Edited by: Sean P. Mullen, University of Illinois at Urbana-Champaign, USA
Reviewed by: Martin P. Paulus, University of California San Diego, USA; Amanda Lea Rebar, Central Queensland University, Australia
*Correspondence: Samuele Marcora, Endurance Research Group, School of Sport and Exercise Sciences, University of Kent at Medway, Medway Building, Chatham, Kent ME4 4AG, UK e-mail:
This article was submitted to the journal Frontiers in Human Neuroscience.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution and reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
The psychobiological model of endurance performance proposes that endurance performance is determined by a decision-making process based on perception of effort and potential motivation. Recent research has reported that effort-based decision-making during cognitive tasks can be altered by non-conscious visual cues relating to affect and action. The effects of these non-conscious visual cues on effort and performance during physical tasks are however unknown. We report two experiments investigating the effects of subliminal priming with visual cues related to affect and action on perception of effort and endurance performance. In Experiment 1 thirteen individuals were subliminally primed with happy or sad faces as they cycled to exhaustion in a counterbalanced and randomized crossover design. A paired
The psychobiological model of endurance performance (Marcora,
According to this effort-based decision-making model, any factor that influences perception of effort and/or potential motivation influences endurance performance, even when the physiological capacity to perform endurance exercise is unchanged. This proposal is in contrast to the muscle fatigue model of endurance performance in which exhaustion is thought to be caused by central and/or peripheral muscle fatigue (Allen et al.,
The proposal that the termination of endurance exercise is a conscious decision determined by perception of effort and potential motivation does not exclude the possibility that endurance performance may be influenced non-consciously. In fact, research over the last three decades has illustrated a variety of contexts in which human behavior can be altered by non-conscious psychological manipulations. Of particular relevance to the psychobiological model of endurance performance are studies showing that subliminal reward priming (Pessiglione et al.,
The capacity of the non-conscious perceptual system is considerably large in humans when compared to the limited capacity to attend to conscious information (Dijksterhuis and Nordgren,
Approximately 90% of the human capacity to process non-conscious information is occupied by the visual system (Dijksterhuis and Nordgren,
Interestingly, non-conscious manipulation of visual cues related to affect and action in the form of subliminal priming has been shown to influence effort during cognitive tasks within the framework of motivational intensity theory. For instance, individuals subliminally primed with happy faces exerted greater effort during challenging cognitive tasks in comparison to when primed with sad faces (Silvestrini and Gendolla,
Although it is evident that subliminal priming with visual cues related to affect and action can alter effort during cognitive tasks, the effects of such non-conscious visual cues on perception of effort and endurance performance are currently unknown. Establishing these effects is important for endurance athletes and would build on the recognized associations between affective states and endurance performance (Lane et al.,
The current investigation consists of two experiments. The aim of Experiment 1 was to establish the effects of subliminal priming with affective facial expressions (happy and sad faces) on rating of perceived exertion (RPE) and endurance performance. The aim of Experiment 2 was to establish the effects of subliminal priming with action and inaction words on RPE and endurance performance. Experiment 1 utilized a traditional group design whereas Experiment 2 employed a single-subject design. The latter was used to illustrate the application of randomization tests to assess the effects of non-conscious psychological manipulations in individual athletes as this approach may have important practical uses (see Section General Discussion). In both experiments a TTE test was used to establish the effects of each subliminal priming procedure on RPE and endurance performance without the confounding effect of individual pacing (Hopkins et al.,
A clear relationship exists between affective states and sports performance, with positive affect generally associated with better performance and negative affect generally associated with poorer performance (Beedie et al.,
Fourteen healthy and recreationally trained individuals volunteered to take part in the study. One female participant was excluded due to a computer malfunction that revealed one affective facial expression during the final visit. Hence 13 participants were included in the final data analysis [7 males, mean ±
The experiment consisted of a single blind, randomized and counterbalanced crossover design in which all participants visited the laboratory on four separate occasions. All exercise tests were conducted at the same location, at a similar time of day, on the same electromagnetically braked cycle ergometer (Excalibur Sport, Lode, Groningen, Netherlands). Saddle and handlebar specifications on the cycle ergometer were adjusted to suit the preference of each participant. These specifications were then maintained for every visit thereafter.
