Humans acquire an amazing variety of skills during lifetime, including playing piano and dancing. Yet, the neural plasticity underlying learning and long-term persistence of skills are surprisingly little understood. As neural plasticity is not measurable in healthy humans, this study adopted a behavioral approach using detailed kinematics as a window to assess learning and retention. Two groups of subjects practiced a bimanual polyrhythmic skill. They oscillated their forearms in the horizontal plane supported by two rotating bars, which recorded angular displacement of each forearm. Subjects were instructed to oscillate their forearms in a 3:2 ratio with the dominant arm performing the fast movement. One group performed the skill at their preferred tempo, while the other group was paced by a metronome to a comfortable but imposed frequency. Importantly, only sparse feedback about their average frequency ratio was provided to allow individuals to develop their idiosyncratic realization. All subjects underwent extensive practice: 20 sessions (15 trials each). Retention of the skill was tested three months after practice. Continuous performance was quantified by relative phase between arms and trajectory in 4D phase space. Results showed that all subjects acquired the average 3:2 ratio, although with very different accentuations. At retention, subjects in the self-paced group retained not only the average frequency ratio but also their idiosyncratic movement trajectories. In contrast, the metronome group showed increased variability in their hand coordination. These results highlight that humans retain detailed kinematic patterns over a long time. However, temporal constraints affect the consolidation of movement patterns.