The central nervous system intrigues us. Despite its complexity it guarantees the control of balance and walking with seemingly little effort. Unravelling how the central nervous system accomplishes this is a challenging task. Over the last two centuries, a large body of literature has established that motor control relies on the recruitment of a low-dimensional spinal organisation. Chapter 1 provides a brief overview of how modular functional units at the spinal cord steer muscle groups. It covers historical and contemporary views of this spinal modularity and highlights the concept of muscle synergies. Anticipating the experimental assessments compiled in the thesis, this chapter also emphasised the role of the motor cortex in establishing motor coordination and provides a glance of cortico-synergy coherence. Chapter 2 entails an experimental assessment on balance control. It is the first study that combines cortico-muscular coherence and muscle synergies. It revealed prominent coherence between the cortex and muscle synergies for beta- (13-30 Hz) and Piper-band (~40 Hz) oscillations. This coherence was visible in the cortical motor network, but most pronounced in the paracentral lobule – a medial part of the sensorimotor cortex. In Chapter 3, the focus shifts to walking. In this study, beta-band coherence turned out significant for two muscle synergies that are active during the double support phases. Two muscle synergies with temporal patterns timed to the single support phases did not yield such coherence. The maximal coherences of the double support synergies were spatially resembled at the sensorimotor areas. The two patterns that did interact with the motor cortex resembled those that emerge in toddlers when they start walking independently. By contrast, the two muscle synergy patterns without significant cortico-synergy coherence were similar to those observed during neonatal stepping. There may therefore be associations between the motor cortex and the two later-emerging muscle synergies during early walking development. In the study underlying Chapter 4 I therefore addressed the functional cortico-synergy organisation during walking by contrasting independently walking toddlers with adults. This study again revealed a separation of muscle synergies; only two muscle synergies were coherent to activity in the motor network. Given the bipartite entrainment in the beta- and Piper-rhythms, in Chapter 5 I discard conventional coherence and focused instead on relationships across frequencies. In this methodological study, I approached connectivity by bicoherence and phase-amplitude coupling and investigated their pros and cons. This first assessment paves the way to employ cross-frequency coupling to study walking and postural control. Future studies will incorporate this technique to investigate the possible interplay between the beta- and Piper-rhythms, which expectably clarifies how independent walking is realised with its balance demands. In Chapter 6, I reflect on the applications of cortico-synergy coherence acquired during motor control tasks. It sketches some experimental and methodological outlooks that may shed even more light on the functional relationship between the motor cortex and muscle synergies than the earlier chapters do. It includes preliminary findings on a longitudinal study comprising early walking development of muscle synergy representations in the cortex. In line with the preceding studies, a two-by-two grouping of muscle synergies was found, with the synergies that are coherent with cortical activity being those that emerge around the first independent steps. This is a promising result but to achieve an encompassing view on how independent walking is accomplished, the analysis will need to be extended towards cortico-synergy bicoherence. Taken together, this thesis revealed that brain rhythms manifest in the muscle synergies during postural and locomotor control. It provides support that some of the spinally organised muscle synergies do strongly interact with the sensorimotor cortex. The corresponding spinal circuitries are hence not regulated locally.