During Visit 1, each participant completed the informed consent questionnaire and a checklist to ensure compliance with pre-task instructions; anthropometric measurements were then recorded. After this, an incremental ramp test was carried out on the cycle ergometer to establish PPO and
Visit 2 was a familiarization session in which participants completed all questionnaires (see Section Psychological Questionnaires) and the TTE test to be used during Visits 3 and 4. Upon arrival for Visit 3 participants completed mood and motivation questionnaires followed by the TTE test. For this test, participants were positioned on the cycle ergometer (set to hyperbolic mode) and instructed to remain in the saddle at all times. The test began with a 3 min warm up at 30% of the participants PPO. After 3 min the power output was automatically increased to a power output corresponding to 65% PPO. Pedal cadence was freely chosen between 60–100 RPM and was recorded every minute during the test, as was heart rate (see Section Additional Physiological Measures). RPE was recorded at 2 min intervals during the test (see Section Rating of Perceived Exertion). TTE was defined as the time accrued from the onset of 65% PPO until the point at which either the participant voluntarily terminated the test or pedal cadence had fallen below 60 RPM for five consecutive seconds. No verbal encouragement was provided at any point during the TTE test to avoid experimenter bias. Furthermore, to avoid bias from facial mimicry, the experimenter stood behind participants at all times (Tassinary et al.,
Throughout the TTE test participants were exposed to a scanning visual vigilance task (see Section Scanning Visual Vigilance Task). This computerized cognitive task commenced at the onset of the TTE test and stopped at exhaustion. Corresponding to a randomized and counterbalanced order, subjects were allocated to priming with either happy or sad faces which were subliminally delivered within the scanning visual vigilance task for the duration of the TTE test (see Section Subliminal Priming Procedure). During the TTE test the computer screen was placed at eye level 60 cm away from participants. After the TTE test and the blood sample, participants completed the mood questionnaire for the second time.
All procedures during Visit 4 were identical to Visit 3, other than participants being subliminally primed with the alternative affective facial expression. At the end of Visit 4, participants underwent a standardized funneled debriefing procedure (Bargh and Chartrand,
The two experimental visits were separated by a minimum of 5 days and a maximum of 14 days. During this time, individuals were instructed to maintain their normal training program. As requested prior to each visit, participants preserved similar dietary patterns during the preceding 24 h while consuming an amount of water equivalent to least 35 ml·kg−1 body weight and attaining at least 7 h of sleep the night before. Participants also avoided any heavy exercise in the 24 h prior to testing and refrained from the consumption of caffeine and nicotine 3 h before each exercise test. Finally, participants voided before all exercise tests and performed them in similar clothing during every visit. Participants remained unaware of their TTE for the familiarization visit and for Visits 3 and 4 until the final debriefing procedure.
The scanning visual vigilance task developed by Lieberman et al. (
Subjects were subliminally primed during the scanning visual vigilance task throughout each TTE test. One prime was presented sequentially every 4996 ms. Each prime sequence consisted of a white fixation cross that was displayed on a black background in the center of the computer screen (1000 ms). This was instantly followed by a facial expression (16 ms) that was backward masked by a briefly flashed black and white pattern (130 ms). Following the backward mask, the screen either remained black (3850 ms) or alternatively a green circle of 3 cm diameter appeared against the black background in a random location (3850 ms). The next prime sequence was commenced immediately after this. In order to prevent habituation to the affective facial expressions, two thirds of the prime sequences consisted of a neutral face with the remaining one third consisting of the relevant affective facial expression (Silvestrini and Gendolla,
The funneled debriefing procedure was administered as a manipulation check to ensure that participants were not consciously aware of the affective facial expressions during the subliminal priming procedure. This method was adopted from previous recommendations from subliminal priming research (Bargh and Chartrand,
Perception of effort was measured using the Category Ratio 10 (CR10) scale developed by Borg (
Heart rate was recorded throughout the TTE test using wireless chest strap radio telemetry (S610, Polar Electro, Kempele, Finland). Before testing, the chest strap was dampened and securely fastened to the participant’s chest according to the manufacturer’s guidelines. Lactate concentration was measured by collecting 5 μl of whole fresh blood from the earlobe 3 min after the TTE tests. Each blood sample was immediately analyzed using a calibrated device (Lactate Pro LT-1710, Arkray, Shiga, Japan).
