1/27/2024 0 Comments Muscle synergyInterestingly, the switch in coordination during walking depends on whether the walking speed is increased or decreased ( Schöner et al., 1990 Van Emmerik and Wagenaar, 1996). In the vicinity of a phase transition, one may expect the dynamics’ dimensionality to be drastically reduced and muscle activity patterns to stay on low-dimensional manifolds. The methodological benefit of investigating such changes in coordination is that they arguably share characteristics of classic phase transitions, in the sense of non-equilibrium thermostatistics ( Kelso, 1995 Beek et al., 2002) al., 2002 Kelso, 1995). This switch is accompanied by a change in the phase relationship between the arms from in-phase to antiphase phase locking ( Wagenaar and van Emmerik, 2000), and in the immediate vicinity of the transition the variability of frequency (phase) locking drastically increases 1. For example, if speed is increased from loaf (very slow) to normal walking, one can observe a switch in frequency locking from a 2:1 to a 1:1 ratio between the arm and leg swing ( Craik et al., 1976 Schöner et al., 1990 Van Emmerik and Wagenaar, 1992, 1996): At very low speeds, the arm swing is phase locked to the step cycle, while at fast speeds it locks to the stride cycle. To understand the emergence of coordination patterns and, by this, the way muscle activity is orchestrated, one typically challenges the stability of phase locking by altering a control parameter. The degree of interlimb coordination can be characterized by the strength of frequency and phase locking between limbs. Human locomotion requires a well-organized activation of multiple muscles to coordinate movements of upper and lower limbs. These findings suggest that the stability of arm-leg coordination is associated with modulations in long-distant neuromuscular connectivity. Intermuscular coherence at 4–22 Hz between upper and lower body and within the legs was particularly pronounced for 1:1 arm-leg coordination and was diminished when switching between modes of coordination. The community structure of the multiplex network revealed four modules, which clustered functional and anatomical linked muscles across modes of coordination. NMF of the coherence spectra distinguished three EMG frequency bands: 4–8, 8–22, and 22–60 Hz. The corresponding multiplex network contained a single module indicating pronounced muscle co-activation patterns across the whole body during a gait cycle. We found five relevant muscle synergies that significantly differed in activation patterns between 1:1 and 2:1 arm-leg coordination and the transition period between them. We decomposed EMG envelopes and intermuscular coherence spectra using non-negative matrix factorization (NMF), and the resulting modes were combined into multiplex networks and analyzed for their community structure. Walking speed was varied from very slow to normal. Subjects walked on a treadmill with their arms swinging along their body while kinematics and surface electromyography (EMG) of 26 bilateral muscles across the body were recorded. We examined whether these switches are accompanied by changes in functional connectivity between multiple muscles. When walking speed is increased, the frequency ratio between the arm and leg swing switches spontaneously from 2:1 to 1:1. 3Neuroscience Research Australia, Randwick, NSW, Australia.2Department of Neuropsychology and Psychopharmacology, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands.1Department of Human Movement Sciences, Faculty of Behavioural and Movement Sciences, Amsterdam Movement Sciences & Institute for Brain and Behavior Amsterdam, Vrije Universiteit, Amsterdam, Netherlands.Boonstra 2,3, Andreas Daffertshofer 1* and Nadia Dominici 1
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