Regular physical activity invokes biochemical, morphological, and functional changes in various systems of the human body (11,14,32,39). In addition to improving exercise performance and reducing the risk of many chronic diseases, aerobic exercise training has emerged to be effective in improving some aspects (e.g., attention, memory, and executive function) of cognitive function in school-aged children, young adults, and the elderly (17,37).
Interval training is a time-efficient alternative to traditional moderate intensity exercise training; both modes of training are efficacious in augmenting endurance capacity. Until now, interval training has been primarily aimed at improving aerobic power, and is currently widely recommended for athletic and general or recreational populations (18,40). Interval training is characterized by repeated bouts of cyclic exercise (such as cycling and running). Periods of rest between exercise bouts allow exercise to be performed for prolonged total duration (14,24). Although interval training may be superior to traditional moderate intensity training in improving aerobic fitness (15,18), other benefits of interval training such as improved cognitive performance are poorly investigated.
High-intensity exercise might have a temporal negative effect on cognitive functions (6), which could be explained at least partly by elevated stress hormones and restricted cerebral hemodynamics (40). Although improved cognitive function is associated with pleasurable sensations experienced during or shortly after moderate intensity exercise (31), early and repeated fatigue caused by the intense and repetitive nature of intervals may induce unpleasant sensations (32), and in the short-term, could preclude some exercise training benefits in cognitive functions (6). However, as the training progresses, the benefits may outweigh the acute short-term effects and the program could be as effective as, or even superior to moderate intensity exercise training. Indeed, it has been shown that interval training improves aerobic fitness (maximal oxygen uptake) in just 6 weeks (39). It has been recently found that a single interval training session at the intensity corresponding to 80% of heart rate reserve could acutely improve selective attention (1). As better cognitive functions are associated with physical activity levels (21) and higher aerobic fitness (17), it has been hypothesized that because of increased aerobic fitness, completing a regular interval training program would improve cognitive function. Therefore, the aim of this study was to assess the effect of 7 weeks of interval training on aerobic fitness and cognitive performance in active young individuals.
Experimental Approach to the Problem
A short-term interval training study was designed to increase aerobic power and running endurance in young amateur dinghy sailors, and to examine whether a positive effect on cognition could also be obtained by this intervention. Age- and sex-matched active individuals who did not change their activity pattern during the same 7 weeks served as a control group. The program was developed by a professional athletics coach who also supervised all the training sessions and conducted the running tests. The dependent variables in the study were changes in maximal oxygen uptake, running capacity, and cognitive abilities such as cognitive flexibility (the ability to adjust to changing demands), working memory (the ability to hold information in the mind and manipulate it), and short-term memory (the ability to hold a limited amount of information in a very accessible state temporarily) (10). The independent variable was training intervention.
Twelve young dinghy sailors (9 boys and 3 girls, the age of the subjects was 15–18 years). From a sailing sports school, with 3–4 years of training history were involved in the study during their winter preparatory training period (off-sailing season). During the 7-week training period, 4 subjects (3 boys and 1 girl) dropped out of the study because of injury or other reasons. Thus, the final experimental group consisted of 8 subjects (6 boys and 2 girls) who completed all the training and testing. The study was preceded by 6 weeks of easy training 2–3 times a week after the end of the competitive season. A control group of 10 healthy secondary school students (8 boys and 2 girls) were age-, height-, and weight-matched to the experimental subjects (Table 1). Except for testing, subjects of the control group did not receive any form of supervised or regular training and they were asked not to deviate from their normal activity pattern until completion of the study.
The study complied with the ethical principles for experiments with humans as laid down in the Declaration of Helsinki 2013 update. The protocol of the research was approved by the Regional Ethics Committee for Biomedical Research. All subjects and the parents of those subjects who were under 18 years at the start of the study read and signed the informed consent form and parental consent was obtained.
One week before the exercise training intervention, the subjects attended a familiarization session, and were introduced to and performed the experimental procedures for cognitive testing. The subjects were instructed to refrain from consuming any food for at least 12 hours, and from heavy exercise and caffeine for at least 24 hours before the experiment, and instructed to sleep at least 8 hours the night before baseline experiment testing. The cognitive functioning results revealed a nonsignificant difference between familiarization and experimental sessions before intervention, which indicates that subjects reached a learning plateau. After baseline cognitive testing, subjects were weighed and performed an aerobic power test on the cycle ergometer to measure maximal oxygen uptake (V̇o2max). Three days before the start of the exercise intervention, subjects performed 200 m and 2,000 m running tests separated by 30 minutes of rest. The experimental group then started a 7-week supervised training program. After the 7 weeks, both the experimental and control groups performed cognitive tasks and V̇o2max and running tests in the same order as at baseline. In the experimental group, cognitive tasks and V̇o2max tests were performed 3 days after, and 200 m and 2,000 m running tests were done 5 days after the last training session.