In order to evaluate differences in conscious affective state before and after the TTE test, mood was assessed using two self-reported items of the positive (
Unless otherwise noted, data are shown as mean ±
During the funneled debriefing procedure, the description given by all participants regarding their understanding of the study rationale conformed to the study rationale that they were provided with before the experiment. No participant was able to report anything unusual that was related to the real experimental manipulation, no participant suggested that the computer screen affected what they did during the TTE test and no participant was able to decipher the real reason for the black and white patterned backward mask. Three participants mentioned a possible brief facial silhouette on one occasion within the flashed black and white pattern that acted as the backward mask. However when probed further, no participant was able to elaborate on the facial expression of this silhouette. As no participant was suspicious of the study’s true purpose at its culmination and no participant was aware of any facial expression within or prior to the flashed backward mask, this was considered to provide sufficient evidence of a successful experimental manipulation.
We created separate global mood ratings for pre-exercise and post-exercise mood by adding the positive hedonic tone scale items to the reverse scored negative hedonic tone scale items (Table
Pre-exercise | Post-exercise | |
---|---|---|
Happy | 23.2 ± 2.6 | 23.6 ± 2.7 |
Sad | 22.9 ± 2.7 | 23.0 ± 2.3 |
Similar to mood, ratings for success motivation (happy faces = 18.4 ± 5.1, sad faces = 17.1 ± 5.6,
As predicted, subliminal affective priming had a significant effect on TTE, with participants cycling for 178 s (12%) longer when they were subliminally primed with happy faces (1519 ± 787 s) in comparison to when they were subliminally primed with sad faces (1342 ± 585 s),
At exhaustion, there were no significant differences between subliminal priming with happy and sad faces in heart rate, blood lactate concentration, and RPE (Table
Sad | Happy | ||
---|---|---|---|
Blood [la] (mmol·l) | 7.13 ± 1.86 | 6.97 ± 2.51 | 0.60 |
Heart Rate (beats·min−1) | 180 ± 10 | 181 ± 10 | 0.53 |
RPE (0–10+) | 9.92 ± 0.28 | 9.96 ± 0.32 | 0.33 |
Mean Cadence (RPM) | 75.8 ± 4.7 | 78.0 ± 3.3 | 0.01 |
RPE increased significantly during the TTE test (main effect of iso-time:
Heart rate increased significantly during the TTE test (main effect of iso-time:
The findings that non-conscious visual cues related to affect can alter perception of effort and performance during whole-body endurance exercise are in line with previous studies that have effectively used similar subliminal priming procedures to manipulate performance (Freydefont et al.,
The present findings provide experimental evidence to support the hypothesis that positive affect is associated with better sport performance and negative affect is associated with poorer sport performance. This is important as support to this hypothesis is seemingly limited to correlational findings (Lane et al.,
The aim of Experiment 2 was to replicate the findings of Experiment 1 with a different type of subliminal visual cues: action and inaction words. This type of non-conscious visual cues is relevant for a number of reasons. Specifically, the effects of subliminal priming with action or inaction words on perception of effort and endurance performance may be mediated by alterations in pre-motor and motor areas of the brain. Activity of these cortical areas during muscle contractions is associated with perception of effort in humans (de Morree et al.,
One healthy male participant (age 22 years, PPO 287 W,
The experiment was a single blind, blocked randomization tests design (Dugard et al.,
All exercise tests were conducted at the same location, at a similar time of day, on the same electromagnetically braked cycle ergometer (Excalibur Sport, Lode, Groningen, Netherlands). Saddle and handlebar specifications on the cycle ergometer were adjusted on the first visit to suit participant preference and these specifications were then maintained for every visit. Visit 1 consisted of an incremental ramp test to establish PPO and
Visits 1, 2, and 3 were separated by a minimum of 7 days each, while all visits between Visits 3 and 14 were separated by a minimum of 6 days and a maximum of 15 days. All pre-visit instructions were the same as those for Experiment 1. The participant remained unaware of his TTE value for the familiarization visit and for every subsequent visit until the final debriefing procedure.
