Mental fatigue (MF) is a psychobiological state induced by demanding cognitive tasks that is characterized by subjective tiredness or lack of energy and impaired cognitive function.1,2 Exercise physiologists have confirmed its potential to impair physical performance2 and sport-specific psychomotor performance.3 Interestingly, in these studies, poorer performance was not accompanied by any peripheral physiological changes but, rather, by increased rating of perceived exertion (RPE). Therefore, RPE has been proposed as the cause of reduced endurance performance for tasks that are vulnerable to MF.2 In contrast, physical activities that require brief bursts of strength/power, such as sprints, jumps, and maximum voluntary contractions, appear to be largely unaffected by MF.2,4,5 For example, there is evidence that MF does not impair strength and power exercises lasting up to 3 minutes.2,6 Conversely, MF can impair submaximal resistance strength exercise. One study documented that MF hastened the time to failure when performing a submaximal contraction at 20% maximum voluntary contraction.7 Similarly, other studies noted that cognitive tasks impaired subsequent performance of calisthenic exercises, including fewer push-ups and sit-ups as well as shorter wall-sits.8,9 Taken together, these findings suggest that MF could negatively impact resistance training. Despite the large body of research concerning the effects of MF on physical performance, there is scant evidence on the effect of MF on resistance exercise performance. Recent studies found that MF, induced by cognitive tasks, reduced the number of repetitions and sets of resistance exercise performed.10,11 Accordingly, there is preliminary evidence that weight training may be negatively impacted by MF. Impairments in physical performance that accompany states of MF appear to be uncoupled from physiological changes, such as oxygen consumption, heart rate (HR), and blood lactate.2 However, the role of muscle activity is less clear-cut. Some studies report no effect of MF on electromyography (EMG),6,7,12,13 whereas other studies document increased vastus lateralis14 and rectus femoris15 EMG when in a state of MF. Accordingly, further research is warranted to better understand the effect of MF on muscle function. The vast majority of studies on MF and physical performance have examined the effect of a classic cognitive task (eg, Stroop) designed to induce MF, on a subsequent exercise task. However, in sport a variety of cognitive tasks, which vary in terms of duration and scheduling relative to the sporting activity, such as coach briefings, social media, work commitments, and the mental demands of sport itself,11,16 have been identified as sources of MF. During physical training with scheduled rest breaks (eg, resistance training, high-intensity interval training, and track sprinting) and multistage competitions, athletes may engage in cognitive activities (eg, social media or games on a smartphone) during these inactive periods and thereby put themselves in a mental state that impairs any subsequent physical performance. In other words, such athletes can be considered as engaging in a form of intermixed mental and physical loading. To date, no laboratory study has examined the effect of such intermixed loading on performance. Moreover, studies that have examined the effect of performing the cognitive task during the physical exercise task (ie, concurrent loading) are rare.17,18 Importantly, the evidence generated by these studies is mixed, and therefore, the effect of a cognitive load added to a physical load on subsequent physical performance awaits clarification. To address this gap in our understanding of the MF-performance relationship, we explored the effect of intermixed mental and physical loads, which were designed to create a state of cognitive and physical fatigue, on a subsequent physical endurance task. The purposes of the current 2-part study were 3-fold. Our first study purpose (part 1) was to determine the effect of a prior isolated cognitive task on subsequent MF, mental alertness, and perceived effort associated with performing a series of submaximal weight lifting exercises. We hypothesized that cognitive loading with an isolated cognitive task would induce MF, reduce mental alertness, and increase RPE during weight lifting. Our second study purpose (part 2) was to determine the effect of cognitive tasks intermixed with resistance training exercises on MF, alertness, and perceived effort. We hypothesized that intermixed cognitive and physical training would increase cognitive load, induce MF, reduce alertness, and increase sessional RPE of the training session. Finally, our third study purpose was to determine the effect of intermixed cognitive and physical training on subsequent cycling time trial (TT) performance. We hypothesized that the cognitive loading and MF produced by the intermixed cognitive and physical training would reduce subsequent cycling power output and distance covered.
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