Participant characteristics

All experimental procedures in this investigation were reviewed and approved by the Westmont College Institutional Review Board (IRB) prior to the beginning of this study and conformed to the principles of the Declaration of Helsinki. The protocols and procedures were explained, and all participants provided written informed consent prior to testing. Participants (N = 33) consisted of recreationally trained individuals recruited by word of mouth (males = 21, females = 12; mean ± SD; age 20.7 ± 1.3 years, body weight 72.3 ± 9.3 kg, height 176.5 ± 8.9 cm, peak oxygen consumption (VO2peak) 47.9 ± 9.3 ml/kg/min). Six additional participants began the study, but withdrew before completion and were excluded from the analysis. Participants were excluded if any of the following applied: current acute musculoskeletal injury; medications known to interfere with the sympathetic nervous system; unwillingness to comply with training interruptions mandated by the protocol; any uncontrolled chronic health conditions.

Protocol overview and exercise protocols

The study consisted of a controlled pre-test–post-test design in which participants visited the laboratory on six separate occasions and were randomized into two independent groups using a random number generator (Mental training (MT) n = 16; 9 males, control (CON) n = 17; 12 males) during the first visit. The first TTE (performed on study day 3) was classified as the pre-training TTE and was compared to the final TTE, classified as post-training, (performed on study day 6) for all data. All exercise tests were conducted in the same location on the same electromagnetically braked cycle ergometer (Excalibur Sport, Lode, Groningen, the Netherlands), with the saddle height adjusted to suit the preference of each participant and maintained for each visit. Each time trial was performed at the same time of day to avoid circadian rhythm differences between trials. Participants were asked to avoid caffeine before each exercise trial and all participants refrained from exercise for 24 h before each trial. None of the participants were trained cyclists to account for any variance cycling experience would add to time to exhaustion nor were there any monetary incentives for participating in or completing the study.

Graded exercise test protocol

During visit one, each participant completed an incremental cycling test beginning at 100 W (50 W for females) with resistance increasing 5 W every 15 s until volitional exhaustion to establish VO2peak and to determine ventilatory threshold. Participants pedaled at their preferred pedaling rate, and the test was terminated when the cadence dropped below 70 rpm for more than 5 s, despite strong verbal encouragement. Metabolic data were collected using open circuit calorimetry (Vista MX, Vacumed, Ventura, CA). Peak oxygen consumption (VO2peak) was recorded as the highest VO2 recorded in a 15 s period. Subsequently, the ventilatory equivalent method, or power output corresponding to a systematic increase in the ventilatory equivalent of oxygen (VE/VO2) without a concomitant increase in the ventilatory equivalent of carbon dioxide (VE/VCO2), was used to determine the power output at ventilatory threshold (Wasserman and McIlroy 1964).

Time to exhaustion (TTE) protocol

The subsequent five visits consisted of a time to exhaustion test at a wattage of 10% above the determined ventilatory threshold. The time to exhaustion test began with a 5-min warm-up at 100 W (75 W for women). Following the warm-up, participants were asked to cease pedaling while the power was set on the ergometer. Two minutes after the warm-up ended, participants were asked to begin the TTE. They were encouraged to stand for the first 3–5 s of the TTE to start the flywheel spinning, but they were required to stay seated for the remainder of the trial. Time to exhaustion was defined as the time from the onset of pedaling until the point at which cadence had fallen below 70 rpm for more than 5 s. If cadence was below 70 rpm, researchers tapped on the cycle ergometer’s tachometer to alert the participant to increase cadence. No verbal encouragement was provided at any point during the time to exhaustion test to eliminate any external motivation. Heart rate was recorded every minute throughout the time to exhaustion test using a wireless chest strap (Polar Electro Inc., Bethpage, New York, USA).

Visit two was a familiarization session and was separated from visit three by 1 week. Visits three through six were each separated by 7 days during which the MT interventions took place. MT interventions consisted of watching an initial training video immediately following the exercise test during visits 3–5. During the subsequent weeks, MT participants were asked to watch one of three videos each day for the following week (ending with watching all three videos twice during the week, for a total of watching each video six times each over the course of 3 weeks). Prior to visit 3 and visit 6, participants in each group were instructed to complete the CD-RISC 10 and GRIT-S surveys.