Physiological and Performance Measurements
V̇o2max was measured using the ramp incremental test on an electronically braked cycle ergometer (Ergometrics 800S; Ergoline Medical Measurement Systems, Bitz, Germany) at a pedaling cadence of ∼70 rpm. Before the test, a 10-minute warm-up was performed using a combination of light pedaling and muscle stretching. The test was started with 3 minutes at 20 W and continued with 5 W load increments for each 10 seconds until the pedaling cadence could not be maintained. Subjects breathed through a low resistance mouthpiece, and oxygen consumption and carbon dioxide production were measured breath-by-breath using a wireless portable gas analyzer (Oxycon mobile; Jaeger, Höchberg, Germany). Before each test session, the gas analyzer was calibrated using standard gas mixtures provided by the manufacturer. V̇o2max was measured as the highest oxygen consumption intensity over consecutive 15-second periods of the test and was expressed per body mass kilogram of the subject. For sufficiency of the efforts used during V̇o2max test, at least 2 of the following criteria had to be fulfilled: V̇o2 plateau, maximal heart rate >90% of the predicted for age (220 − actual age in years), and respiratory exchange ratio >1.1 by the end of exercise (19). Heart rate (HR) was recorded using a Polar monitor (model T31-coded; Kempele, Finland). Before each V̇o2max test, body weight of the subjects was measured using electronic scales (Tanita TBF-300; Tokyo, Japan), and body height was measured using Harpenden anthropometer.
Running tests were performed on the same 200 m athletics indoor track as the training program. After a supervised warm-up of ∼20 minutes consisting of jogging, stretching, and some specific drills, subjects individually ran 200 m with maximal efforts. After 10–15 minutes of rest, subjects completed a self-paced 2,000 m in groups of 4–6 subjects. Subjects were instructed to cover both distances as quickly as possible. Time was recorded manually with a stopwatch. Air temperature during the testing and training was ∼18° C. Subjects completed running tests and trained in light running shoes and racing gear.
Cognitive Function Tests
All tests were performed in a quiet and dimly lit laboratory using a laptop (HP Compaq 6730b; Hewlett Packard, Palo Alto, California, USA) screen ∼0.40 m in front of the subject. The test battery took ∼15 minutes to perform and it included tests to assess executive function domains (i.e., cognitive flexibility and working memory) and nonexecutive function domain (i.e., short-term memory) in random order (10). The same programmed cognitive test battery was used in previous study (3).
Unpredictable Task Switching
The odd-even test (30) was used to measure cognitive flexibility by the reaction time to make a choice and the number of incorrect choices made for an unpredictable digit-choice protocol. Forty randomized single digit stimuli from 0 to 9 were displayed with varying interstimulus intervals in the middle of the screen during the test period of 3 minutes. Subjects had to react as quickly as possible by pressing a button for an even (right button) or an odd (left button) digit. The average reaction time of the correct choices was calculated, and the number of incorrect choices expressed in percentages.
Predictable Task Switching
The Schulte-Gorbov test (27) is a modified version of the same color Schulte table, and was used to measure cognitive flexibility. A square table appeared in the middle of the screen, which contained random red numbers from 1 to 25 and black numbers from 1 to 24. The subject had to find and click the mouse button on red numbers in ascending order and on black numbers in descending order as fast and as accurately as possible. On finding each number, they also had to switch the color and order of numbers in the following sequence: 1—a red number, 24—a black number, 2—a red number, 23—a black number, etc. The duration of task performance was measured and the percentage of mistakes (incorrect choices) was calculated.
Free Recall Task
The free recall task (34) was used to measure working memory. Subjects were exposed to random and different 10 pairs of digits; each new pair appeared for 1.5 seconds on the screen before being replaced by the next pair. Immediately after exposure to all pairs, subjects were asked to recall as many of the studied digit pairs as they could in any order by entering them in a table. The task was repeated twice and the percentage of correct answers was calculated (maximum 20 answers corresponding to 100%).
Forward Digit-Span Task
The forward digit-span task (10) tests the ability to remember a sequence of digits in a short time, and was used to measure short-term memory. The subject was instructed to remember a 7-digit sequence displayed for 3 seconds in the middle of the screen. The subject then immediately entered the digits using a numeric keyboard in the same consecutive sequence as presented. If the digits were identified correctly, for the next attempt, the sequence was one digit longer; if an error was made, the next sequence was one digit shorter. There were 16 sequences. The mean number of digits identified successfully was recorded.
