The present study examined the effect of placebo on the results of an exercise aerobic stress test in normal weight children. We found that the use of deliberate positive information before an exercise test (in this case, water described as a drink that strengthens muscles and increases energy) led to a significant increase in running duration and the maximal heart rate achieved. In addition, we found that although performing a longer and more intense exercise test, the use of placebo was associated with a lower rate of RPE during the activity and a significantly shorter recovery time.
In the past few years, several studied tested the power of placebo effect in the context of physical activity , with the assumption that the placebo effect can contribute to the effect of physical activity on the body and constitute a potent factor in this context. These studies examined different types of placebo, but primarily focused on placebo in the form of information. Most studies focused on adults, few were done on adolescents  and to the best of our knowledge, none have been conducted on pre pubertal children .
In an adult study , female hotel room attendants were informed that their daily work constitutes healthy physical activity as recommended for their age group. A control group was not given this information. After 4 weeks, the informed group showed a decrease in body weight, blood pressure, body fat, waist-to-hip ratio, and body mass index, even though there was no change in their workload and they did not introduce any new physical activity into their daily routine. The control group showed no significant change. The authors concluded that these improved health metrics are due to the informed room attendants’ change in perception following the information they received. This study supports the assumption that the placebo effect may have an integral part in physical activity promotion and its influence on health.
In two other studies [13, 14], weightlifters consumed placebo pills they believed to be anabolic steroids and received positive information stating that the pills would improve motor performance. Later, the researchers revealed to the subjects that they merely received a placebo and not real steroids. The results found significant improvements in motor performance, such as heavier weights lifted and an increased exercise repetition rate, when the weight lifters believed they were taking steroids. Conversely, these improvements disappeared when the weight lifters discovered they had merely taken a placebo. Our study also found that in prepubertal children, the deliberate positive information regarding a drink consumed prior to exercise improved significantly running stage, running duration and led the children to reach a higher peak heart rate.
The mechanisms behind the placebo effect are yet to be delineated, but one of the most prominent ones is expectation; namely, the expectation of reward . The term expectation of reward describes the phenomenon in which hope for improvement, and the belief in it, brings it about. The subject holds this expectation consciously based on past experience or for other reasons, such as persuasion, belief, learning or an explanation .
In sports, Beedie et al.,  also focused on the expectation of a reward mechanism. In their study, cyclists performed a cycling activity after drinking a caffeine-free placebo drink. Different groups received different information, suggesting they were receiving either a caffeine-free drink, a drink with a low dose of caffeine (4.5 mg/kg), or a drink with a high dose of caffeine (9 mg/kg). Accordingly, subjects who believed they had received a high dose of caffeine showed an improvement, subjects who believed they had received a low dose of caffeine showed a smaller improvement, and subjects who believed they received a caffeine-free drink showed no significant improvement. Later, the researchers revealed to the subjects that all of them received only a caffeine-free placebo, and the subjects’ performance decreased.
Expectation of reward is known to affect pain and anxiety. The expectation of a negative outcome may cause subjects to anticipate a threat, and thus to increase anxiety, while the expectation of a positive outcome may reduce anxiety, and activate the neural networks of the brain’s associated with positive reward mechanisms . This idea is clearly observed in studies of the analgesic effects of placebos, as placebo reduces anxiety, and in turn, reduced levels of anxiety leading to higher pain tolerance . Studies show that subjects that expected a positive treatment effect experienced a more significant change in the brain’s metabolic activity compared to a subject who believed they were receiving a placebo, despite both groups receiving a medication with a known inherent bio-chemical effect . Our study also found a decrease in the rate of perceived exertion (RPE) following the administration of the placebo. Despite the fact that subjects reached higher running stages, longer running time and a higher peak heart rate, they reported less physical exertion.
One can easily apply the same logic to the placebo effect on physical activity, the subjects’ expectations could have reduced their anxiety about the stress test, leading to a better tolerance of the exercise, and as a consequence to a faster recovery.
Children today do not meet the physical activity recommendations for their age [6, 7]. Low levels of physical activity may increase future metabolic risk in both normal weight and obese children . Improving the experience of exercise, by reducing stress, and improving the duration and effectiveness of physical activity is of utmost importance. Using the placebo effect may be a promising tool.
Our results also highlight the possible bias with interpreting the results of a “maximal” exercise testing in children, since placebo information as well as other motivating aids and fatigue distractors may lead the child to a better performance .
Limitations to our study included the relatively small sample size of pre-pubertal normal weight children, and the fact that the study was performed in a laboratory setting. Moreover, additional components of fitness were not evaluated in our study.