Mental training intervention

The mental training intervention was designed to examine the primary research question regarding the physiological underpinnings of traditional mental skills training. Mental training can also be called psychological skills training, and, as defined by the American Psychological Association, is a program of instruction and practice in the use of relaxation, concentration, imagery, goal setting, and energizing to enhance athletic training. A Certified Mental Performance Consultant © (CMPC) who has more than 10 years of field experience working with endurance and team athletes designed the mental training protocol. The mental training protocol was video recorded to ensure consistent delivery to participants.

The mental training intervention consisted of four videos containing exercises for participants to use to enhance their performance: an introduction to mental skills training and breathing techniques to reduce stress and anxiety while allowing increased confidence and feelings of well-being (Wilson and Taylor 2014) (video time 9 min and 43 s), a “controlling the controllables” lesson to focus more on controllable aspects of performance to reduce stress (video time 8 min), self-talk and confidence intervention to combat negative thinking and doubt (video time 11 min and 25 s), and finally, an imagery intervention to mentally prepare for performances and challenging moments (video time 4 min and 40 s). The goal of the mental training intervention was to equip participants with breathing techniques, cognitive behavioral strategies, and mental preparation strategies to endure fatigue and enhance endurance. Each of the videos, other than the introduction, were watched two times during the week at home for a total of 3 weeks. The introductory video was watched three times in total, immediately after each TTE on days 3–5. In addition, the participants were instructed to follow along with the video, performing the instructed activity, to practice the different skills being taught and to keep them as active listeners. Adherence to watching the videos was assessed weekly by asking participants a question on what they had found most interesting/beneficial from the videos for that week. Total time of the mental training intervention was 173 min and 39 s. The control group were not given any videos during the study. However, each CON participant was given the videos at the end of the study.

Psychological measures

The ten-item Connor–Davidson Resilience Scale (CD-RISC; (Connor and Davidson 2003)) was employed to measure resilient characteristics in the participants. The 10-item CD-RISC was used in place of the original 25-item scale because the 10-item scale has stronger psychometric properties in sport and performance contexts (Gucciardi et al. 2009; Gonzalez et al. 2016)). Participants were directed to indicate how much they agreed with statements as they apply to their lives. Each response was given on a five-point Likert-Type Scale (0—not at all true to 4—true nearly all the time). Example items included “I can deal with whatever comes my way,” Having to cope with stress can make me stronger,” and “I tend to bounce back after illness, injury, or other hardships.” Scores were summed and ranged from 0 to 40 with higher totals indicating more resilient characteristics. Cronbach’s alpha for the CD-RISC in this study was 0.778 at pre-test and 0.862 at post-test.

Grit was assessed with the Short Grit Scale [Grit-S; (Duckworth and Quinn 2009)]. The Grit-S consists of two, four-item subscales that measure interest and effort. Each participant was asked to answer the following statements honestly on a five-point Likert-Type scale (1—not at all like me to 5—very much like me). Examples of items included “I have difficulty maintaining my focus on projects that take more than a few months to take” (interest subscale) and “Setbacks don’t discourage me” (effort subscale). Scores were summed for each subscale and ranged from 4 to 20 with higher totals indicating more interest or effort. Cronbach’s alphas for the interest subscale was 0.765 at pre-test and 0.616 at post-test and for the effort subscale was 0.853 at pre-test and 0.507 at post-test.

Neuromuscular testing

Central and peripheral contributions to muscle fatigue, 5–10 min before and 1 min after each exercise test (TTE), were examined by superimposing a supramaximal magnetic stimulation of the femoral nerve during and 5 s after a series of maximal voluntary contractions (MVCs) of the quadriceps. Participants sat in a semi-reclined position on a table, with the upper body and lower back supported at a hip angle of 45°, and the knee joint angle set at 90° of flexion with the arms folded across the chest. A magnetic stimulator (Magstim 2002; Wales, UK) connected to a 70 mm double coil was used to stimulate the femoral nerve. The evoked quadriceps twitch force was obtained from a calibrated load cell (MLP-300; Transducer Techniques, Rio Nedo Temecula, CA) connected to a noncompliant strap which was placed around the participant’s right leg, just superior to the malleoli.

