CrossFit® – Development, Benefits and Risks

CrossFit is a form of high-intensity interval training and comparable to other high-intensity exercises. CrossFit exercises increase VO2max, strength, endurance, and improves the body composition (e. g. lean body mass). The injury risk in CrossFit is similar to that of other, comparable sports such as Olympic weightlifting, powerlifting and gymnastics and lower than that of competitive contact sports such as rugby union and rugby league. Shoulder joint, shoulder girdle and spine injuries are predominating in CrossFit and require special attention from physicians and coaches. In particular, this includes expert instruction for CrossFit beginners as well as an initial orthopedic examination before starting CrossFit training. The involvement of well-educated coaches correlates with a decreased injury rate. For this reason, it is recommended that beginners complete a special program with slow load increase and instructions of the fundamental CrossFit techniques. In CrossFit, previous injuries also increase the risk of re-injury, which is consistent with the findings of other sport-related injury studies, and should also be taken into account. Evidence Level III


Background
CrossFit is a functional training modality that merges high-intensity exercises with functional multi-joint movements [9]. CrossFit is one of the fastest growing high-intensity functional training modes in the world [3]. Its popularity has grown worldwide over the past decade since its inception by Greg Glassman in 1996 in the USA with more than 15,000 affiliated gyms worldwide ( Figure 1). CrossFit has gained popularity not only among the civilian population but also among military, firefighters and police personnel, since CrossFit originally was developed among others for military strength and conditioning training and gradually spread among the civilian population [29] (Figure 2). CrossFit was developed as a core strength and conditioning program. The objective of CrossFit is to contribute to an optimization of the general physical competence (including a wide array of adaptations to all three metabolic pathways) without specialization. One of the main features of CrossFit is that the exercises are performed quickly and repetitively while there is limited or no recovery time between the sets [34].

CrossFit -Training
CrossFit training consists of different exercises, and should enable the individual to master various physical tasks (e.g. Olympic lifts, rope climbing, traditional sports, moving large loads over longs distances) and prepares for unknown tasks (e.g. surviving fights or fires). The structure of a CrossFit training may vary between different institutions. Usually each training session lasts about one hour and is comprised of a specific warm-up, strength and/or skill training, a programmed strength or conditioning workout for 10-30 minutes and finished with cooldown and/or mobility exercises [1]. In CrossFit, the training program differs depending on the athlete's individual condition, training location, intensity, duration,

Summary
CrossFit is a form of high-intensity interval training and comparable to other high-intensity exercises. CrossFit exercises increase VO 2max , strength, endurance, and improves the body composition (e. g. lean body mass). The injury risk in CrossFit is similar to that of other, comparable sports such as Olympic weightlifting, powerlifting and gymnastics and lower than that of competitive contact sports such as rugby union and rugby league. Shoulder joint, shoulder girdle and spine injuries are predominating in CrossFit and require special attention from physicians and coaches. In particular, this includes expert instruction for CrossFit beginners as well as an initial orthopedic examination before starting CrossFit training. The involvement of well-educated coaches correlates with a decreased injury rate. For this reason, it is recommended that beginners complete a special program with slow load increase and instructions of the fundamental CrossFit techniques. In CrossFit, previous injuries also increase the risk of re-injury, which is consistent with the findings of other sport-related injury studies, and should also be taken into account. Evidence Level III Orthopaedics and Traumatology organizational form and complexity. CrossFit training contains numerous recurring training elements and a variety of strategies for quantification and competition [24], and is characterized by short and highly intensive exercises ( Table 1). The exercises are performed either for time or for maximum repetitions and can be conducted as individual, partner or group training [3]. Another format is to maintain a specific workout pace for a time frame (e.g. complete a specified number of repetitions every minute on the minute (= EMOM) [24].

CrossFit -Associations and competitions
There's no official association since CrossFit is a branded fitness regimen created by Greg Glassman in 1996. Although there is no official set of rules, there are guidelines depending on the individual competition.

CrossFit -Coaches' demographics
CrossFit coaches are responsible for the training and therefore also for   the health of the participants. In a study by Waryasz, Daniels, Gil, Suric, and Eberson (2016) a survey questionnaire with 208 participants confirmed, that 77.9% of the CrossFit coaches were male with an average age of 34.5 AE 9.1 years. From all participating CrossFit coaches 80.4% had an exercise-related bachelor's degree, 72.4% an exerciserelated master's degree and over 89% a strength and conditioning certification [38]. Within another study by Waryasz, Suric, Daniels, Gil, and Eberson (2016) asking CrossFit coaches (n = 193) 86.6% were certified as instructors and 26.7% had a bachelor's degree in a training-related field. Instructors with a CrossFit certification have significant fewer bachelor's (p = 0.04) or master's (p< 0.001) degrees than those without a CrossFit certification [39].