In conjunction with visit allocation, action or inaction words were subliminally primed within the computerized scanning visual vigilance task for the duration of each TTE test. The procedure for the scanning visual vigilance task was identical to that of Experiment 1 other than the fact that subliminally primed words replaced the subliminally primed affective facial expressions that were used in Experiment 1. During the subliminal priming procedure, a word prime sequence was presented serially every 4996 ms. Each prime sequence first consisted of a white fixation cross that was displayed on a black background in the center of the screen (1000 ms). This was instantly followed by a word prime (16 ms) that was backward masked by a random letter sequence (130 ms). This random letter sequence always consisted of the letters MZKGWB and appeared after every word prime.
Following the backward mask, the screen either remained black (3850 ms) or alternatively a green circle of 3 cm diameter appeared against the black background in a random location on the screen (3850 ms). The next word prime sequence commenced immediately after. To prevent habituation to the subliminal word primes, two thirds of the primes consisted of non-word primes with the remaining one third consisting of the word primes (Silvestrini and Gendolla,
The word primes were obtained from the empirically derived Computerized Edinburgh Associative Thesaurus (Kiss et al.,
The funneled debriefing procedure (Bargh and Chartrand,
The procedures used to measure RPE were identical to those in Experiment 1 except that RPE was measured at 1 min intervals during the TTE test.
The Brunel mood scale (BRUMS) was used to assess mood before the TTE test. This measure of mood has been validated for use with adult populations (Terry et al.,
Unless otherwise noted, data are shown as mean ±
For each randomization test, in order to test for statistical significance, mean values for each treatment condition were first calculated. The difference between these means was then obtained. These values provided the true experimental difference between treatments for each dependent variable. The randomized order of experimental treatments across the 12 visits represented one of many possible ways in which the treatment visits could have been arranged. Using a pre-designed macro (Dugard et al.,
A qualitative evaluation of the funneled debriefing procedure indicated that the participant believed the cover rationale for the study to be genuine throughout. The participant also did not detect any subliminally primed action and inaction words during the 12 visits. The participant therefore remained naive to the true experimental hypotheses during the investigation.
A randomization test for each BRUMS subscale revealed no significant differences in pre-exercise mood between conditions (see Table
BRUMS subscales | Motivation | |||||||
---|---|---|---|---|---|---|---|---|
Anger | Confusion | Depression | Fatigue | Tension | Vigour | Success | Intrinsic | |
Action | 0.00 ± 0.00 | 0.00 ± 0.00 | 0.00 ± 0.00 | 0.00 ± 0.00 | 2.83 ± 0.75 | 14.33 ± 1.97 | 28.00 ± 0.00 | 28.00 ± 0.00 |
Inaction | 0.00 ± 0.00 | 0.00 ± 0.00 | 0.00 ± 0.00 | 0.00 ± 0.00 | 4.17 ± 0.75 | 14.83 ± 0.98 | 27.67 ± 0.52 | 28.00 ± 0.00 |
0.99 | 0.99 | 0.99 | 0.99 | 0.06 | 0.49 | 0.33 | 0.99 |
The participant cycled for 399 s longer when subliminally primed with action words in comparison to inaction words (see Table
Block | Visit | Condition | TTE (s) | HR (beats . min−1) | Blood [la] (mmol·l) | Mean Cadence (RPM) | RPE (0-10+) |
---|---|---|---|---|---|---|---|
1 | 1 | A | 1772 | 140 | 2.2 | 86.5 | 10 |
1 | 2 | I | 915 | 136 | 2.6 | 80.6 | 10 |
1 | 3 | I | 1835 | 137 | 3.1 | 85.1 | 10 |
1 | 4 | A | 1910 | 144 | 2.7 | 86.9 | 10 |
2 | 5 | I | 2304 | 146 | 2.0 | 82.3 | 10 |
2 | 6 | I | 2781 | 153 | 2.6 | 84.8 | 10 |
2 | 7 | A | 2822 | 148 | 1.9 | 80.7 | 10 |
2 | 8 | A | 2975 | 142 | 2.3 | 81.9 | 10 |
3 | 9 | I | 2705 | 139 | 1.9 | 81.1 | 10 |
3 | 10 | A | 3291 | 138 | 2.4 | 81.0 | 10 |
3 | 11 | I | 2528 | 139 | 2.4 | 82.2 | 10 |
3 | 12 | A | 2692 | 136 | 1.9 | 81.9 | 10 |
2577 ± 605 | 141 ± 4 | 2.23 ± 0.31 | 83.1 ± 1.4 | 10 ± 0 | |||
2178 ± 706 | 142 ± 7 | 2.43 ± 0.44 | 82.7 ± 1.7 | 10 ± 0 |
Randomization tests between conditions at iso-time (15th minute) revealed that RPE was significantly lower (
The results of Experiment 2 bolster those of Experiment 1 by demonstrating that non-conscious visual cues related to action were able to significantly alter RPE and TTE for the individual involved in this single-subject randomized experiment. These findings are consistent with the previously reported effects of subliminal priming with action or inaction words on effort during cognitive tasks (Gendolla and Silvestrini,
