What determines the presence of upper extremity (UE) flexor synergy versus extensor synergy in stroke patients?

Upper Extremity Synergy Patterns After Stroke

Direct Answer

The presence of flexor versus extensor synergy patterns in stroke patients is primarily determined by the extent and location of corticospinal tract (CST) damage and the subsequent upregulation of brainstem motor pathways, not by a binary choice between the two patterns—most patients with moderate-to-severe hemiparesis exhibit predominantly flexor synergy dominance with limited extensor synergy expression. 1

Neural Mechanisms Determining Synergy Patterns

Corticospinal Tract Integrity as the Primary Determinant

• Early measurement of CST fiber integrity via diffusion tensor imaging (DTI) predicts motor outcome patterns at 12 months, particularly for patients with initially more severe impairment. 1
• Higher fractional anisotropy (FA) of the ipsilesional CST in the acute stage correlates with better motor recovery and less stereotypical synergy expression. 1
• CST lesion load in the acute stage has predictive value for poor motor outcome—greater CST damage leads to more pronounced synergy constraints. 1

Brainstem Pathway Upregulation

• Abnormal flexion and extension synergies emerge because of upregulation of diffusely projecting brainstem motor pathways following stroke-induced damage to corticofugal pathways. 2
• The pattern of muscle activation during synergies reflects brainstem motor pathway organization rather than cortical control. 2
• Patients with severe CST damage rely more heavily on these alternative brainstem pathways, resulting in stereotypical movement patterns. 2

Why Flexor Synergy Predominates

Proximal-to-Distal Gradient

• Synergy-driven contractions are strongest when elicited via proximal joints and weakest when elicited via distal joints. 2
• For some patients, joint torque and muscle activation generated during maximal voluntary contractions are lower than during maximal synergy-induced contractions, particularly for wrist and fingers. 2
• This explains why flexor synergy (involving shoulder adduction, elbow flexion, forearm supination) is more commonly observed and more limiting than extensor patterns. 2

Muscle-Specific Alterations

• In stroke survivors, the anterior deltoid is abnormally coactivated with medial and posterior deltoids within the shoulder abductor/extensor synergy. 3
• The shoulder adductor/flexor synergy is dominated by pectoralis major activation, with limited anterior deltoid activation. 3
• Impaired control of individual deltoid heads contributes to the predominance of flexor over extensor patterns. 3

Clinical Severity and Synergy Expression

Severity-Based Patterns

• Patients can be categorized by flexor synergy impairment: mild (10-12 points on Fugl-Meyer flexor synergy subscale), moderate (6-9 points), and severe (0-5 points). 4
• Significantly higher recovery occurs in proximal components (Brunnstrom Recovery Stage-Arm: 4.13 ± 1.07) compared to distal components (BRS-Hand: 2.47 ± 1.45). 5
• Upper arm Fugl-Meyer scores (29.03 ± 4.31) are consistently higher than wrist-hand scores (11.50 ± 5.88), reflecting the proximal-to-distal gradient of recovery. 5

Motor Evoked Potential Correlation

• The presence of an upper limb motor evoked potential (MEP) in response to transcranial magnetic stimulation at the hyperacute and acute stages strongly predicts good motor outcome and identifies patients who will follow proportional recovery rather than synergy-constrained patterns. 1
• Patients without an MEP are more likely to exhibit persistent synergy constraints. 1

Functional Implications

Movement Component Linkage

• Very high correlation (r = 0.9 to 1.0) exists between specific FMA components like wrist circumduction and spherical grasp. 5
• Moderate to high correlation (r = 0.5 to 0.89) exists between flexor synergy, extensor synergy, movement combining synergies, and movement out of synergy. 5
• This linkage means that recovery of one component facilitates recovery of related components within the same synergy pattern. 5

Associated Reactions

• Associated reactions in the wrist/finger flexors are stronger than those of other paretic muscles and depend on whether the non-paretic contraction is at a proximal or distal joint. 2
• This demonstrates the whole-limb nature of synergy constraints and explains why isolated joint control is difficult. 2

Critical Clinical Pitfall

The key misconception is that patients have "either" flexor "or" extensor synergy—in reality, most patients with moderate-to-severe impairment have both patterns present but with flexor synergy being more dominant and functionally limiting due to the proximal-to-distal gradient of brainstem pathway influence. 2, 5 The extent of CST damage determines how constrained the patient is to these stereotypical patterns, with greater damage producing more rigid synergy expression and less voluntary control outside these patterns. 1