CrossFit -Health benefits
Overall, the effectiveness of CrossFit in improving physical fitness as well as health related effects is not well explored [13]. A systematic review revealed 13 studies that examined the safety and health benefits of CrossFit [26]. Some of those recent studies showed, that CrossFit training can be effective for the improvement of several physical fitness parameters as enhanced endurance, increased maximal aerobic capacity VO 2max , aerobic and anaerobic capacity and a positive influence on participants' body composition, strength, flexibility, power and balance, BMI, fat mass and waist circumference, not only in adults, also in adolescents ( [3,4,14]). Besides positive physical effects of CrossFit training, also effects on mood state and social aspects were examined and found. Among other things, it has been shown that CrossFit training has a positive influence on motivational factors and the sense of community ( [10,16,22,33,41]).

CrossFit -Incidence and location of CrossFit related injuries
Because of the increasing popularity of CrossFit, questions about the health benefits and risks arise from a medical point of view, especially from an orthopedic perspective ( [7,14,18]). Due to the high intensity of the CrossFit training exercises, sports physicians expect an increased incidence of injuries, especially among beginners. Previously published studies have reported injury rates ranging from 0.74 to 3.3 per 1000 h CrossFit training ( [9,15,21]). On the basis of these studies, the injury incidence in CrossFit is comparable or slightly higher to related sports, including gymnastics, weightlifting and powerlifting with a 2b evidence level [5,15,20]), but lower than in game sports and other traditional sport disciplines [35]. However, injuries to the shoulder appear to be more common in CrossFit [20]. Other researchers found a higher incidence of CrossFit related injuries compared to traditional weightlifting. Elkin, Kammerman, Kunselman, and Gallo (2019) showed that those who were following a CrossFit routine were 1.3 times more likely to be injured and 1.86 times more likely to seek medical attention than those who were following a traditional weightlifting program. In a multivariate logistic regression analysis adjusted for sex and age, an injury was 2.26 times more likely in the CrossFit group. In both groups, shoulder injuries were most common (46.4%), followed by lower back (38.3%) and hip injuries (9.1%). The authors concluded, that CrossFit athletes are more likely to be injured and to seek medical treatment compared with athletes in traditional weightlifting [5]. Larsen et al. (2020) conducted a prospective cohort study with novice members of a CrossFit Center who began an eight-week, free-of-charge membership period. Among the 168 included participants, a total of 28 injuries (14.9%) were reported. The injury rate per 1000 h of exposure was 9.5 and therefore significantly higher than in previous studies. Therefore, the risk of injuries may be higher among novice participants than among experienced CrossFit athletes. This is consistent with further study showing a higher injury incidence in beginners during the first six months of CrossFit training ( [25,27]). Therefore, a care of CrossFit beginners is particularly important, as well as an orthopedic initial examination before starting CrossFit training. A high injury rate in CrossFit particularly affects the shoulder, spine, hip and knee [40]. In a four-year analysis of a descriptive epidemiology study, 30.5% of the CrossFit athletes experienced an injury over the previous 12 months [9]. Injuries to the shoulders were the most common (39%), followed by back (36%), knees (15%), elbows (12%), and wrists (11%) for both male and female athletes [9]. Elkin

CrossFit -Rare injuries and diseases
In addition to typical and frequent injuries, the scientific literature also reports on less frequent injuries and diseases, although these are significant in individual cases (Table 3).

CrossFit -injuries of the spine with neurological symptoms
Injuries of the spine and a 'Numb-Legí n a CrossFit athlete were reported from Esser, Thurston, Nalluri, and Muzaurieta (2017) [8]. The impact of CrossFit-related injuries was examined by Hopkins et al. (2017). Between June 2010 and June 2016, they evaluated all cases of spine injuries complaining of an injury caused by performing CrossFit. However, it must be noted that in this study the data were collected in only one hospital with neurological specialization (Northwestern University,  Table 4). The most common injury types were spinal injuries, accounting for 20.9% of all reported injuries. Of these spinal injuries, the lumbar region was the most commonly injured reporting 83.1% of all spine injuries. Although the most commonly prescribed treatment modality was physical therapy (40.4%), surgery was required in six patients (6.7%), and is comparable to other high-intensity competitive weightlifters as described in previous and actual studies ([2,30,36,37]).

CrossFit -Rhabdomyolysis
Hopkins, Li, Svet, Kesavabhotla, and Dahdaleh (2019) examined 523 patients incurred injuries associated with CrossFit activities [19]. They reported, that 11 patients received a diagnosis of rhabdomyolysis. The average age was 34.9 AE 9.4years, with 81.9% of the patients being male and 54.5% of the patients being beginners. Routman, Triplet, Kurowicki, and Singh (2018) reported in two cases about isolated rhabdomyolysis of the infraspinatus muscle. As reported, all cases of rhabdomyolysis are attributed to inappropriate training planning and too high intensity [31].