The purpose of the present research was to investigate the effects of non-conscious visual cues on perception of effort and endurance performance. The two different types of visual cues utilized in Experiments 1 and 2 respectively were able to alter both perception of effort and endurance performance during cycling exercise. Specifically, as hypothesized in Experiment 1, subliminally priming participants with happy faces as they cycled to exhaustion at 65% PPO significantly reduced RPE in comparison to subliminal priming with sad faces. Correspondingly TTE was significantly greater when participants were subliminally primed with happy faces. Similarly, the findings from the single subject approach used in Experiment 2 demonstrated that subliminal priming with action words significantly reduced RPE and enhanced TTE in comparison to subliminal priming with inaction words.
The findings that non-conscious visual cues can affect perception of effort and whole-body endurance performance extend previous reports that non-conscious psychological manipulations have significant effects on effort and behavior during both cognitive and physical tasks (Bargh et al.,
Based on motivational intensity theory, the psychobiological model of endurance performance (Marcora,
This psychobiological model of endurance performance provides a single theoretical framework to explain the effects of many different physiological and psychological factors known to affect endurance performance. These factors include muscle fatigue (Marcora et al.,
The significant effects of subliminal visual cues on TTE also challenge the proposal that endurance exercise terminates when the fatigued neuromuscular system (Amann and Dempsey,
The present findings also provide further evidence against one of the main hypotheses of the central governor model of endurance performance: the subconscious brain, based on interoception and previous experience, calculates the maximum time a person can exercise without a catastrophic failure of homeostasis, and regulates RPE and TTE accordingly (St Clair-Gibson and Noakes,
Given the pivotal role played by perception of effort in mediating the effects of subliminal visual cues on endurance performance, it is important to establish the mechanisms that may be responsible for changes in perception of effort during the exposure to subliminal visual cues. Although the present experiments were intended to be exploratory and not designed for this purpose, some potential psychological explanations for the current findings are worthy of consideration. For instance, within the framework of the broaden and build hypothesis (Fredrickson,
In conjunction with the theme of non-consciously activated mental representations, it has also been reported that the sensitivity to non-conscious reward priming increases as effort becomes more pronounced (Bijleveld et al.,
In addition to the similar pre-exercise mood ratings, conscious appraisals of mood did not change from pre to post-exercise and were not different between affective priming conditions. These similarities in conscious mood ratings might initially imply that the subliminal manipulation of affect in the present study was not effective. However, the present finding is consistent with other studies that have utilized non-conscious affective priming (Winkielman and Nowak,
From a practical perspective, the importance of the present findings are best signified when their effects are compared to the effects of physiological factors known to alter endurance performance. For instance the 12% difference in TTE resulting from subliminal priming with happy or sad faces and the 17% difference in TTE between subliminal priming with action and inaction words can be likened to the negative effects of inspiratory muscle fatigue (14%; Wüthrich et al.,
Despite the theoretical and practical implications of the present experiments, it is also important to recognize some potential limitations. For instance, because a control condition was not utilized in either experiment it is not possible to establish whether the difference in TTE resulted from an increase in endurance performance following subliminal priming with happy faces or action words, a decrease in endurance performance following subliminal priming with sad faces or inaction words, or a combination of both effects. However, it was decided that establishing the existence of an overall effect of non-conscious visual cues on endurance performance was the most pertinent aim of each of these proof of principle studies. Hence, on this occasion, the use of one experimental condition and a control condition was not implemented. In addition, due to the crossover design of both investigations, a forced prime recognition check was not carried out at the culmination of each study. This was again however a known consequence of each methodological design. Specifically, while it is acknowledged that this manipulation check is important, in this instance it was reasoned that a within participant design would more effectively answer our research questions and that this approach would confound such a check. This is because implementing a forced prime recognition check only after the final subliminal priming visit may have introduced recall bias owing to the fact that participants would have been required to recall facial expressions not only for the task that they had just completed, but also for those of the previous visit(s).
