Abstract
Objective. Stroke affects the expression of muscle synergies underlying motor control, most notably in patients with poorer motor function. The majority of studies on muscle synergies have conventionally approached this analysis by assuming alterations in the inner structures of synergies after stroke. Although different synergy-based features based on this assumption have to some extent described pathological mechanisms in post-stroke neuromuscular control, a biomarker that reliably reflects motor function and recovery is still missing. Approach. Based on the theory of muscle synergies, we alternatively hypothesize that functional synergy structures are physically preserved and measure the temporal correlation between the recruitment profiles of healthy modules by paretic and healthy muscles, a feature hereafter reported as the FSRI. We measured clinical scores and extracted the muscle synergies of both ULs of 18 chronic stroke survivors from the electromyographic activity of 8 muscles during bilateral movements before and after 4 weeks of non-invasive BMI controlled robot therapy and physiotherapy. We computed the FSRI as well as features quantifying inter-limb structural differences and evaluated the correlation of these synergy-based measures with clinical scores. Main results. Correlation analysis revealed weak relationships between conventional features describing inter-limb synergy structural differences and motor function. In contrast, FSRI values during specific or combined movement data significantly correlated with UL motor function and recovery scores. Additionally, we observed that BMI-based training with contingent positive proprioceptive feedback led to improved FSRI values during the specific trained finger extension movement. Significance. We demonstrated that FSRI can be used as a reliable physiological biomarker of motor function and recovery in stroke, which can be targeted via BMI-based proprioceptive therapies and adjuvant physiotherapy to boost effective rehabilitation.
Highlights
MethodsWe denominated the mean of these five nCC coefficients (from the set of five healthy synergy structures) as the FSRI, resulting in FSRIGlobal and task-specific FSRI values (FSRISF,shoulder external rotation (SER),EE,WF,finger extension (FE) for individual movements).We considered this feature as the degree of temporal similarity of the activation profiles of healthy functional control modules manifested in the paretic muscles in reference to a healthy homologous pattern
FSRI We considered the synergy structures extracted from the healthy limb before treatment in each subject as the patient-specific reference set of functional synergies (FSRI computation approach is represented in figure 2(B)) whose structures have not been influenced by any familiarization/adaptation process to the evaluated motor task [51]
Effect of the BMI feedback contingency on muscle synergy features When separated by feedback, the interaction between the intervention type and time was significant for FSRI variable during the finger extension (FE) movement only (FSRIFE), which coincides with the motor task that was trained by all patients and most intensively in the BMI therapy
Summary
We denominated the mean of these five nCC coefficients (from the set of five healthy synergy structures) as the FSRI, resulting in FSRIGlobal and task-specific FSRI values (FSRISF,SER,EE,WF,FE for individual movements).We considered this feature as the degree of temporal similarity of the activation profiles of healthy functional control modules manifested in the paretic muscles in reference to a healthy homologous pattern. This feature was calculated separately for the PRE and POST assessment sessions. For non-Gaussian variables, we compared the pre-post difference in the analyzed scores between intervention groups by applying a two-tailed unpaired test (normal distribution) or a non-parametric Mann–Whitney U test
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