<p>Spaced practice in declarative memory tasks consistently yields greater learning than massed practice, but spacing effects are less consistently observed for motor skills. This study evaluates factors that may determine spacing effects on motor skill learning, including: (1) extant theories of declarative spacing effects, (2) reactive inhibition, which transiently impairs performance and may also impair learning, and (3) the micro-consolidation hypothesis, which posits that motor skill learning takes place exclusively during brief performance breaks. Across two experiments, we varied the number of correct sequences per trial and the length of breaks during training, while keeping the total correct sequence count constant, using a widely studied motor sequence task. A pronounced performance advantage was observed for the spaced groups by the end of training. However, on a later test in which the task conditions were equated, group performance was statistically indistinguishable. Hence, spaced practice yielded no or minimal learning advantage and the large reactive inhibition effect in the massed group appears to be a transient performance phenomenon without consequence for learning. Furthermore, we found no evidence for the most straightforward micro-consolidation account, which predicts greater learning with more breaks. Our results are consistent with a simple account advanced by Gupta and Rickard^1,2, according to which learning occurs entirely online (i.e., concurrently with performance) and is independent of spacing and reactive inhibition. Finally, our findings indicate that proposed mechanisms for declarative spacing effects, such as memory reactivation and contextual variability, do not generalize to motor learning, highlighting fundamental differences between the two learning systems.</p>