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Flexion synergy pattern after stroke9/9/2023 There is evidence that use of the CST is generally dominant all the time after the onset of stroke if the initial CST integrity is retrievable 18, 19, 20. Regardless of whether the CST is in use, stroke survivors are seamlessly capable of closing their hands. Here, a question arises as to whether there is a difference in the ability to flex the fingers according to the usabilities of the CST and its alternative tracts. The evidence that involuntary grasping is in correlation with increased activation in contralesional cortical areas, not ipsilesional cortical areas, implies that hand movements are governed by the CRST 12. Executing hand opening and closing with an arm posture under a working synergy may be a difficult task when the stroke survivor preferentially uses alternative tracts other than the CST 4. It is very difficult for stroke survivors who are unable to employ the CST to perform a movement out of synergy. When alternative tracts are assumed to be activated, monotonous synergistic movement is dominant 12, 17. Generally we observe that while use of alternative tracts leads to stroke-caused synergies, the CST enables motion out of those synergies 13. Several studies demonstrate that the influence of the CRBT is relatively low in humans 10, 16. Diffusor tensor imaging (DTI) studies showed the inverse relationship between CST integrity and CRST integrity 10, 14, 15. There is extensive evidence that the cortico-reticulospinal tract (CRST) especially enhances flexion synergy expression, among cortico-bulbospinal motor pathways including the CSRT and the cortico-rubrospinal tract (CRBT) 9, 10, 11, 12, 13. As an adaptive strategy, the reliance on cortico-bulbospinal motor pathways increases that are closely related to flexion and extension synergy expressions. Such pathological synergies largely originate from damage to the corticospinal tract (CST). The so-called flexion synergy and extension synergy are representative stroke-caused characteristics in the affected individuals and involve abnormal co-activation of shoulder abductor and adductor with distal limb flexor and extensor, respectively 6, 7, 8. It is typical that stroke re-groups muscle groups and causes abnormal couplings among the muscle groups across the limbs. Stroke creates abnormal synergies through regrouping muscle groups that are innervated together in the context of executing movements. Originally, synergy is a concept that describes low-dimensional movement expressed in a higher dimensional space of possible muscle activations. Hand opening and closing, even this simple motion is possible only if the motor drive to the appropriate joints of the hand needs to overwhelm the synergy that is dominant over the hand 4. Hand dexterity is substantially influenced by synergies caused by stroke 1, 2, 3, 4, 5. We present evidence that responses of flexors of the MCP joints following stroke depend on the degree of impairment which is hypothesized to originate from preferentially use of different neural motor pathways. the CRST) results in a degradation in movement smoothness and slow activation of MCP flexors. The results imply that use of alternative tracts (i.e. We found that participants with the UEFM score above a certain value, who are assumed to use the CST, moves the MCP joints more smoothly ( Pā<ā0.05) and activates the flexors to flex the joints faster ( Pā<ā0.05), in comparison to participants with low UEFM scores, who are assumed to preferentially use alternative tracts. UEFM scores have been perceived as an indirect indicator of CST integrity. Here we note that hand closing is enabled by alternative tracts as well as the CST, and a research question arises: Does motor characteristics while closing the hand depend on the integrity of the CST? In this study, we evaluate the abilities of 17 stroke survivors to flex and relax the metacarpophalangeal (MCP) joints and investigate whether motor characteristics can be distinguished based on CST integrity which is estimated using upper-extremity Fugl-Meyer (UEFM) scores. It is universally accepted that alternative tracts including the cortico-reticulospinal tract (CRST), employed in the case that the corticospinal tract (CST) is damaged by stroke, brings about such abnormal synergies. Abnormal synergies govern the arm and hand in stoke survivors with severe impairment, so hand opening, which is required to overcome the working synergy, is an extremely difficult task for them to achieve. While stroke survivors with moderate or mild impairment are typically able to open their hand at will, those with severe impairment cannot.
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