Directional EMG patterns reveal impairment-related disruptions in upper-limb force control after stroke

 

Big fucking whoopee.

 

  You delivered NOTHING that gets survivors recovered!

Hope you enjoy NOT RECOVERING from your stroke when you become the 1 in 4 per WHO that has a stroke.  

Directional EMG patterns reveal impairment-related disruptions in upper-limb force control after stroke

• Sungjin Bae,
• Hyung-Soon Park,
• Jeong-Ho Park &
• William Z. Rymer 

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Abstract

Background

Hemiparetic stroke disrupts upper-limb motor control, leading to direction-specific deficits and abnormal compensatory strategies. By quantifying the angular range and preferred direction of muscle activation, directional electromyography (EMG) analysis has shown altered directional tuning in stroke survivors. However, it remains unclear how impairment severity affects direction-specific difficulty and compensation, and how these disruptions change with force level and task direction. This study used directional EMG mapping during a multi-directional isometric task at varying force levels to examine these issues in detail.

Methods

Eight chronic stroke survivors performed isometric force-matching tasks in eight directions (0°–315° at 45° intervals) in the plane orthogonal to the forearm at 20%, 40%, and 60% of maximum voluntary contraction (MVC) using a wrist fixture. All forces and torques applied to the fixture and surface EMG from eight upper-limb muscles were recorded. For each muscle, EMG vectors were calculated using EMG amplitudes and actual force directions to analyze directional tuning and angular concentration. Task accuracy was quantified as the angular error between the target and actual force directions, and compensatory strategies were estimated from off-axis forces and torques. Generalized linear mixed-effects models were applied to these metrics to examine the effects of force magnitude, task direction, and their interactions. Spearman correlations were used to assess muscle coactivation and the relationship between impairment severity, based on Fugl-Meyer Assessment in Upper Extremity (FMA-UE), and task accuracy.

Results

Severely impaired participants showed restricted force generation in lateral directions, and activation tuning constrained to a narrow medial range. These disruptions accompanied larger angular errors, which were negatively correlated with FMA-UE scores. Greater force demands progressively recruited the deltoid muscles, indicating significant directional tuning; however, task accuracy changed in a direction-specific manner, improving in some directions and deteriorating in others. Increasing force demand also produced direction-specific shifts in off-axis forces and torques that diverged from classic flexor/extensor synergies, suggesting enhanced shoulder-driven control that may promote more independent joint control. Coactivation analysis showed potential joint-stabilizing coactivation within the deltoids and triceps in nearly all directions. In contrast, severe impairment was associated with fewer significant coactivation pairs and loss of biceps–brachioradialis coupling.

Conclusions

These findings suggest that directional control deficits and compensatory strategies after stroke are modulated by complex interactions among impairment severity, force magnitude, and task direction, creating patient-specific vulnerable zones. Directional EMG mapping provides quantifiable directional targets that may be useful for designing personalized rehabilitation interventions.

Trial registration This study was retrospectively registered at ClinicalTrials.gov (NCT07113457) on 08 August 2025.