To understand the biological, psychological, social, and environmental factors that contribute to individuals’ participation in extreme sport.

Study sample size varied from one (26–28) to 7,109 participants (29). Although many studies did not disclose mean ages (n = 12), in those that did (n = 22), mean age of participants ranged from 19.68 years (29) to 36 years (30). Seven studies used male-only participants (18, 28, 31–35), three studies used female-only participants (26, 27, 36) and the remaining studies used mixed-gender or did not disclose participants' gender. Inclusion of transgender or gender fluid participants was not reported by any study.

Sixteen studies recruited participants using purposive sampling technique [e.g., (18, 37, 38)], three using convenience sampling (27, 39), two using snowballing (40, 41), one using a combination of convenience and snowballing (42) and twelve studies did not disclose sampling methods despite having details of the inclusion criteria in their methods [e.g., (43–45)].

The existential and extrinsic theme consisted of eleven papers and was developed to encompass factors within the included studies that related to extreme sports that are extrinsically oriented. “Existential” in this context refers to the existence of individuals as free and responsible for determining their own development (53), as it directly relates to human existence. “Extrinsic” refers to the type of motivation that involves striving towards an external goal or being driven by external factors (54). Sub themes such as engaging with nature [e.g., (30, 37, 47, 50, 55)], freedom [e.g., (2, 34, 47, 51)] social interactions [e.g., (30, 37)], and challenge (34, 51, 55, 56) were encompassed within this theme.

The sub-theme of nature was seen as a key motivator for individuals to engage in extreme sports (30, 37, 50), as individuals felt they could explore and appreciate new and remote spaces, giving them a sense of freedom from everyday life (2, 51). When immersed in nature, extreme sports athletes reported a sense of tranquillity and transformation of time when engaging in their sport (41). This was reported to increase their feeling of being present in the moment, which is analogous to the concept of mindfulness (56). Focusing on the immediate task in this way was said to distract them or allow them to shift attention away from unpleasant and/or stressful feelings (56). This distraction in nature could also be seen as a sense of freedom. For example, Brymer and Schweitzer (2) found six key elements of freedom, which included; freedom from constraints; freedom as movement; freedom as letting go of the need for control; freedom as the release of fear; freedom as choice and personal responsibility; and freedom as being at one. The social interaction was seen to motivate individuals as they could enjoy challenges with friends and meet new people, allowing them to feel a sense of “belonging” (37). The subtheme of challenge was discussed by studies in the review as a key motivation for participation [e.g., (34, 56)]. For example, Frühauf et al. (51), found challenge motivated athletes in many ways (encountering new places, exploring personal limits, experiencing skill, overcoming the challenges of environmental conditions). Pushing limits was seen to be both a mental and physical demand (56).

Personality included ten papers and was discussed in relation to participation/motivation for extreme sports, either as individuals having a sensation seeking (SS) trait or the stable trait alexithymia leading to the use of extreme sports as a means of emotion self-regulation. Sensation seeking (SS) is a trait typified by the propensity of individuals to seek high-risk activity that provides thrills and excitement (57). Alexithymia is typified by individuals who have difficulty identifying and describing their emotions (17), which can lead many individuals to use extreme sport as a means of emotion self-regulation (17, 42).

The theme motivation includes seven papers and aims to explain individuals' motives for participation in extreme sport. Self-determination theory's (SDT) elements of competence, autonomy and relatedness all connected to athletes' motivations. Athletes have reported to experience all three elements of SDT when engaging in challenging situations, leading them to seek out these experiences more (58). Extreme sport athletes reported feeling a sense of autonomy over choices regarding their activity, increasing their perception of control over their experience. This is said to give athletes the feeling of complete freedom to decide when, where and how to engage in these activities (39). Extreme sport athletes reported that motivation linked to goal achievement in the form of winning competitions (59), potentially contributing to the fulfilment of competence. The element of relatedness can be fulfilled through social support and the sense of belonging with like-minded individuals. In the studies reviewed this was said to create strong feelings of relatedness due to the camaraderie experienced (60).

Constructs such as self-efficacy refer to beliefs in one's capabilities to organise and execute a particular skill or activity (61). As confidence and self-efficacy increase, research suggests individuals may take more risk, challenge themselves to harder goals, expand their effort and persist in the face of adversity as they believe they are better equipped to cope with the situation (33, 62). Extreme sports athletes reported a sense of achievement when participating, particularly from setting goals or winning competitions (33, 46). This could be attributed to achievement goal theory which suggests that an individual's vision can be converted into goals, initiatives and activities that help direct behaviours and actions in specific ways (63).

