Short latency stretch reflexes depend on the balance of activity in agonist and antagonist muscles during ballistic elbow movements

The spinal stretch reflex is a fundamental building block of motor function, modulating sensitivity across tasks to augment volitional control. Stretch reflex sensitivity can vary continuously during movement and changes between movement and posture. While there have been many demonstrations of reflex modulation and investigations into the underlying mechanisms, there have been few attempts to provide simple, quantitative descriptions of the relationship between the volitional control and stretch reflex sensitivity throughout tasks that require the coordinated activity of several muscles. Here we develop such a description and use it to test the hypothesis that the modulation of stretch reflex sensitivity during movement can be explained by the balance of activity within the relevant agonist and antagonist muscles better than by the activity only in the muscle homonymous with the elicited reflex. We applied continuous pseudo-random perturbations of elbow angle as subjects completed approximately 500 movements in elbow flexion and extension. Measurements were averaged across the repeated movements to obtain continuous estimates of stretch reflex amplitude and background muscle activity. We also ran a control experiment on a subset of subjects performing postural tasks at muscle activity levels matched to those measured in the movement task. For both experiments, we assessed the relationship between background activity in the agonist and antagonist muscles controlling elbow movement and the stretch reflexes elicited in them. We found that modulation in the stretch reflexes during movement can be described by modulation of the background activity in the agonist and antagonist muscles, and that models incorporating agonists and antagonists are significantly better than those considering only the homonymous muscle. Increases in agonist muscle activity enhanced stretch reflex sensitivity whereas increases in antagonist activity suppressed reflex activity. Surprisingly, the magnitude of these effects was similar, suggesting a balance of control between agonists and antagonist that is very different than the dominance of sensitivity to agonist activity during postural tasks. This greater relative sensitivity to antagonist background activity during movement is due to a large decrease in sensitivity to homonymous muscle activity during movement rather than substantial changes in the influence of antagonist muscle activity.

Reducing the Soleus Stretch Reflex With Conditioning: Exploring Game- and Impedance-Based Biofeedback

People with spasticity, i.e., stretch hyperreflexia, have a limited functional independence and mobility. While a broad range of spasticity treatments is available, many treatments are invasive, non-specific, or temporary and might have negative side effects. Operant conditioning of the stretch reflex is a promising non-invasive paradigm with potential long-term sustained effects. Within this conditioning paradigm, seated participants have to reduce the mechanically elicited reflex response using biofeedback of reflex magnitude quantified using electromyography (EMG). Before clinical application of the conditioning paradigm, improvements are needed regarding the time-intensiveness and slow learning curve. Previous studies have shown that gamification of biofeedback can improve participant motivation and long-term engagement. Moreover, quantification of reflex magnitude for biofeedback using reflexive joint impedance may obtain similar effectiveness within fewer sessions. Nine healthy volunteers participated in the study, split in three groups. First, as a reference the “Conventional” group received EMG- and bar-based biofeedback similar to previous research. Second, we explored feasibility of game-based biofeedback with the “Gaming” group receiving EMG- and game-based biofeedback. Third, we explored feasibility of game- and impedance-based biofeedback with the “Impedance” group receiving impedance and game-based biofeedback. Participants completed five baseline sessions (without reflex biofeedback) and six conditioning sessions (with reflex biofeedback). Participants were instructed to reduce reflex magnitude without modulating background activity. The Conventional and Gaming groups showed feasibility of the protocol in 2 and 3 out of 3 participants, respectively. These participants achieved a significant Soleus short-latency (M1) within-session reduction in at least –15% in the 4th–6th conditioning session. None of the Impedance group participants showed any within-session decrease in Soleus reflex magnitude. The feasibility in the EMG- and game-based biofeedback calls for further research on gamification of the conditioning paradigm to obtain improved participant motivation and engagement, while achieving long-term conditioning effects. Before clinical application, the time-intensiveness and slow learning curve of the conditioning paradigm remain an open challenge.

Disentangling acceleration-, velocity-, and duration-dependency of the short- and medium-latency stretch reflexes in the ankle plantarflexors

Previous research and definitions of the stretch reflex and spasticity have focused on velocity dependence. We showed that perturbation acceleration, velocity, and duration all shape the M1 and M2 response, often via nonlinear or interacting dependencies. Consequently, systematic execution and reporting of stretch reflex and spasticity studies, avoiding uncontrolled parameter interdependence, is essential for proper understanding of the reflex neurophysiology.