biceps brachii), which will in turn lead to larger stretch reflex amplitudes and lower velocity thresholds as preactivation increases for a given starting angle. Given the graded nature of flexion synergy and stretch reflex expression, it is hypothesized that progressive increases in shoulder abduction loading will lead to incremental increases in the electromyographic activity of the elbow flexors (e.g. Increases in muscle preactivation are known to impact the stretch reflex by increasing its amplitude as well as decreasing the velocity and joint angle at which a reflex is evoked. ![]() elbow extension) motions that necessitate shoulder abduction loading. Specifically, expression of the flexion synergy leads to abnormally increased elbow flexor activity during reaching (i.e. It should be noted that despite the aforementioned dependence of the stretch reflex on position, velocity, and preactivation, the work to be presented here will focus exclusively on the latter two components.Īlthough the interplay between the flexion synergy and stretch reflexes remains largely uninvestigated (however see for a recent study on this subject), clear hypotheses can be drawn. Due to these reductions, there is an increased likelihood that the combination of a slower speed, shorter muscle length, and lower preactivation level stretch may elicit a reflex. Following stroke, it has generally become accepted that the velocity, joint angle and preactivation thresholds necessary for reflex evocation decrease (although it should be noted that the physiological mechanisms responsible for these shifts remain unclear). shorter muscle length) to elicit a reflex, while a reflex can be elicited at a lower level of preactivation if the stretch velocity or muscle length is increased. ![]() Importantly, these reflex parameters are also inherently linked: an increase in muscular preactivation allows a lower velocity stretch or a stretch beginning at a more acute joint angle (i.e. ![]() Reflex amplitude is directly proportional to all three of these parameters, such that greater stretch velocities, joint angles or higher levels of muscular preactivation result in greater reflex amplitudes. Stretch reflexes are typically defined by three characteristics: velocity sensitivity, position sensitivity and a dependence upon the level of muscle activation present prior to the stretch. In addition to the flexion synergy, stretch reflexes may also play a role in movement disorders associated with chronic hemiparetic stroke. This graded expression pattern therefore makes it more difficult, for example, to extend at the elbow while lifting a book than while supporting the weight of the limb alone. Importantly, it should be noted that the expression of the flexion synergy is progressive that is, as the amount of shoulder abductor activity is increased, so too is activity in the elbow flexors. This coupling is particularly limiting because it leads to the loss of independent joint control, hampering volitional elbow extension when shoulder abduction is concurrently required. ![]() Originally described clinically, the flexion synergy has subsequently been quantified in individuals with chronic hemiparetic stroke and is defined as the involuntary neural coupling of shoulder abductor activity with activation of elbow flexors in the paretic upper limb. Therefore, of particular relevance to the work to be presented herein are the so-called ‘flexion synergy’ and the stretch reflex. While it has been suggested that this impairment may be due in part to changes in muscle properties following stroke, it is likely that the predominant contributors to the overall functional deficit are abnormal muscle coactivation patterns and/or altered reflexes. Chronic hemiparetic stroke is frequently associated with a compromised ability to reach with the paretic upper limb especially when supporting the weight of the limb.
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