In our study, we show the impact of suggestions given under hypnosis on influencing strength on both objective and subjective levels. We were able to increase the feeling of subjective strength significantly within participants with our hypnosis intervention and also found significant effects between the hypnosis and the control group for the measurement point after the hypnosis intervention. Our data therefore indicate that our hypnosis intervention significantly influences mental processes like the perception of strength.
Due to the feedback of our participants, our hypnosis intervention made them feel stronger and more self-confident. On day 8, some participants said that they used the post-hypnotic anchor while climbing and felt trust in their own skills and confidence. One participant also told, she would feel like Hulk, when activating the anchor. This feedback shows the possible influence of such interventions and supports our results of a changed participant’s perception.
Objective strength was increased on day 8 after activating the post-hypnotic anchor (long-term) compared to the subject baseline, but not on day 1 immediately after the hypnosis intervention compared to the subject baseline. This might be due to the relaxing effect of hypnosis that interfered with the task of pressing the hand dynamometer. Some participants felt still very relaxed after the hypnosis session (carry-over-effect of relaxation) and still had to process the experience, even though they were already fully awake again. Although we did not see any significant differences in objective handgrip strength between the hypnosis and the control group, we found a tendency toward increased strength in the hypnosis group, supported by decreased strength measurements in the control group. The significant decrease in objective handgrip strength in the control group on day 1 after reading compared to the subject baseline indicates that the reading part did not have a motivational influence on the participants.
Our data do not show a discrepancy between objectively measured strength and perceived strength, but subjective strength was the parameter that changed the most. Based on these results, we suggest that our hypnosis intervention first and foremost acts on the mental state of participants and that physical parameters are changed as a consequence of these alterations. Schmidt et al. (2017) [29,30] could show that visual and auditory perception can be influenced by hypnosis and that primary perceptual processes are not affected, but subsequent cognitive processes are. Our data thus support these findings of the previous studies. An altered mental state might lead to altered physical parameters via altered information flow in the brain [30,31]. It would also confirm the literature that a voluntary muscle reserve can be activated through hypnosis [32]. Increased objective handgrip strength might be due to a higher recruitment of motor units by a changed neural control within the muscles [33].
We examined our study due to good scientific practice guidelines and determined the cohort size by means of an a priori power analysis. Conducting the study in two cities also meant that the interventions were conducted by two different people. We used a SOP and standardized the given instructions, but it cannot be excluded to the fullest, that these circumstances may have influenced our study. Our data do not suggest it and therefore rather point to the generalizability of our findings.
We used handgrip strength as a parameter for muscle strength, because it is an important clinical parameter – recognized and easy-to-measure that indicates not only muscle strength but also the health status of patients, e.g. lung function [34].
The participant’s gender was not given fully equal, but it can be seen as no influencing factor since the main focus was on within-subject effects and in general, no effects were expected for the gender distribution.
We also did no suggestibility tests before; everybody was invited to take part in our study. Schmidt et al. (2024)[18] could show that suggestibility has no influence on the efficacy of given suggestions under hypnosis and is not necessarily a predictor for the outcome of a hypnosis intervention [35]. However, most of our participants reached a medium to high state of trance, indicating that the hypnosis itself worked well.
The literature states that very experienced participants, who may have already reached their physiological limits, would no longer be able to improve their performance through hypnosis (ceiling effects; [36]). We did not exclude trained participants; on the contrary, several people interested in climbing and bouldering participated in our study, because they wanted to improve their handgrip strength. The sportiness of the participants had no influence on our data, but it is very likely that all our participants had not yet reached their physiological limit, as they were all recreational athletes.
Many potential influencing factors were not excluded, e.g. training between the two appointments or the use or non-use of the post-hypnotic power anchor within the week between the two appointments. Another influencing factor might have been habituation with the hand dynamometer as well as acclimatization to the situation or the general well-being of the participants. Participants may also have deliberately influenced their strength in order to meet the expectations of the experimenters. Furthermore, the study took place in the winter, meaning that participants arrived with cold hands and had to warm up their hands first, especially after cycling.
Our study design excluded the possibility of doing a (double) blinded study, which meant that both the participants and the experimenters knew which group the participants belonged to, either the control group or the hypnosis group. Therefore, we cannot exclude a possible influence on subjective evaluations due to altered expectations on both sides.
Although we used suggestions relating to already existing data, it might have helped to use not only motivational suggestions, but also more knowledge, e.g. that there is a 30% reserve within voluntary muscles that can be activated through hypnosis [32].
