Post-Stroke Abnormal Synergies Are a Resultant Behavior of the Interplay of Weakness, Spasticity, and Environmental Demands

Abnormal synergies coexist with primary motor impairments (weakness and spasticity) after stroke. They are observed during voluntary movements and are characterized by stereotyped movement patterns, a loss of independent joint control, and abnormal muscle coordination (eg, involuntary activation of distal muscles led by proximal joint movement). The structure and expression of abnormal synergies, however, are highly context-dependent (eg, influenced by gravity, body posture, and task constraints) and are not homogeneous across patients. Current evidence suggests that abnormal synergies are pathophysiologically related to damage to corticospinal tracts, upregulation of brainstem pathways (particularly the medial reticulospinal tract, but also involves the vestibulospinal tract), and maladaptive changes in spinal circuitry. These same mechanisms may overlap with those underlying spasticity and weakness. Spasticity is believed to result from overexcitability of descending excitatory brainstem pathways, the reticulospinal tract in particular, and the consequent exaggerated stretch reflexes, while muscle weakness is attributed to the reduction in corticospinal output. Recent research and clinical observations have advanced our understanding of abnormal synergies. Post-stroke abnormal synergies are not fixed motor impairments. A new framework is proposed that suggests post-stroke abnormal synergies could constitute the behavioral expression of primary impairments, including weakness and spasticity, arising from their interactions with environmental constraints, particularly antigravity control mechanisms. The term "synergy" has been used across various frameworks, with different definitions and quantification approaches. Identifying appropriate methods to quantify abnormal synergies is therefore essential to investigate their relationship with spasticity and muscle weakness, and evaluate potential abnormal synergy improvements following interventions, even if they primarily target other motor impairments.