Under some experimental conditions spinal cord microstimulation (SCμstim) produces 4 or 5 patterns of isometric force fields. This is interpreted as evidence for the existence of 4 or 5 movement primitives hard-wired in the spinal cord (Giszter et al., J Neurosci, 1993; Lemay & Grill, SFN Proc, 1999). However, the biomechanical actions and relative strengths of available muscles may limit the number of possible force vectors, making movement primitives a product of the actuator properties rather than the controller. In this study, we compared hindlimb movement vectors produced by SCμstim to those generated by direct nerve and muscle stimulation. In 2 pentobarbital-anesthetized cats, cuff electrodes were placed around nerve branches innervating the main limb flexors and extensors. Pairs of intramuscular (IM) electrodes were placed in the same muscles. SCμstim was delivered through microwires inserted to depths of 3-4.5mm along the lumbar enlargement. The foot was attached to a spring-loaded lever presenting...