Reciprocal inhibition versus unloading response during stretch reflex in humans.

Rotation of an upper limb joint produces excitatory stretch reflex peaks M1 and M2 in the stretched muscles and simultaneous decrease in electromyographic (EMG) activity in the shortened muscles. The objective of this study was to examine whether the decreased activity in the antagonists (rINHIB) is purely from unloading of the spindles or receives active inhibition involving inhibitory interneurons. If rINHIB is due only to unloading, then the termination of rINHIB should vary with the duration of perturbation used to elicit stretch reflex, namely shorter stretches should result in shorter values of decreased periods of EMG. To examine this question, rectangular pulses, ranging in duration from 25 to 150 ms, were used to stretch wrist flexors or extensors with a torque motor. These rectangular pulses resulted in joint rotations which peaked at times (T(peak)) ranging from approximately 75-160 ms. As shown by previous authors, when the duration of rotation was shortened, the magnitude of M1 did not change, while the magnitude of M2 decreased. However, termination time of rINHIB in the shortened muscles did not change with change in T(peak), implying thereby that unloading of spindles of the antagonist muscles is not the only mechanism for the reduction in activity and that inhibitory reflex pathways most likely contribute. Possible sources of inhibition are discussed for the short- and long-latency inhibition.

Exploratory behavior during stance persists with visual feedback.

Recent evidence showing center of pressure (COP) displacements increase following an external stabilization of the center of mass (COM) supports the theory that postural sway may be exploratory and serve as a means of acquiring sensory information. The aim of the current study was to further test this theory and rule out potential confounding effects of sensory illusions or motor drift on prior observations. Participants stood as still as possible in an apparatus which allowed movements of the COM to be stabilized ("locked") without subject awareness, and they were provided real-time visual feedback of their COM or COP throughout the trial. If there was an influence of sensory illusions or motor drift, we hypothesized that the change in COP displacement with locking would be reduced when participants were provided visual confirmation of COM stabilization (COM feedback), or when they were aware of the position of the COP throughout the trial (COP feedback). Confirming our previous results, increases in COP displacement were observed when movements of the COM were stabilized. In addition, our results showed that increases in COP displacement could not be explained by the presence of sensory illusions or motor drift, since increases in COP were observed despite being provided convincing evidence that the COM had been stabilized, and when participants were aware of their COP position throughout the trial. These results provide further support for an exploratory role of postural sway. The theoretical basis of current clinical practices designed to deal with balance control deficits due to age or disease is typically based on the opinion that increases in sway are a consequence of a failing balance control system. Our results suggest that this may not be the case, and if sway is in fact exploratory, a serious re-evaluation of current clinical practices may be warranted.

Balance problems with Parkinson's disease: are they anxiety-dependent?

Non-motor symptoms, such as fear of falling and anxiety, are frequently reported in Parkinson's disease (PD). Recent evidence of anxiety and fear directly influencing balance control in healthy young and older adults, raises the question whether fear of falling and anxiety also directly contribute to the balance deficits observed in PD. The goal of the current study was to examine whether PD patients and controls responded similarly or differently to experimentally induced increases in anxiety. For this purpose, 14 PD patients (tested during a subjective optimal ON state) and 16 healthy age-matched control subjects stood in three conditions of different levels of postural threat: normal threat (quiet standing at ground level); medium threat (standing at the edge of a surface elevated to 80 cm); and high threat (same, but to 160 cm). Outcome measures included mean position, mean power of frequency (MPF) and root mean square (RMS) of centre of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions. Physiological and psychosocial measures of fear and anxiety were also recorded. Increased threat changed postural control similarly in PD patients and controls; MPF of AP and ML COP increased and the mean COP position was shifted backward in both groups. These results indicate that during the ON state, static balance in PD patients and controls is equally susceptible to the influence of anxiety. Significant correlations observed between COP changes and measures of fear and anxiety provide evidence to support the proposed neural links between structures controlling emotion and postural control. Future studies should further address this issue by including more severely affected patients, by testing the influence of dopaminergic medication, by including more anxious patients, and by using dynamic measures of balance.

Shifting the balance: evidence of an exploratory role for postural sway.

Humans and other species are unable to stand perfectly still; their bodies continuously sway during stance even during concentrated efforts to avoid such movement. Traditionally, this phenomenon has been viewed as an inability of the central nervous system (CNS) to maintain perfect equilibrium because of its reliance on feedback from sensory signals to control corrective ground-reaction forces. Using a novel method to minimize movements of the body during stance without subject awareness, we have made the unique discovery that ground-reaction forces are generated independent of body sway, as evidenced by observations of increased centre of pressure variability when postural sway is minimized experimentally. Contrary to traditional views, our results suggest that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems. This novel paradigm has the potential to significantly shift long-standing views on balance, and questions the theoretical basis behind conventional treatment strategies for balance deficits associated with age and disease.

Recovery of human motoneurons during rotation.

During prolonged contractions, few studies have reported rotation among low threshold motoneurons. The question arises whether a motoneuron stops firing due to an increase in firing threshold or whether it is due to regional switching of activity among muscle fascicles. We postulated that if the rest period resulted from an increase in firing threshold, a progressive recovery in the excitability of the motoneuron would be observed during the rest period. The excitability of soleus or tibialis anterior motoneurons was tested during the rest periods. The results showed that a previously tonic motoneuron that had dropped off during rotation, rarely responded to Ia or TMS inputs in the initial parts of the rest period; however, its response probability increased significantly in the second half. Based on these data, we suggest that the observed rotation is due to changes in firing thresholds of motoneurons during prolonged firing.