The training program was developed by a professional athletics coach who also supervised all of the training sessions and conducted the running tests (Table 2). Training consisted of interval running 3 times per week for 7 weeks. Training of similar frequency and duration has been shown to be effective at improving aerobic power in other studies (15,26,35,39). Each workout comprised 200–1,000 m intervals that increased progressively in the number of repetitions each week. Total workout time increased gradually from ∼42 minutes per session on week 1 to ∼90 min by the end of the program (range, 36 and 106 minutes); high intensity exercise time increased from only 12 minutes per session at week 1 to 30 minutes per session by week 7 on the average. Running speed was calculated for each subject based on the average running velocity of the pretraining 2,000 m time trial, and was 95, 105, 115, and 145% of the average for 1,000, 600, 400, and 200 m intervals, respectively. The distance covered (but not the pace) during the session increased as the training program progressed. Sitting and slow walking was used between intervals for recovery. Training was done on the 200 m athletics indoor track. During intervals, average HR increased to ∼190 b·min−1, and recovered to ∼130 b·min−1 by the next interval.
Data are presented as means and SDs. To test whether the changes in measured parameters with training (experimental group) and with the time (control group) was significant, a nonparametric exact Wilcoxon's test was used to compare experimental and control groups at baseline. The level of significance was set at 0.05. Statistical analysis was carried out using SPSS v.21.0 (IBM Corp., Armonk, NY, USA).
There were no differences between the groups in any of the measured variables at baseline (p > 0.05). All subjects in the experimental group who had completed the training study complied with the training program. There was a slight but significant increase of V̇o2max in response to training in the experimental group (p ≤ 0.05), but no change was evident in the control group. Absolute values of V̇o2max increased from 2.83 ± 0.57 to 2.97 ± 0.58 L·min−1 (p = 0.018) in the experimental group, and remained unchanged in the control group (3.03 ± 0.54 to 3.06 ± 0.50 L·min−1 pre- and post-training, respectively, p > 0.05). Relative V̇o2max before and after training in experimental group and control group values are presented in Figure 1. No significant changes in the body mass were found in either the experimental (64.6 ± 9.5 and 64.1 ± 9.2 kg pre- and post-training, respectively) or control group (68.8 ± 7.2 and 67.3 ± 6.7 kg).
Running performance in the 2,000 m improved from 539 ± 79 to 499 ± 80 seconds (i.e., by 40 seconds) by the end of the training in the experimental group (p ≤ 0.05; Figure 1), but did not change during the 7 weeks in the control group (553 ± 65 to 546 ± 71 seconds; p > 0.05; Figure 2). Performance in the 200 m sprint also improved in the experimental group (33.0 ± 3.3 to 31.0 ± 3.3 seconds pre- and post-training, respectively; p ≤ 0.05), but not in the control group (32.0 ± 3.0 and 31.8 ± 2.9 seconds; p > 0.05).
In the experimental group, the number of correct answers in the unpredictable task switching test increased (p ≤ 0.05) whereas complex reaction time remained unchanged after the training (Table 3). The predictable task switching test duration decreased (p ≤ 0.05) in response to training, but there was no change in the percentage of mistakes made. In the control group, no indexes of cognitive function changed (Table 3). The results of the free recall test and forward digit-span task did not change in either group (Table 3).
The main finding of the current study is that cognitive flexibility, but neither working nor short-term memory domains of cognitive function, improved after interval training program of 7 weeks. Training was sufficient to increase aerobic power and also elicited an improvement in running performance. To the best of our knowledge, our study is the first to show how cognitive functions are affected by short-term interval training program, because previous exercise training intervention studies on cognitive function implemented moderate intensity continuous aerobic exercise alone or in combination with interval training.
The intervention slightly, but significantly increased V̇o2max in our experimental group by ∼2 ml·kg−1·min−1. At baseline, V̇o2max values of the sailors and active nonathletic individuals in our study were similar, which is consistent with a previous study that showed no difference in treadmill V̇o2max between controls and elite sailors (23). Interestingly though, no change in treadmill V̇o2max had been observed in elite sailors during the season (23), whereas our study demonstrated that running intervals could be efficient in improving aerobic power, at least in subelite young sailors. In this study, gas exchange analysis was done during cycle ergometry, which is rather a different muscular activity from running. This choice of activity mode for testing was intentional, so that it could be explored whether the benefits of the training were transferred to other types of activities such as cycling or sailing.
As anticipated, training also improved the 2,000 m running performance of the amateur sailors involved in the study. It should be noted that running was not a training mode that was used frequently by our subjects (sailors) before the study. The applied training was designed mainly to improve performance over distances of about 2,000 m, but was also efficient at increasing aerobic power (cycling V̇o2max). The training program probably enhanced running abilities in a vast range of distances, as evidenced by the improved 200 m sprint performance. In response to interval training, adaptation occurs in many systems throughout the body and encompasses, but is not limited to, improved cardiovascular function, neural recruitment patterns, skeletal muscle energetics, and altered morphology (4,5,24).
