Characterization of motor function recovery using muscle synergies in stroke patients undergoing upper limb rehabilitation

In stroke rehabilitation, muscle synergies have been investigated to provide information on the level of upper limb motor impairment, but not yet for explaining motor recovery after therapy. In this study, we analysed muscle synergies parameters extracted from 62 people with stroke who underwent a specific upper limb treatment (20 sessions, 1h/day, 5d/week, 4 weeks) consisting of virtual reality, robotic or conventional treatment. Overall, participants improved upper limb motor function (Fugl-Meyer Assessment Upper Extremity-FMA-UE: Δ= 7.14 ± 7.46, p< 0.001) but the number of muscle synergies of the impaired side (N-aff) did not change after treatment (N-aff: T0= 8.8 ± 1.5; T1=8.7 ± 1.4; p=0.374). Then, we stratified the sample into Responder (No.=34) and Non-Responder (No.=28) participants, based on the Minimal Clinical Important Difference value of FMA-UE (Δ >5 points). We investigated merging and fractionation indexes in both subgroups and only the Responder subgroup significantly decreased the percentage of merged muscle synergies (p=0.004). No significant changes in the fractionation index resulted in either subgroup.

Finally, we identified vocabularies of affected upper limb motor synergies, before (No. 13 synergies) and after treatment (No. 14 synergies), and in unaffected upper limb (No. 16 synergies). We identified upper limb motor functions associated with each muscle synergy in each vocabulary based on the muscles represented in the muscle synergy. There were no differences in motor functions characterizing Responder patients. However, after therapy, both Responder and Non-Responder subgroups were characterized by the same distribution of motor functions across muscle synergies observed in the unaffected limb.

Trial registration

The trial is registered in ClinicalTrial.gov, identifier number NCT03530358 ( https://clinicaltrials.gov/study/NCT03530358 ).

Key messages

• Rehabilitation therapy for the upper limb induces reduction of muscle synergies merging in people with stroke expressing clinically important improvement of motor function. This muscular pattern is in accordance with motor control mechanisms underpinning functional recovery.

• Motor function of the affected upper limb at baseline did not characterize the muscular profile of patients responding to rehabilitation treatment (Responder).

• After therapy, all patients regardless the amount of motor function improvement (i.e., Responder, Non-Responder) express a muscular profile of the affected upper limb like the unaffected upper limb.