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This review describes methods for interfacing motor neurons from muscle recordings and their applications in studies on the neural control of movement and in the design of technologies for neurorehabilitation. After describing methods for accessing the neural drive to muscles in vivo in humans, we discuss the mechanisms of transmission of synaptic input into motor neuron output and of force generation. The synaptic input received by a motor neuron population is largely common among motor neurons. This allows linear transmission of the input and a reduced dimensionality of control by the central nervous system. Force is generated by low-pass filtering the neural signal sent to the muscle. These concepts on neural control of movement are used for the development of neurorehabilitation technologies, which are discussed with representative examples on movement replacement, restoration, and neuromodulation. It is concluded that the analysis of the output of spinal motor neurons from muscle signals provides a unique means for understanding the neural coding of movement in vivo in humans and thus for reproducing this code artificially with the aim of restoring lost or impaired motor functions.