In the literature reviewed, the concept of risk was examined with regards to individuals’ motives for participating in extreme sports (three papers). Risk can be split into two main mechanisms: analytic and experiential. Analytic risk refers to conscious, rational, and logical decisions that require information to justify a conclusion. Whereas experiential is unconscious, automatic, and requires no justification or prior knowledge. Participating in extreme sports is thought to put an individual at both physical {injury [e.g., (29)] or death} and psychological (high-stress, competitiveness, and perfectionism) risk (64). However, some participants appear attracted to risk situations as desirable rather than something that should be minimised (65).

The analogies with addiction and withdrawal theme encompassed three papers that explain an alternative motivation for participating in extreme sport. Withdrawal states such as anhedonia, craving and negative affect reported by extreme sport participants [e.g., (18)] bare similarities to the emotional experience of people with addictions (i.e., substance and behavioural). Of relevance, behavioural addictions are classified by excessive engagement, mood modification, withdrawal, conflict, and relapse (66). Some of the literature [e.g., (18)] indicated that individuals continually engage in extreme sports as when they do not participate, they experience mood disturbance (e.g., anhedonia), which may lead to a powerful desire to continually engage (e.g., craving) [e.g., (28, 40)]. When individuals stop or break from participating (e.g., rest days), they may experience physical and psychological responses (i.e., withdrawal).

Several reviewed studies highlighted extrinsic motivators such as nature [e.g., (27, 30, 47, 50, 51, 55)], freedom [e.g., (2, 47, 51)] social interactions [e.g., (27, 30, 37)] and challenge (51, 55, 56). Studies on these factors were predominately interview based [e.g., (2, 30, 51)], allowing rich insights into how they related to participation. However, these extrinsic motivators can be deemed subjective, making it difficult to compare across individual data. It is therefore unclear if individuals within extreme sport perceive the outdoor element of nature the motivation for their participation, or whether any outdoor sport would give the same reward. Given athletes highlighted that extreme sport gives them a feeling of freedom from constraints [e.g., (2)], it is also unclear if the feeling of freedom is due to being immersed in the outdoors or because of their sport.

In relation to social connection, extreme sports athletes appear to be motivated by the camaraderie with like-minded others, giving a sense of belonging [e.g., (27)]. This bares similarities with research on those in the military, another high-risk domain (67). Individuals in military environments often suggest they feel part of a “family” due to their shared experience with like-minded individuals in the same career, often described as “brothers or sisters” (68). However, research has also suggested low risk sports can also lead to camaraderie leading to feeling part of a team collective (69). Thus, camaraderie may not be unique to extreme sport participation and individuals could experience this benefit from low-risk sports.

Personality is commonly explored in studies attempting to understand the motivation to participate in extreme sport. The studies reviewed here indicate that personality motivates individuals to participate in extreme sport due to the sensations of thrill (45) and the ability to “feel” and therefore describe feelings (49). Two core themes relating to personality were identified: (1) sensation seeking and (2) alexithymia and emotion self-regulation.

Some studies found individuals are motivated to take part in extreme sports for the thrills and excitement experienced whilst participating [e.g., (45, 48, 52)]. For example, Slanger and Rudestam (45) explored a range of sports and found those in extreme sport scored significantly higher in the thrill and adventure seeking aspect of the sensation seeking scale than low risk sport participants. Furthermore, some authors have suggested that extreme sport athletes may become desensitised to “thrill” with continual exposure, which might lead them to continually seek out either new or more extreme thrills (70). For example, Fruhaüf et al. (30), found that individuals continually expanded and relocated their perceived limits to seek new thrills. This suggests that when extreme sport athletes participate for reasons related to sensation seeking, their participation becomes self-perpetuating.

Most of the studies on sensation seeking reviewed in this SR used Zuckerman's sensation seeking scale as a measurement tool (31, 45, 48, 52). This scale was initially developed to look at individuals' propensity to engage in activities rather than motivation to participate (42). This makes it difficult to conclude that participants in these studies were motivated by the sensations they feel [e.g., (31, 45, 48, 52)]. For example, Cronin (52) found mountain climbers scored higher on experience seeking and thrill and adventure seeking than the control group, however, it is not clear how (if at all) sensation seeking motivates extreme sports athletes.

