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

Postural displacement induced by electrical stimulation; A new approach to examine postural recovery

Background

Controlling upright posture entails acute adjustments by the neuromuscular system to keep the center of mass (COM) within the limits of a relatively small base of support. Sudden displacement of the COM triggers several strategies and balance recovery mechanisms to prevent excessive COM displacement.

New method

We have examined and quantified a new approach to induce an internal neuromuscular perturbation in standing posture on 15 healthy individuals to provide an insight into the mechanism of loss of balance (LOB). The method comprises eliciting an H-reflex protocol while subjects are standing which produces a contraction in soleus and gastrocnemius muscles. We have also defined analytical techniques to provide biomarkers of balance control during perturbation. We used M-Max unilaterally or bilaterally and induced a forward or sideway perturbation. The vector analysis and the Equilibrium Point calculations defined here can quantify the amplitude, direction, and evolution of the perturbation.

Results

Clear patterns of loss of balance due to stimulation was observed. Compared to quiet standing, the density of the EPs substantially increased in the perturbation phase. Leftward stimulation produced significantly higher number of EPs compared to the bilateral stimulation condition which could be due to the fact that the left leg was the nondominant side in all our subjects.

Comparison and conclusion

In this study we provide a proof-of-concept technique for examining recovery from perturbation. The advantage of this technique is that it provides a safe perturbation, is internally induced at the spinal cord level, and is free from other factors that might complicate the recovery analysis (e.g., locomotion and the integration of the spinal pattern generator and cutaneous pathways in mediating changes). We have shown that the perturbation induced by this method can be quantified as vectors. We have also shown that the density of instantaneous equilibrium points (EPs) could be a good biomarker for defining and examining the perturbation phase. Thus, this protocol and analysis provides a unique individual assessment of recovery which can be used to assess interventions. Finally, given that the maximal motor response is used as the perturbation (e.g., M-max) it is highly reliable and reproducible within an individual patient.

The inflow of sensory information for the control of standing is graded and bidirectional

The control of upright standing is accomplished through the integration of different sources of sensory information and by providing an appropriate motor program to control both expected and unexpected perturbations imposed on the system. However, the dynamic characteristics of postural sway and its interplay with the regulation of Ia sensory information within the spinal cord are largely unknown. Here, using a stochastic technique for analyzing the dynamics of upright standing, we demonstrate that the changes in the dynamics of postural sway were accompanied by modulation of the soleus H-reflex during quiet standing. While the causality of this relation was not established, the results showed that these changes were independent of the sway of the center of pressure and were bidirectional and purposeful. With this novel perspective, the appropriate reflex gain, which is important for balance control, can be predicted from the dynamic characteristics of postural sway. Our current findings provide the first human behavioral evidence to suggest the contribution of the spinal cord in fulfilling the desired motor programming of a complex task. This contribution is, by conventional guess, carried out through interneuronal adjustments, which are under the control of different brain areas.

Long-term effect of prednisolone on functional blink recovery after transient peripheral facial motor paralysis

OBJECTIVE

To determine the functional recovery in patients with severe transient peripheral facial motor paralysis (Bell palsy).

STUDY DESIGN

Prospective controlled trial.

SETTING

Academic medical center.

SUBJECTS AND METHODS

Blink recovery was studied in 2 groups of severely affected Bell palsy patients during a follow-up period of 84 weeks. The patients in one group received prednisolone within the first week after the onset of symptoms. No medication was given to the other group. A control group of healthy subjects was also included. Simultaneous orbicularis oculi muscle activity and eyelid kinematics were recorded by surface electromyographic (EMG) recording and eyelid search coils, respectively.

RESULTS

At the beginning of the paralysis, very little integrated orbicularis oculi muscle activity and eyelid movement was measured at the palsied side of the face. Thirteen weeks later, the integrated orbicularis oculi EMG and functional blink recovery gradually improved until 39 weeks. Beyond, only the integrated orbicularis oculi EMG slightly increased. At 84 weeks, the integrated orbicularis oculi EMG was significantly larger in the prednisolone group compared with the control group. The integrated EMG of the nonmedicated group recovered to normal values. Curiously enough, the functional blink recovery at the palsied side remained reduced to 64% compared with the healthy controls in the prednisolone-treated group and to 36% in the nonmedicated group.

CONCLUSION

The authors demonstrate that prednisolone significantly increased the orbicularis oculi muscle activity and significantly improved functional blink recovery in severely affected Bell palsy patients. However, the increase of muscle activity was insufficient to restore functional blinking to normal values.