In response to the training performed, sailors augmented their ability to adjust to changing task demands (improved cognitive flexibility), which was reflected by the increased number of correct answers in unpredictable task switching and decreased task duration in predictable task switching tests. However, training had no effect on the results of memory tests. The outcome of our study is somewhat consistent with that of similar studies which applied exercise training of moderate intensity and also showed an improvement in executive functions paralleling an increase in aerobic fitness (28,37). Indeed, with exercise training, attention seems to improve most consistently among indices of cognitive function whereas an effect on memory is rarely found (7,36). Improved attention shifting, which is indicative of superior cognitive flexibility, may have occurred through several different adaptation mechanisms. Exercise training may increase cardiovascular capacity, induce more efficient regulation of cerebral blood flow, and enhance delivery of oxygen and nutrients to the brain areas crucial for executive control (33). Colcombe et al. (9) found that in response to aerobic training, which improves endurance capacity, increased activity of the prefrontal cortex leads to superior executive function. Aerobic fitness and some aspects of cognitive performance have also been shown to improve after 8 weeks of military training (16).
Even a single interval training session consisting of 10 cycling bouts (1 minute at the intensity corresponding to 80% of heart rate reserve interspersed with 1 minute of active recovery) has been shown to acutely improve selective attention in healthy middle-aged individuals (1). Improved cognition after the training was probably partly a result of an increased threshold for exercise intensity (or in fact any other type of stress) on cortisol release, rendering the body more resilient to the effects of stress (29). A recent pilot study on the effects of 4-month mixed exercise training, which also involved interval sessions, has revealed, among other benefits, a positive effect on the cognition of obese patients (11).
The overall activity in young healthy adults is higher on average than in the older population (38), whereas decreased participation in social, physical, and other types of activities by the elderly or disabled may contribute to a loss in their cognitive function, which provides more opportunities for faster and larger improvement in their cognition with environmental and training interventions (11,33). Indeed, when compared with younger peers, elderly people tend to benefit more in their cognitive abilities from exercise training (8). This age dependency in the ability to alter cognitive function by exercise training could be explained by the inferior baseline cognition of older individuals. It has been reported that cognitive function in the elderly is improved when moderate intensity exercise training is performed for 6 months or longer (12). An improvement in cognitive performance of young active individuals in only 7 weeks of interval training is inspiring and warrants further investigation in other populations, including with the elderly.
One of the limitations of this study was a final small number of subjects, in addition to them being of both genders. However, the number of subjects is similar to previous field training studies (15,35) and was large enough to observe statistically significant changes in cognitive flexibility, aerobic power, and running performance. One possible reason for the relatively modest improvement of cognitive function in our study could be associated with the young age of the subjects. The rate of improvement in cognitive function peaks during young adulthood (25), which suggests that there may be little room for cognitive function augmentation by any means during this period of life, including exercise training, in healthy humans (17,25).
It must be recognized that the training program applied was not sufficient to affect aspects of cognition other than cognitive flexibility, and thus a revised stimulus such as longer duration of training may be required to achieve a more comprehensive adaptation, including improved memory. Additional studies may be required to clarify how exercise duration, frequency, and intensity affect cognitive performance. Of note, our subjects were young, healthy, motivated amateur athletes with a history of several years in sports. High intensities of interval training should be prescribed with caution for other populations such as novice athletes and patients. Moreover, prolonged exposure to interval training must be applied with caution in any population because of the substantial burden of psychological stress (monotony) and associated increase in the risk for overtraining (2,13,22).
In conclusion, this study has shown that interval running training for 7 weeks had a positive effect on endurance performance and the ability to effectively shift attention in young active individuals, but did not affect short-term and working memory.
Our study confirms that a relatively short-term interval training program could provide multiple benefits (4,5) which are not only limited to augmented physical working capacity but also extend to improved mental abilities. Although in general it is believed that cognitive functions are at their peak during young adulthood (17), it has been shown here that 7 weeks of interval training improved the ability to shift attention in young active individuals. Increased attention shifting ability is one of the priorities in dinghy sailing, as well as many other sports (20). Therefore, our findings suggest that coaches should be encouraged to regularly include interval training into training routine to enhance performance in sports such as sailing where both cognitive and aerobic performance are highly important. It is also of note that short-term interval training performed by dinghy sailors in running mode resulted in improved performance of sprint (200 m) and endurance (2,000 m) running, and translated into increased aerobic power in cycling mode of exercise. These findings indicate an improvement in “general” aerobic fitness in young amateur athletes. When having time constraints like, if few weeks are left before the start of competitive season, interval training could be especially attractive because as it requires only limited amount of time in a weekly schedule and possesses similar or superior benefits to moderate intensity high-volume exercise training. For instance, voluminous and time-consuming military multitraining for 8 weeks resulted in gains in physical fitness and benefits in cognition (16) similar to those achieved in our study with only 8 hours of intense running during 7 weeks.
The authors thank Juozas Bernotas for his technical support.
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