CrossFit -Dissection of the cervical carotid artery
Three cases of dissection of the cervical internal carotid artery were reported that are associated with CrossFit workouts [23]. Patient 1 suffered a distal cervical internal carotid artery (ICA) dissection near  Overhead press (any variation: strict press, push-press, push-jerk, split-jerk, etc) 13 Total, weightlifting 26 Unknown origin 14 a Participants could choose more than one option for each injury.
the skull base and a small infarct in Wernicke's area. Patient 2 suffered a proximal cervical ICA dissection that led to arterial occlusion and recurrent middle cerebral artery territory infarcts and significant neurological sequelae. Patient 3 had a skull base ICA dissection that led to a partial Horner's syndrome but no cerebral infarct. It is discussed that CrossFit's emphasis on speed and high-intensity movements and lack of proper guidance on technique may be one of the main reasons for injuries. Specific exercises, such as those that require rapid twisting movements involving weights, may be particularly dangerous, as they may lead to cervical vascular dissection [23]. reported in a case study a bilateral traumatic internal carotid artery dissection after CrossFit training [11], although committing that these cases are rare (2.5 to 3 per 100.000 athletes) [32].

CrossFit -Females and incontinence
CrossFit training and competitions are performed at high intensity, with high biomechanical impacts on the skeletal system and often with pressurized breathing. Therefore the exercises increase intra-abdominal pressure and cause (involuntary) urinary incontinence (UI), especially in women. In a population-based internet study of 551 female CrossFit athletes a prevalence of UI of 29.95% was shown. Most women with UI reported loss of urine during at least one exercise (16.70%). Double under, single under and box jumps are the most frequently mentioned exercises that lead to UI [28]. High, Thai, Virani, Kuehl, and Danford (2019) interviewed 314 women through an anonymous questionnaire in CrossFit centers. The prevalence of UI was 26.1%, which is only slightly above the prevalence in the general female population [17]. Elks, Jaramillo-Huff, Barnes, Petersen, and Komesu (2020) found a much higher incidence of UI among 322 women surveyed. They compared the incidence of UI between participants in CrossFit classes with female participants of other fitness classes. CrossFit athletes more frequently reported UI (84% vs 48%, p< 0.001), and higher severity of UI (Urinary Distress Inventory score: 20.8 vs 12.5, p< 0.001). Weightlifting, jumping movements (single and double unders) and box-jumps with high intra-abdominal pressure were the most common exercises and reasons associated with UI in CrossFit athletes; also age and participation in CrossFit are significant and independent predictors of UI as number of vaginal deliveries ( [6,12]).

Future directions and implications
According to previously mentioned studies, the sub-disciplines, gymnastics, powerlifting and Olympic weightlifting, were detected as primary cause for the most prevalent injury locations in CrossFit, the shoulder joint, shoulder girdle, spine, hip and knees ( [27,35,40]). The shoulder and lower back were the most commonly injured in gymnastic and powerlifting movements, respectively; primarily acute and fairly mild injuries [40]. Due to the frequency of injuries shoulder and spine must be given special attention. This concerns both sports medical preliminary examination/ preventive screening by sports physicians familiarized with CrossFit injury profiles and the adaptation of training exercises to the athlete's health and physical condition by CrossFit coaches. To prevent injuries, the scope and intensity of the exercises in the CrossFit must be increased slowly, scalable and carefully. At this point the injury reducing influence of supervision by qualified coaches becomes apparent [40]; especially among novice participants who seem more prone to injury than experienced participants [21]. Also, this includes the adequate exercise selection for females with urinary incontinence problems which are more likely to occur in CrossFit [6]. Due to the worldwide growth of CrossFit participation since 2008 and a limited amount of empirical research/evidence of injury rates and mechanisms there might be an increased risk of injury for CrossFit participants [20]. Most of the available research on injury rates is based on cumulative retrospective cohort studies [35] or single case-studies on (severe) injuries without proven direct causality to CrossFit [3]. For this reason, we recommend that more prospective studies should be conducted to provide a differentiated survey of CrossFit associated acute and/or chronic injuries and to allow the identification of potential risks.

Checkbox
As a conclusion, athletes who are willing to train CrossFit are advised to -check their suitability in a preliminary examination by sports physicians based on their current physical condition and medical background. participate in an affiliated gym's on-ramp program (= introduction class) before starting CrossFit training (getting used to techniques/movements and intensity). -train in an affiliated gym under supervision of qualified trainers. -increase the scope and intensity slowly and carefully in accordance with the trainer.

Important note
On June 6, 2020, CrossFit's CEO and founder, Greg Glassman, commented on the murder of African-American George Floyd by police officers with a racist, mocking statement. In connection with the current coronavirus pandemic and the public health situation, he described the issue as ''FLOYD-19'' [43]. CrossFit's main sponsor, Reebok, announced that it was terminating its collaboration with the CrossFit Company and its representative, Greg Glassman.
Meanwhile, hundreds of fitness studios around the world have distanced themselves from Greg Glassman and his statements [42]. As a result of the fierce criticism, Greg Glassman has resigned as CEO of the CrossFit Company.
In this context, we would like to emphasize that at the time of writing this article, we were not aware of these incidents, and we dissociate ourselves from racist statements of any kind.