Flexion synergy overshadows flexor spasticity during reaching in chronic moderate to severe hemiparetic stroke

Abstract

Objective: Pharmaceutical intervention targets arm flexor spasticity with an often-unsuccessful goal of improving function. Flexion synergy is a related motor impairment that may be inadvertently neglected. Here, flexor spasticity and flexion synergy are disentangled to determine their contributions to reaching dysfunction.

Methods: Twenty-six individuals participated. A robotic device systematically modulated shoulder abduction loading during ballistic reaching. Elbow muscle electromyography data were partitioned into windows delineated by elbow joint velocity allowing for the separation of synergy- and spasticity-related activation.

Results: Reaching velocity decreased with abduction loading (p<0.001) such that velocity was 30% slower when lifting the arm at 50% of abduction strength compared to when arm weight was supported. Abnormal flexion synergy increased with abduction loading (p<0.001) such that normalized activation ranged from a median (interquartile range) of 0.07 (0.03-0.12) when arm weight was supported to 0.19 (0.12-0.40) when actively lifting (large effect size, d=0.59). Flexor spasticity was detected during reaching (p=0.016) but only when arm weight was supported (intermediate effect size, d=0.33).

Conclusion: Flexion synergy is the predominant contributor to reaching dysfunction while flexor spasticity appears only relevant during unnaturally occurring passively supported movement.

Significance: Interventions targeting flexion synergy should be leveraged in future stroke recovery trials.

Keywords: Arm; Biomechanical phenomena; Movement; Muscles; Neurology; Rehabilitation; Robotics; Stroke.

Figure 1

Setup of a participant in the ACT^3D (A). Visual feedback of the arm avatar reaching toward an outward target with a display of the reaching trajectory (B).

Figure 2

Example position data from a single reaching trajectory under 50% abduction loading (A) with the peak elbow angular velocity labeled with a red circle followed by a single elbow extension angular velocity profile (B) with time 0s (truncated) representing the successful lift and acquisition of the home position target (large green dot in A). The reaching target is indicated with a large black dot whereas the endpoint of the actual reach is labeled with a small black dot. The grey dotted lines represent the accepted range of reaching movements. Movement onset is labeled with a green vertical line while peak angular velocity is labeled with a red vertical line in (B) and a red circle in (A). EMG data was evaluated at lift (from time 0s to green vertical line), first 25ms of movement onset (between green and black vertical lines), and from the end of movement onset window (black vertical line) to peak angular velocity (red line) in order to determine the EMG contributions of synergy-related flexor activation (lift) and spasticity-related flexor activation (end of movement onset to peak velocity).

Figure 3

Mean (standard error) elbow angular velocity decreases as a function of abduction loading. The asterisks indicate significant decreases in velocity from the 0% abduction loading condition.

Figure 4

Mean (standard error) biceps brachii EMG (A) is larger following the onset of elbow extension for all abduction loading levels except the 50% condition representing spasticity-related flexor activation superimposed upon initial synergy-related flexor activation. Asterisks indicate a significant effect of time period. Mean (standard error) lateral head of triceps brachii EMG (B) (shown here for ease of subsequent comparison) is larger following the onset of elbow extension for all abduction loading levels representing consistent maximum volitional drive of extensors during the ballistic reaching task. Asterisks indicate a significant difference between each individual time period. Change in EMG, while not directly illustrated, can be visually appreciated and was not modulated by abduction loading.

Figure 5

Mean (standard error) biceps brachii EMG increases as a function of abduction loading when first lifting the arm but prior to the onset of reaching (Lift) representing abnormal progressive synergy-related flexor activation. Mean (standard error) change in biceps brachii EMG in the first 25ms of reaching movement (Onset) and then up to peak elbow extension angular velocity (Reflex) is not modulated by abduction loading.