With our study, we show that strength can be influenced by a one-time hypnotic intervention, and effects can be seen for both objective and subjective increases in strength. We also show that the effects can be activated through a post-hypnotic anchor and last over a longer period of time, at least one week. With regard to the research question, we were able to contribute interesting results to the current state of research, emphasizing the power of the mind to influence perception and eventually physical parameters. Our study also indicates practical relevance, using hypnosis as an effective, non-invasive method to increase strength, both subjectively and objectively, for professional athletes to improve their performance and the medical sector to help patients regain their health.
Implications for practice and future research
Further research might focus on the comparison between-groups, investigate the duration of the long-term effects of the hypnosis intervention and suggestions given in the hypnotic state should contain both motivation and knowledge. Studies could also include a third group of participants receiving suggestions only (without hypnotic trance), meaning that different effect sizes could potentially be found between the groups (control, hypnosis, suggestions only). Further researchers should also pay attention to the relaxing after-effects of hypnosis when recording parameters immediately after the intervention. As far as long-term effects are concerned, it would also be interesting to investigate for an even longer period of time and perhaps also find a limit to its effectiveness.
Studies could already show the influence of hypnosis on physiological parameters [37, 12, 38]. Further research might focus on various physiological parameters, shed light on the detailed neuronal mechanisms that lead to these alterations and might be able to explain the chain from mental alterations over neuronal mechanisms to changed behavior or changed physical parameters. Understanding the effects of hypnosis on muscle strength can contribute to scientific knowledge and may uncover the effects of the impact of the mind on physical functions to further understand the complex interplay. It might help understand psychological factors that affect the body and eventually be used for therapeutic purposes.
This could be proof of both: the effectiveness of hypnosis and the relevance of using this tool in sports and the medical sector.
Our data indicate that physiology can be influenced by changes in the mental state and that hypnosis is an appropriate tool to reach this goal. This study exemplifies the mind-body-interaction and emphasizes the influence of mental processes on physical performance, as shown by an increased feeling of strength that led to physiological changes. It has already been shown that hypnosis can influence vision [29,30] and hearing [31]. With our study, we now also show that one does not have to train and can still become stronger.
Our intervention can change subjective strength and eventually even influence objective parameters. This means that the mind has a huge influence on physical performance. Our intervention can help athletes improve their performance and patients regain their health after a medical procedure. As a non-invasive therapy, it might help patients after a long disease or a heavy surgery to regain their strength. Patients then suffer from weak bodies [39-41] and often also bad mental states (e.g. depression, [42,43]). Clinicians repeatedly report that the patient's mental state and inner feelings are essential influencing factors for recovery and outcome [44-47]. The feeling of the own ability to reach these goals is crucial. Our intervention offers an excellent opportunity for this. Especially for rehabilitation patients, this intervention could help to improve the overall feeling of a patient’s body through mental changes and eventually change strengthparameters. The intervention might also help to motivate patients and to regain trust in the body, a better sense of self and self-confidence in their ability to get healthy again. Also, patients who suffer from chronic fatigue and/or post-exertional Malaise such as ME/CFS-patients (and also Long COVID- and Post-vac-patients) might profit from our intervention. Patients often experience their bodies as weak [48] and lose confidence in the body [49]. ME/CFS is often associated with anxiety, which can impair fatigue symptoms [50]. In addition, patients with ME/CFS in particular are still often marginalized as psychosomatic [51] and often do not receive adequate treatment [52]. Interestingly, in ME/CFS patients, fatigue intensity seems to correlate with handgrip strength and cytokine level [51]. Our intervention can help to rebuild trust in one's own strength and to become physically more stable again via the mental level. Studies could already show that hypnosis can help improve fatigue symptoms in chronic fatigue patients (self-hypnosis and neurofeedback [53]).
Also, professional athletes could profit by using our intervention to prepare for a competition. Athletes have tried to push their limits more and more over the past few years. This has also led to athletes overtraining and breaking down [54]. The increase in performance according to the motto 'more is more' has reached its limits. Cognitive strategies to improve performance are already used in professional sports. Athletes usually have an individual mental state of maximum performance [55]. Suggestions and key words are already used to achieve this [56] or remember a positive experience of success [57]. Tod et al. (2015)[58] mention that these strategies probably alter the central-nervous system, leading to altered motor unit recruitment, synchronization and firing rates within neurons: implying that changes in the central nervous system could possibly influence the activity of the sympathetic nervous system. This could lead to changes in peripheral elements, such as the ability of muscles to contract. These alterations in muscle function might manifest in the main muscles driving the movement, the opposing muscles, or any other muscles involved in the motion. Our intervention represents an opportunity to use inner resources by combining these approaches and using the focused attention and heightened sensory and imaginative understanding within a hypnotic trance state in a way that has not yet been harnessed.