also between different subjects. This normalisation process fits the size of the measured M-responses and H-reflexes over the entire stimulus range with model curves to better facilitate the calculation of important parameters. This approach allows normalisation of not only the size of the response but also the relative stimulus intensity required to elicit the response. This eases the comparison of the reflex responses under various situations, and is capable of bringing out any genuine differences in the reflex in a reliable manner not previously possible. This study illustrates that comparison of the reflex between days is problematic, even in the same subject, as both the reflex size and the relative stimulus intensity required to obtain this reflex changed in all subjects. We suggest that H-reflex studies need to use normalisation not only for size of the reflex but also for the stimulus intensity, and also that all experiments for a single subject should be performed in the same session or during the same day using some level of background muscle activity in the muscle concerned as the variability of the muscle at rest was found to be larger.

Effects of wrist position and contraction on wrist flexors H-reflex, and its functional implications

In order to determine whether joint position exerts a powerful influence on length-tension regulation in multiarticulate wrist flexors, three wrist positions (neutral, flexion and extension) and four levels of flexor contraction [0%, 10%, 20% and 30% maximum voluntary contraction (MVC)] were manipulated. There were significant differences in H-reflex amplitudes according to wrist positions and levels of flexor contraction. H-reflex increased linearly as a function of contraction in all three wrist positions. H-reflex was consistently larger in the wrist flexion than in the wrist extension position. The strength of the relationship (omega2) indicated that wrist position had a greater effect on H-reflex than force of muscle contraction. The interaction between wrist flexors contraction and joint position was significant only in the wrist flexion position. Trend analysis showed that, in the wrist flexion position, a low level of contraction was sufficient to maximally facilitate the H-reflex; however, a quadratic component was seen at higher contraction levels. The above findings may reflect the length-tension relationship of the multiarticulate wrist flexors. Therefore, this paper will discuss the functional implications related to the larger H-reflex in flexion position and the depressed H-reflex in the wrist extension position.

Motoneuron pool excitability of hemiplegic patients: assessing recovery stages by using H-reflex and M response

OBJECTIVES

To compare the excitability of the motoneuron pools of both the spastic and the unimpaired sides of patients with hemiplegia with a new method by using H-reflexes and M responses. The method determines the ratio of the developmental slope of the H-reflex (Hslp) to the slope of the M response (Mslp). We also examined the relation between the Brunnstrom stages and the Hslp/Mslp.

DESIGN

Experiment.

SETTING

Electrophysiologic experimental laboratory in a Japanese medical school.

PATIENTS

Fifteen hemiplegic patients (9 men, 6 women; age range, 48-71yr; mean, 60yr) with spasticity caused by a stroke.

INTERVENTIONS

The subject was instructed to relax while seated in a reclining chair with his foot fixed on an immobile pedal. After the soleus H-reflex and M responses on one side were recorded, the same experimental procedures were carried out on the other side.

MAIN OUTCOME MEASURES

Hslp/Mslp Brunnstrom stages.

RESULTS

Hslp/Mslp had better predictive value than conventional indicators of motoneuron pool excitability. Hslp/Mslp appeared to be a better match for the bell-shaped pattern of the Brunnstrom stages.

CONCLUSION

Hslp/Mslp is the preferred index for evaluating the motoneuron pool excitability of the spastic side of hemiplegic patients.

Neural influences on sprint running: training adaptations and acute responses

Performance in sprint exercise is determined by the ability to accelerate, the magnitude of maximal velocity and the ability to maintain velocity against the onset of fatigue. These factors are strongly influenced by metabolic and anthropometric components. Improved temporal sequencing of muscle activation and/or improved fast twitch fibre recruitment may contribute to superior sprint performance. Speed of impulse transmission along the motor axon may also have implications on sprint performance. Nerve conduction velocity (NCV) has been shown to increase in response to a period of sprint training. However, it is difficult to determine if increased NCV is likely to contribute to improved sprint performance. An increase in motoneuron excitability, as measured by the Hoffman reflex (H-reflex), has been reported to produce a more powerful muscular contraction, hence maximising motoneuron excitability would be expected to benefit sprint performance. Motoneuron excitability can be raised acutely by an appropriate stimulus with obvious implications for sprint performance. However, at rest H-reflex has been reported to be lower in athletes trained for explosive events compared with endurance-trained athletes. This may be caused by the relatively high, fast twitch fibre percentage and the consequent high activation thresholds of such motor units in power-trained populations. In contrast, stretch reflexes appear to be enhanced in sprint athletes possibly because of increased muscle spindle sensitivity as a result of sprint training. With muscle in a contracted state, however, there is evidence to suggest greater reflex potentiation among both sprint and resistance-trained populations compared with controls. Again this may be indicative of the predominant types of motor units in these populations, but may also mean an enhanced reflex contribution to force production during running in sprint-trained athletes. Fatigue of neural origin both during and following sprint exercise has implications with respect to optimising training frequency and volume. Research suggests athletes are unable to maintain maximal firing frequencies for the full duration of, for example, a 100m sprint. Fatigue after a single training session may also have a neural manifestation with some athletes unable to voluntarily fully activate muscle or experiencing stretch reflex inhibition after heavy training. This may occur in conjunction with muscle damage. Research investigating the neural influences on sprint performance is limited. Further longitudinal research is necessary to improve our understanding of neural factors that contribute to training-induced improvements in sprint performance.

Inhibition of the soleus H-reflex during dorsiflexion is dependent on individual differences in maximal soleus H-reflex as a test reflex

The quantitative differences among individuals in the natural reciprocal inhibition of the soleus H-reflex during dorsiflexion were examined, in conjunction with the maximal H-reflex as the test reflex size in each individual. Maximal H-reflex was expressed relative to the maximal M-response (H(max)) when compared among individuals. Analysis showed that with increases in H(max) at rest in each individual, the inhibitory effect was first enhanced, then reached a peak, and was finally alleviated. This pattern was similar to the intraindividual pattern of the inhibitory effect induced by specific conditioning stimulus as a function of the test reflex size.

Evident difference in the excitability of the motoneuron pool between normal subjects and patients with spasticity assessed by a new method using H-reflex and M-response

The excitability of the motoneuron (MN) pool in the resting state was compared between normal control subjects and patients with spasticity resulting from HTLV-I-associated myelopathy, using a new parameter, Hslp/Mslp, and the conventional parameters Hmax/Mmax and Hth/Mth. Differences in the excitability of the MN pool between these two groups reached a high degree of statistical significance only when assessed with the new parameter. This suggests the methodological advantage of the Hslp/Mslp over both Hmax/Mmax and Hth/Mth for evaluation of the excitability of the MN pool in the resting state.