In the second experiment a single subject design was implemented to exhibit the use of randomization tests as an important and effective methodological approach to single subject research. It should be noted however that randomization tests lack any ecological validity beyond that of the individual investigated. Wider generalization of the present findings is therefore not possible. Nonetheless, the strength of these tests also resides in this focused individual approach. Importantly, the repeated assessment approach that is inherent in Experiment 2 also permits some interpretation of the manner in which the priming intervention itself may have worked. For instance, the findings hint at the possibility that the effects of action and inaction word priming is acute as opposed to long lasting. As such this approach may be repeatedly used to acutely manipulate performance. It should also be noted that the TTE in Visit 2 was markedly shorter than all others. This occurred despite the participant having adhered to all their instructions prior to the visit. Although the data from this visit may be a genuine result of the treatment manipulation, and should not be regarded as otherwise, it is also acknowledged that it could be anomalous. To account for this the raw performance value for this visit was substituted with the highest TTE value of the corresponding visit within that block. Importantly, despite doing so the difference between conditions remained significant.
Notwithstanding the methodological considerations, obvious directions for future research include the addition of a control condition to the present design to help establish the directional effects of the non-conscious visual cues used in Experiments 1 and 2. Likewise, replicating Experiment 2 from a group based perspective would facilitate wider generalization regarding the effects on non-conscious action and inaction words on RPE and endurance performance. Such approaches might clarify whether non-conscious visual cues can be used as a performance-enhancing strategy during training and competitions in endurance athletes, e.g., by using contemporary technology such as smart glasses. This would also establish whether non-conscious visual cues may be an effective tool to reduce perception of effort and, thus, improve exercise adherence in recreational exercisers. In addition, it would be useful to assess whether these subliminal priming effects remain when individuals are aware that they are being subliminally primed as this would mitigate the ethical considerations associated with subliminal priming. Moreover, the investigation of pre-event subliminal priming may permit a direct approach to performance enhancement in competitive events by eliminating the necessity for visual display technology during the event itself. Finally, the impact of alternative visual cues such as anger or disgust may provide further invaluable insights into some of the factors that non-consciously influence endurance performance.
In conclusion, the collective findings of Experiments 1 and 2 are the first to show that subliminal visual cues can influence perception of effort and endurance performance. These novel findings corroborate the suggestion that endurance performance is regulated by psychobiological factors. Together, this supports and extends the explanatory power of the psychobiological model of endurance performance and the suggestion that any physiological or psychological factor affecting perception of effort and/or potential motivation will affect endurance performance (Marcora et al.,
Anthony Blanchfield, James Hardy, and Samuele Marcora contributed to the conception and design of the work. Acquisition was performed by Anthony Blanchfield and analysis and interpretation of the work was conducted by Anthony Blanchfield, James Hardy, and Samuele Marcora. Anthony Blanchfield, James Hardy, and Samuele Marcora were involved in all stages of the drafting and revision of the work as well as final approval of the version to be published. All authors (Anthony Blanchfield, James Hardy, and Samuele Marcora) agree to be accountable for all aspects of the work.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.