Sequential Activation of Muscle Synergies During Locomotion in the Intact Cat as Revealed by Cluster Analysis and Direct Decomposition

Abstract

During goal-directed locomotion, descending signals from supraspinal structures act through spinal interneuron pathways to effect modifications of muscle activity that are appropriate to the task requirements. Recent studies using decomposition methods suggest that this control might be facilitated by activating synergies organized at the level of the spinal cord. However, it is difficult to directly relate these mathematically defined synergies to the patterns of electromyographic activity observed in the original recordings. To address this issue, we have used a novel cluster analysis to make a detailed study of the organization of the synergistic patterns of muscle activity observed in the fore- and hindlimb during treadmill locomotion. The results show that the activity of a large number of forelimb muscles (26 bursts of activity from 18 muscles) can be grouped into 11 clusters on the basis of synchronous co-activation. Nine (9/11) of these clusters defined muscle activity during the swing phase of locomotion; these clusters were distributed in a sequential manner and were related to discrete behavioral events. A comparison with the synergies identified by linear decomposition methods showed some striking similarities between the synergies identified by the different methods. In the hindlimb, a simpler organization was observed, and a sequential activation of muscles similar to that observed in the forelimb during swing was less clear. We suggest that this organization of synergistic muscles provides a means by which descending signals could provide the detailed control of different muscle groups that is necessary for the flexible control of multi-articular movements.

Modeling discrete and rhythmic movements through motor primitives: a review

“…Under the hypothesis that the fields can be summed, and since the intensity of stimulation does not change the pattern of force orientation (Giszter et al (1993)), the space of possible end-effector target positions could be spanned through the weighted summation of a limited set of force fields. Note that similar results were obtained with rats (Tresch et al (1999)) and cats (Krouchev et al (2006); Ting and Macpherson (2005)). …”

Combining modules for movement

“…Such patterns, in which a group of muscles are activated in a fixed balance, have previously been considered as muscle synergies". Other investigators have generated corroborative evidence in cats (Lemay et al, 2001;Ting and Macpherson, 2005;Krouchev et al 2006). A clear-cut example of a recombination of synergies is from locomotion with the different limb CPGs.…”

Synergy temporal sequences and topography in the spinal cord: evidence for a traveling wave in frog locomotion

“…With respect to activations of specific groups of muscles, synergies have been proposed as building blocks of motor control (Grillner 1981;Ting and Macpherson 2004;Cheung et al 2005Cheung et al , 2009d'Avella et al 2006;Krouchev et al 2006;Yakovenko et al 2011;Overduin et al 2012;Berger et al 2013;Bizzi and Cheung 2013;Krouchev and Drew 2013). There is also evidence that encoding of muscle synergies already takes place in the spinal cord (Saltiel et al 2001;Stein 2008;Hart and Giszter 2010;Roh et al 2011).…”

“…Caudal extensions and lateral forces primarily rely on hip and knee extensors, respectively. This sequence of forces is a priori a good starting point to study locomotion, since hip extensors finish earlier than knee extensors in the cat hind limb stance (Krouchev et al 2006), and shoulder retractors and elbow extensors, respectively, dominate in early and late stance of cat forelimb fictive locomotion (Saltiel and Rossignol 2004a). With NMDA, the caudal extension-lateral force sequence is particularly seen when the caudal extension is of the type based on synergy B, which is linked to flexions starting with synergy F (Saltiel et al 2005).…”