AbstractCognitive control exertion leads to mental fatigue and has been shown to impart negative effects on subsequent physical performance. Although the mechanisms underlying these effects are not well understood, previous research suggests mental fatigue causes neuromuscular perturbations which may impair physical performance. This study examined the effects of a mentally-fatiguing, cognitive control task on physical performance and muscle activation while performing a resistance exercise task. The study employed a randomized, cross-over design. On Visit 1, participants (N = 10) performed a barbell biceps curl one-rep maximum (1RM) test. On Visits 2/3, participants performed 20 biceps curls at 50% of their 1RM, followed by their respective 10-minute cognitive experimental manipulation (high vs. low cognitive control), and then performed a second set of biceps curls at 50% of their 1RM to failure. Total repetitions performed on the post-manipulation exercise task, and muscle activation (electromyography amplitude) of the biceps, triceps and lumbar erector spinae muscles were recorded. Mental fatigue was significantly greater following the high cognitive control manipulation (d = 1.09). Total repetitions performed following the high (M=24.9) and low cognitive control manipulations (M=25.2) did not differ (p > .05). However, there was a significant increase in muscle activation for the biceps, triceps and lumbar erector spinae during repetitions performed while mentally fatigued that indicate greater involvement of the low-back musculature and co-contraction of the triceps while lifting. Findings suggest mental fatigue may alter motor unit recruitment, reducing the efficiency of movement patterns in ways that may leave people susceptible to injury.