Development of a quantitative assessment for abnormal flexor synergy index in patients with stroke: a validity and responsiveness study

Background: Arm-lifting movements (shoulder flexion) are essential for upper extremity rehabilitation after a stroke. Abnormal flexor synergy (elbow flexion) is frequently observed during shoulder flexion, impeding functional improvement. However, no quantitative method exists for assessing abnormal flexor synergy. This study investigated the validity and responsiveness of a newly developed index to quantitatively evaluate abnormal flexor synergy.

Methods: Participants included 103 patients (mean age: 58.0 ± 10.1 years; 64 men, 39 women) with stroke. Using three-dimensional coordinate data during shoulder flexion obtained from a depth sensor camera, we calculated the abnormal flexor synergy based on our developed index. The abnormal flexor synergy index decreases with increasing flexion of the elbow joint during shoulder flexion (the maximum value is 100% without abnormal flexor synergy). The validity of the abnormal flexor synergy index was assessed by analyzing the correlation between the index and both the Fugl-Meyer Assessment of the Upper Extremity (FMA-UE) four-category scores and the Modified Ashworth Scale (MAS) scores for elbow, wrist, and finger flexors, using Pearson's and Spearman's correlation coefficients. Responsiveness was studied in 17 inpatients (mean age: 59.5 ± 8.1 years; 7 men, 10 women) who underwent proximal upper extremity intervention for approximately 3 weeks, evaluating change from admission to discharge using the standardized response mean (SRM).

Results: Significant correlations were observed between the abnormal flexor synergy index and FMA-UE scores: A (r = 0.625, p < 0.001), B (r = 0.433, p < 0.001), C (r = 0.418, p < 0.001), and D (r = 0.411, p < 0.001), as well as MAS scores for elbow flexors (r = -0.283, p = 0.004) and proximal interphalangeal flexors (r = -0.201, p = 0.042). The highest responsiveness was observed in the FMA-UE A score (SRM = 0.81), followed by the abnormal flexor synergy index (SRM = 0.79).

Conclusions: The newly developed index for assessing abnormal flexor synergy demonstrated superior validity and high responsiveness. These results suggest the potential for using this index to evaluate upper extremity function in patients with stroke.