Effective control of trunk muscles is fundamental to perform most daily activities. Stroke affects this ability also when sitting, and the Modified Functional Reach Test is a simple clinical method to evaluate sitting balance. We characterize the upper body kinematics and muscular activity during this test. Fifteen chronic stroke survivors performed twice, in separate sessions, three repetitions of the test in forward and lateral directions with their ipsilesional arm. We focused our analysis on muscles of the trunk and of the contralesional, not moving, arm. The bilateral activations of latissimi dorsi, trapezii transversalis and oblique externus abdominis were left/right asymmetric, for both test directions, except for the obliquus externus abdominis in the frontal reaching. Stroke survivors had difficulty deactivating the contralesional muscles at the end of each trial, especially the trapezii trasversalis in the lateral direction. The contralesional, non-moving arm had muscular activations modulated according to the movement phases of the moving arm. Repeating the task led to better performance in terms of reaching distance, supported by an increased activation of the trunk muscles. The reaching distance correlated negatively with the time-up-and-go test score.

Sit-to-stand (STS) is a fundamental functional task frequently impaired after stroke and widely used in rehabilitation to assess motor control and balance. While lower-limb kinematic and muscular asymmetries during STS have been documented, the contribution of trunk muscle coordination to compensatory strategies has received limited attention. We investigated STS performance in seven individuals with chronic right-sided hemiparesis under two conditions (free arms and crossed arms) to characterize phase-dependent kinematic asymmetries and side-dependent trunk muscle modulation relevant to rehabilitation practice. Optoelectronic motion capture was synchronized with bilateral surface electromyography, providing time-aligned kinematic and neuromuscular signals for sensor-based assessment of STS. Participants exhibited prolonged and highly variable STS durations, along with ankle asymmetries during the rising and lowering phases and hip asymmetries during upright standing, indicating increased reliance on the less impaired limb. Electromyography revealed side-dependent modulation of trunk muscles, notably latissimus dorsi, erector spinae longissimus, and multifidus, characterized by a prolonged relative contribution on the more impaired side. These findings suggest that altered trunk muscle modulation contributes to compensatory STS strategies after stroke and highlight the importance of trunk-focused neuromuscular assessment to guide individualized rehabilitation interventions aimed at improving symmetry, postural stability, and movement efficiency.