There are several additional issues with Zuckerman's scale. First, there are concerns regarding the outdated nature of the scale. The vocabulary used such as “swingers” and “queer” highlights this problem, as these words may be seen as offensive and stigmatising (71). This could result in individuals not feeling comfortable to answer certain questions or the questionnaire as a whole. Second, the use of forced response may leave many individuals feeling they could have responded to either both options or neither (72), this increases the likelihood of misrepresentation in individuals' responses. Lastly, when understanding extreme sport athletes motivations, the only relevant aspect of the scale is the thrill and adventure seeking element. For example, the disinhibition element of sensation seeking focuses solely on participating in drug use, alcohol and sex which is not relevant to extreme sport motivation. However, if an extreme sport version of the sensation seeking scale for disinhibition was developed this could look at disinhibition in relation to disregard for risk.

Despite these weaknesses, the scale has been continually developed across the years to reduce these issues (73). Whereas it was initially focused on examining the need for novelty and complexity, this has changed to a focus on novelty and intensity, as Zuckerman considered this a defining characteristic of sensation seeking (73). The intensity aspect is important as it determines the vigour of attention and therefore the duration and length of time a sensation persists. However, despite the changes to the sensation seeking scale over time, the role of sensation seeking in explaining extreme sports participation remains unclear. For example, Jack and Ronan (74) found swimmers scored higher on measures of sensation seeking than some high-risk sports participants (hang gliding & motorsports). Anomalies like this suggests that the sensation seeking scale may be limited in its utility to explain extreme sport participation. Therefore currently, sensation seeking may not be a useful differentiation between extreme sports athletes and other groups. More recently, the Sensation Seeking, Emotion Regulation and Agency Scale [SEAS (43)] scale was developed to measure experiences during, after and between participation in extreme sport (43). As contrast to Zuckerman's sensation seeking scale, the SEAS aims to measure and understand motives for seeking emotion regulation and agency through extreme sport participation. It is the first scale that highlights the distinctions in individual differences, behaviour and extreme sports (e.g., skydiving vs. mountaineering). Given the recency of the SEAS scale, it uses appropriate language in comparison to Zuckerman's sensation seeking scale.

Alexithymia is typified by individuals who have difficulty identifying and describing their emotions (17) and is related to emotion self-regulation. Emotion self-regulation refers to one's ability to initiate and maintain the type, intensity and duration of the emotions they feel (75). Three reviewed studies suggest that extreme sport athletes may have difficulty expressing emotions (alexithymia) [e.g., (17, 36)] and their sport provides an opportunity for them to experience greater emotion regulation [e.g., (42, 49, 76)]. Woodman et al. (49) used the Toronto Alexithymia Scale (TAS-20) to assess three aspects of the alexithymia construct [i.e., (1) difficulty identifying feelings (2) difficulty describing feelings (3) externally orientated thoughts]. Woodman et al. (49) suggested that transatlantic rowers exhibited significantly higher scores in describing emotional difficulties compared to normative values derived from an English-speaking adult population. Engaging in rowing appeared to facilitate emotional identification and expression for participants. However, individuals with high levels of alexithymia might struggle to accurately assess their emotional awareness (77). Thus, rather ironically, research relying on the TAS-20 scale to indicate alexithymia may not fully capture an individual's emotional perception. To overcome this issue, an interview method [Toronto Structured Interview for Alexithymia (TSIA)] has been developed, although this exact method (TSIA) was not used in any of the reviewed studies (78).

Studies examining the role of emotion regulation in extreme sport participation used the SEAS [e.g., (43)]. Results across studies indicate that extreme sport athletes experience difficulties in emotion regulation, although this can differ across sports (i.e., mountaineers vs. skydivers. Freeriders vs. slope skiers) [e.g., (38, 43)]. For example, Frühauf et al. (38) found freeriders experienced higher emotion regulation than slope skiers during and after participation in their sport but not between participation.

Reviewed studies suggested that the more self-efficacy individuals have for what they can do in their sport the more risk they were willing to take (62). In general, as confidence and self-efficacy increase, individuals are thought to take more risk, challenge themselves to harder goals, expand their effort and persist in the face of adversity as they believe they can cope with the situation (33, 62). In the context of extreme sport, Jones et al. (32) found an individual's perceived ability was an important factor in decision making when embarking on the most difficult winter climbing routes. Further, Wiersma (35) found when surfers' confidence increased, so too did their desire to strive for bigger waves. They also found that as experience increased, competence and confidence also increased, which led athletes to believe they were taking less risk. This interplay between experience, confidence and risk perception is similar to the definition provided by Boudreau et al. (11).

