Implicit motor learning paradigms aim to minimise verbal-analytical engagement in motor performance. Some paradigms do this by decreasing working memory activity during practice, which reduces explicit processes associated with the search for motor solutions (e.g., hypothesis testing). Here we designed a mentally demanding motor task to fatigue working memory prior to motor practice and then tested whether it reduced hypothesis testing. Fifty-nine participants were randomly assigned to complete the mentally demanding motor task (cognitive fatigue group) or to complete an undemanding motor task (non-fatigued control group). Feelings of fatigue, working memory functions, electroencephalography (EEG) Fz power and vagal control were assessed pre- and post-task to quantify the effect of the mentally demanding motor task on cognitive fatigue. Thereafter, an adapted shuffleboard task was completed to determine the impact on hypothesis testing. Hypothesis testing was assessed by self-report, technique changes and equipment-use solutions. Additionally, verbal-analytical engagement in motor performance was (indirectly) gauged with EEG T7-Fz connectivity and T7 power measures. Participants in the cognitive fatigue group reported more fatigue, and displayed moderated working memory functions and Fz theta power. During practice of the shuffleboard task, participants also displayed more technique changes and higher verbal-analytical engagement in motor planning (EEG T7-Fz connectivity), compared to participants in the control group. The mentally demanding motor task suppressed working memory functions, but resulted in more, rather than less, hypothesis testing during shuffleboard practice. The implications are discussed in the context of implicit motor learning theory.