Feedback protocol of 'fading knowledge of results' is effective for prolonging motor learning retention

[Purpose] "Knowledge of results" (KR) is information about the success of an action relative to a goal. A reduced frequency of "knowledge of results" reportedly promotes motor learning more than a high frequency. However, the effect of gradually diminishing or increasing "knowledge of results" pattern has been rarely studied and is controversial. We investigated the effectiveness of diminishing "knowledge of results" pattern in motor learning. [Participants and Methods] Forty-six healthy adults were randomly assigned to either the 100% KR, 50% KR, or faded KR group. Participants were tasked with exerting 60% of their maximum voluntary contraction of their left shoulder flexion muscle in an isometric exercise. Participants practiced the task 20 times a day for 4 days. A pretest and posttest were conducted before and immediately after the acquisition, respectively. Retention tests were conducted 1 day, 1 week, and 2 weeks after the acquisition. [Results] The absolute error was significantly reduced in the posttest in the faded KR and 50% KR cohorts. However, there was no significant difference in the 100% KR group. In the faded KR subjects, the improvement effect was observed up to 1 week following acquisition. [Conclusion] Faded "knowledge of results" productively prolongs the effect of motor learning.