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Phasic and tonic muscle synergies are different in number, structure and sparseness in upper limb multi-directional movements
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  • Cristina Brambilla ,
  • Marta Russo ,
  • Andrea d'Avella ,
  • Alessandro Scano
Cristina Brambilla
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Marta Russo
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Andrea d'Avella
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Alessandro Scano
Consiglio Nazionale delle Ricerche

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In the last two decades, muscle synergies analysis has been commonly used to assess the neurophysiological  mechanisms underlying human motor control. Despite several synergy models and algorithms have been  employed for processing the electromyographic (EMG) signal, and neural substrates indicate their non?homogeneous origin, EMG patterns are usually preprocessed without separating phasic (movement-related) and tonic (anti-gravity and related to co-contraction) components. Using a comprehensive mapping of upper?limb point-to-point movements, synergies were extracted from phasic and tonic EMG signal separately, estimating the tonic components with a linear ramp model. The goodness of reconstruction (R2 ) as a function  of the number of synergies was compared, and synergies extracted from each dataset at three threshold levels (0.80, 0.85, 0.90) were retained for further analysis. Then, shared, phasic-specific, and tonic-specific synergies  were extracted from the two datasets concatenated. We found only few shared synergies, indicating that phasic  and tonic synergies have in general different structures. Shared, phasic-specific and tonic-specific synergies  were clustered separately and compared for evaluating differences in synergy composition. Phasic-specific  clusters were more numerous than tonic-specific ones and with a higher sparseness, suggesting that they were  more differentiated among subjects. The structure of the clusters indicated that phasic synergies identify  specific patterns related to the movement (sparse composition) while tonic synergies show co-contraction of  multiple muscles for joint stabilization and holding postures. These results suggest that phasic and tonic  synergies should be extracted separately, especially when performing muscle synergy analysis in patients with  abnormal tonic activity and for tuning devices with gravity support