A central goal of neuroscience is to discover how neural circuits control the body’s muscles to produce behavior. However, despite recent advances in tools for studying brain activity, methods for examining the signals that actually control behavior – spiking activity in muscle fibers – have advanced little since the 1950s, fundamentally limiting our understanding of how the brain controls the body. By combining our expertise in electrophysiology (Dr. Sober) and engineering (Dr. Bakir), we have created a new generation of microscale, high channel-count electrode arrays that record muscle activity at unprecedented scale and resolution (single-unit spike trains in muscle fibers) in freely behaving animals and from a range of species including mice, rats, songbirds, and nonhuman primates. This technology is impactful because it allows precise monitoring of motor output in conjunction with recording and manipulation of central neural circuits, consistent with the BRAIN Initiative’s goal to reveal how patterns of activity in the central nervous system are transformed into motor behavior. To create our tools, we have established a design, fabrication, and testing pipeline to create a number of well-tested electrode designs that target forelimb, vocal, and orofacial muscles. Moreover, we surveyed the broader community of researchers examining motor behavior to identify the shortcomings of current methods and to prioritize array designs to address the same. We therefore propose to fabricate and distribute at least 2,500 of our current devices to 100 researchers worldwide (Aim 1) and to develop virtual and in-person training modules to teach users how to use our new technology (Aim 2) efficiently and at scale.