Maximal femoral nerve stimulation was verified in each participant by assessing unpotentiated quadriceps single twitch forces obtained at 70%, 80%, 85%, 90%, 95%, and 100% of maximal stimulator output. A plateau in baseline unpotentiated quadriceps single twitch force with increasing stimulus intensities was observed in every participant and a plateau in M-wave amplitudes was observed in the sub-set of participants in which EMG was recorded. The stimulator was set at 100% for all participants and trials.

A superimposed twitch force during and a potentiated force (Qtw,pot) 5 s after a 5-s maximal isometric voluntary contraction of the quadriceps were measured and this procedure was performed six times. Like others, we found the degree of potentiation was slightly smaller after the first and second MVC (Kufel et al. 2002); therefore, we discarded the first two measurements. Peak force, maximal rate of force development (MRFD), contraction time (CT), and reaction time (RT0.5) were analyzed for all Qtw,pot. Voluntary activation of the quadriceps during the MVCs was calculated using the following equation: 1 − (superimposed twitch force/Qtw.pot force)*100.

Electromyography

Quadriceps electromyogram (EMG) was recorded from the right rectus femoris (RF) using monitoring electrodes with full-surface solid adhesive hydrogel (Delsys Trigno Wireless EMG, Natick, MA, USA) with on-site amplification. Electrodes were placed in a bipolar electrode configuration on the midpoint of the RF with an inter-electrode distance of 100 mm. The EMG electrode was placed in the same location during all visits. The surface EMG electrodes were used to assess the maximal EMG of the RF during a maximal voluntary contraction before each TTE. RF EMG was continuously measured during the subsequent time trial to estimate changes in central neural command.

All EMG recordings were high-pass filtered using fourth order zero-lag Butterworth filters and subsequently smoothed using a root-mean-square (RMS) filter (30-ms symmetrical moving window with successive 1-ms steps). EMG signal amplitudes from the TTE were normalized to the maximum RMS EMG amplitude recorded during MVC testing of the quadriceps. The EMG amplitudes during the TTE were measured over five seconds at baseline and then measured as an average of 0–20%, 20–40%, 40–60%, 60–80%, and 80–100% based on the shorter trial. Due to recording issues (i.e. electrode falling off/coming loose, etc.), data from six participants were excluded from analysis.

Perceptual measures

Perceptual responses for leg specific fatigue and leg specific pain during each TTE were recorded every minute. Numerical ratings of leg fatigue and pain, from 0 to 100 were used to assess the severity of preexisting and exercise-related leg fatigue and pain symptoms, comparable to “perceived discomfort” scales described by (Christian et al. 2014). Fatigue and pain ratings were anchored with 0 being described as no pain or fatigue, 25 being described as mild pain or fatigue, 50 being described as moderate pain or fatigue, 75 as severe pain or fatigue, and 100 as the worst possible fatigue or pain imaginable. Participants provided ratings of perceived exertion (RPE) every 30 s using the Borg 6–20 scale (Borg 1982), explained as the answer to the question “how hard do you feel like you are working?” thus representing the combination of effort and peripheral sensations.

Statistical analysis

Data for all physiological and perceptual measurements were recorded using the “individual isotime” method as described by Nicolò et al. (2019) in order to compare each test at the same absolute timepoint. In short, the worst of the two TTE tests was used to identify 11 timepoints into which the two tests were segmented and normalized as a percentage of the shortest TTE (0–100%). For all participants, each time point varies from the other participants on the basis of their worst TTE.

Time to exhaustion and neuromuscular data were analyzed using 2 (treatment) × 2 (trial) repeated measures ANOVAs using SPSS version 28. Significant treatment effects and time by treatment interactions were followed up with post hoc paired t tests.

Physiological data were analyzed using 2 (treatment) × 2 (trial) × 11 (time) repeated measures ANOVA. To evaluate treatment and time effects for EMG data, a 3 (treatment) × 2 (trial) × 6 (time) repeated measures ANOVA was performed. In instances where Mauchly’s test for sphericity was significant, Hyunh-Feldt correction was used to adjust for degrees of freedom. If time effects were significant, planned contrasts were used to determine which points differed from baseline. Significant treatment effects and treatment by time interactions were followed up with post hoc paired t tests. Psychological data were analyzed using a repeated measures ANOVA. All data were presented as means and standard deviations, with significance set at α < 0.05.