In the reviewed studies, there was also a potential link to achievement goal theory suggesting that individual's visions are converted to goals. Burke et al. (46) found setting goals allowed climbers to feel as prepared as possible going into a specific task and gave a sense of achievement at the end. However, not all individuals felt this, as they believed that having too many outcome-based goals reduced the element of fun. Mackenzie (59) found that participants in varied extreme sports (kayaking, downhill mountain biking, mountaineering, BASE jumping, skydiving and hang gliding) predominately found goal setting useful for self-focus and competitive states but also appreciated goal achievement in the sense of winning competitions and medals. This suggests that extreme sport athletes strive for outcome-based goals, similar to more traditional sports where athletes typically set goals to enhance performance (79, 80).

Extreme sport athletes tend to exhibit a diminished concern for the potential risk and consequences involved, most of this is due to how they manage the risks they take. Such attitudes towards risk are often attributed to athletes ability to mitigate and attenuate the perceived risk through rigorous training, preparation, and control [e.g., (44)]. For example, Brymer et al. (44) found that “outsiders” (low risk and non-sporting individuals) viewed extreme sport as risky but those that participate in them viewed it as a means for emotional clarity and aim to mitigate the risk involved. Despite this Weishaar et al. (29), found that risk seeking and lack of perseverance were the two strongest predictors of extreme sports injury, highlighting that extreme sport does hold some level of risk. The concept of risk is subjective (81) and so it is difficult to compare risk perceptions between individuals, particularly when using interviews as a data collection tool—as was frequently the case in the reviewed studies. As well as being subjective, understanding risk is relative. For example, some individuals may find going for a walk in the mountains as risky, whereas another person in the same sex and age bracket may find this exciting. This is relative to an individual's perception and skill level. In a case study by Kerr (26) the athlete made the decision to withdraw from skydiving following a “freak accident” and death of a friend. The uncontrollable death and therefore risk became too high for the athlete to continue participating. Similarly, a number of high level of extreme sport athletes have quit due to risk perceptions and changes in circumstances. For example, Tim Emmett publicly discussed quitting base jumping following the death of a friend and the birth of his child (82).

In recent years, the measurement of risk has developed with the introduction of the risk-taking inventory scale, which was devised to measure attitude and propensity to take risk (83). However, despite these developments, no studies within this systematic review used this new scale to determine risk, making it difficult to understand whether included studies represent current thinking in the literature or support the inclusion of attitude and propensity.

A small number of studies found evidence for withdrawal states such as anhedonia, craving [e.g., (40)] and negative affect being experienced by extreme sport participants [e.g., (18)]. Heirene et al. (18) found that climbers experienced more frequent and intense anhedonia, craving and negative affect where “nothing compared to climbing.” These symptoms bare similarities to those individuals with behavioural addictions (84). However, it is important to note that there were limited papers in this systematic review that discussed addiction in the context of extreme sport. Popular theories and perspectives in the substance and behavioural addiction literature stipulate that several additional criteria would be required for individuals to be considered as having an “addiction”– namely, impaired control over the behaviour and negative personal, social, or occupational consequences associated with it (85, 86). Several studies in this review examined specific reasons given for participation [e.g., (40)], yet only one (18) explicitly connected it to addiction. However, it was common that symptoms often linked to addiction, such as craving (a desire to experience it again), were prevalent in participants [e.g., (28)]. Without an explicit connection, it becomes challenging to discern whether withdrawal states result from an “addiction” or a deep passion for their sport.

Considering the additional criteria for a behaviour to be viewed as an addiction, a potentially different viewpoint not explored in the literature to date is “passion”. Passion refers to the engagement in an activity an individual finds important and therefore invests time and energy into it (87). Passion can be split into harmonious and obsessive passion. Harmonious passion refers to an individual flexibly and autonomously engaging in a sport. Whereas obsessive passion is when an individual rigidly participates in a controlled way (88). This leads to individuals attaching greater importance on the sport, potentially using it to escape problems and emotions. This in turn makes it difficult to stop the activity, making it hard to conclude if individuals are experiencing withdrawal symptoms similar to those with addiction, or if they are experiencing obsessive passion. However, it is reasonable that obsessive passion could lead to symptoms of withdrawal if an athlete is forced to stop their sport (e.g., injury).