Can we normalize surface electromyography in patients with spasticity of the upper limb?

Spasticity assessment using surface electromyography (sEMG) is challenging in the upper limb of people with limited voluntary motor control. Standardized protocols are limited and it is not clear which method of normalization is the most appropriate for possibly informing clinical decision making.

Elbow flexor muscle activity (spasticity) was assessed in 17 people following traumatic brain injury, stroke, or cerebral palsy. All participants underwent 3D motion analysis with synchronized sEMG during slow and fast passive stretches of the biceps brachii. Methods of comparison included non-normalized sEMG, normalization to maximum voluntary isometric contraction (MVIC), and maximum sEMG activity during activities of daily living (ADL) or active elbow flexion (ROM). A correlation analysis was performed between normalized and non-normalized sEMG parameters and clinical spasticity scales, including the Modified Ashworth and Tardieu Scales.

Normalization to ROM was feasible, in terms of EMG signal quality, only in patients with preserved voluntary control. While normalization to MVIC is commonly used, it was often unreliable due to impaired motor control, and in some cases, non-normalized data performed equally or better. Normalization to ROM showed the strongest correlation with the modified Ashworth Scale. Normalization to ROM or ADL showed the best correlations with clinical spasticity scales, particularly in people with preserved voluntary motor control.

No method of normalization demonstrated consistently superior performance and results were similar to non-normalized sEMG. Findings support adapting sEMG normalization methods based on individual functional abilities to improve spasticity assessment accuracy.