Untangling risk factors including discipline-specific exposure for injuries in preprofessional and professional circus artists in the USA

Participants

Ten circus training facilities across the USA were selected as host sites based on the size of the eligible target population, the presence of long-term intensive training programmes, and willingness to assist with study recruitment. Participants were recruited via flyers at area training centres, social media, American Circus Educator newsletter announcements, emails to host students and staff, and other local circus companies. Eligible participants were preprofessional (training ≥6 hours a week and performing in ≥2 shows per year) and professional (self-identified) circus artists over age 13 years, able to read/comprehend English and fulfil the requirements of the study. Exclusion criteria included a planned absence from the area ≥1 month over the next year. A power analysis showed that 200 was an adequate sample size for a medium-to-large (0.25–0.50) effect size with a p value of 0.05. Adults signed an informed consent form, and adolescents signed an assent form. Parents/guardians completed informed consent.

Design

The prospective, observational cohort design was modelled after a pilot study.11 12 Participants/public were not involved in the study design, conduct or dissemination.

Rolling enrolment occurred from September to December 2018 (cohort 1, four facilities) and from September 2019 to January 2020 (cohort 2, six facilities). Enrolment included online completion of informed consent/assent forms, an intake questionnaire including demographics, training experience and medical history, and a physical examination11 12 of intrinsic factors by a physical therapist (PT). PTs had at least 5 years of orthopaedic or sports-related experience and underwent standardised training for study procedures.

Participants were tracked for 1 year following their enrolment date. Participants were emailed a Qualtrics (V.9/2017–2012/2018, Provo, Utah, USA) survey link to complete a weekly training log where they reported weekly exposure as the total number of sessions (classes, rehearsals, performances or self-directed training by circus discipline and strength or flexibility conditioning). New or ongoing injuries and missed training sessions due to injury were recorded. For weeks without exposure, participants indicated the reason (eg, vacation, illness, COVID-19 closure). For new or recurrent injuries, the PTs conducted an interview and physical examination to determine the injury’s mechanism, body region, tissue and nature. Injuries at study enrolment were not included in the injury frequency calculation unless an exacerbation occurred.

Injury classification

The injury was defined as an anatomical tissue-level impairment.11 12 14 Time loss injury (TL) resulted in the full loss of participation in at least one circus discipline for one or more days after injury onset.11 12 14 Injuries not meeting the TL criteria were defined as non-time loss (NTL). The type of injury and mode of onset followed the circus-specific extension.9 13 New injury represented the first time a specific injury occurred within the study period.9 13 15 16 Subsequent recurrent injury was the same location and nature, and subsequent local injury was the same location but different nature after returning to 100% participation from TL injury.9 13 15 16 Acute sudden onset related to a single traumatic event (eg, fall, awkward landing).9 13 Repetitive sudden or gradual onset was associated with repetitive use.9 13 17 Injuries were defined as directly associated with participation in circus training, rehearsal or performance, or indirectly associated if related to the circus context outside of those activities (eg, spotting), or not related if outside of the circus context.9 13 Disciplines associated with injuries were grouped using the circus arts discipline subgroup classification system, which classified disciplines as aerial acrobatics (aerial, aerial with ground elements), ground acrobatics (human propulsion, apparatus propulsion, balance/control), manipulation and character.9 11

Data analysis

Descriptive statistics were conducted using Microsoft Excel 365 (vV.2211, Redmond, Washington, USA) for baseline intake and examination data, injury rate (frequency per 1000 sessions of circus exposure) and frequency for participant subgroups by age, SAB and professional status. Brief descriptive statistics were also included for the non-binary participants to provide some information on this under-reported group. The remaining analyses used IBM SPSS Statistics (V.26) with significance set at p<0.05. A regression analysis used total training sessions, age and years of circus experience in years as predictor variables for injury incidence. Multiple t-tests compared injury types by discipline subgroups, age, professional status and SAB. Pearson’s correlational analysis tested the relationship between the duration of training experience and injuries. The influence of medical history and prior health status was tested using multiple regression analyses.

Participant characteristics

Select data from study enrolment are in table 1. Average years of circus experience was higher (t(195)=−3.96, p<0.001), accounting for different variances (F=7.26, p<0.008), in professionals (8.63 years; SD=6.01) compared with preprofessionals (5.58 years; SD=3.23). Amenorrhoea was more prevalent in adolescents than adults and preprofessionals than professionals. No adolescents reported an eating disorder, although 36% of adolescent AFAB had amenorrhoea. Fewer participants reported a connective tissue disorder than met the criteria for generalised joint hypermobility (GJH) per their Beighton score.18

Exposure

Circus exposure by discipline subgroup and training context, excluding drop-outs, are in tables 2 and 3. Weekly circus exposure was higher (t(199)=10.76, p<0.001) in professionals (9.27; SD=1.44 sessions) than preprofessionals (6.87; SD=1.65 sessions). Across participant subgroups, exposure was highest for aerial and ground (balance/control), except for adolescents with similar exposure to ground (human propulsion).

Injury rates

Injury rates, excluding drop-outs, are reported per 1000 exposures in table 4. By participant subgroup, AMAB had the highest injury rates (5.69/1000 sessions), and by discipline subgroup, aerial with ground elements, aerial then ground (balance/control) were highest (5.93, 4.26, 3.66/1000 discipline-specific sessions).

Injury types or patterns

Tables 5 and 6 show injury distribution by subgroup. The proportion of injuries was highest for the shoulder (22%), followed by the lumbosacral region (13%) and then the elbow (10%). Muscle/tendon injuries were most common (52%) across tissue types.

COVID-19 effects

The COVID-19 pandemic impacted cohort 2. Training facility and show closures started 1–7 months into their study period. Overall training and performance exposure were lower by 21.83% and 34.53% compared with cohort 1. There were changes in the disciplines trained due to facility access. The environment for training often changed from facility to home or outdoors, and performances became recorded or virtual. Injury assessments were completed virtually with a modified physical examination, sometimes limiting our ability to specify the affected body tissue.

Discipline

Determining extrinsic risk for circus disciplines, including accounting for exposure and artists training multiple disciplines, is challenging9–12 but important to inform injury prevention practices. We found higher injury rates attributed to aerial versus ground acrobatics, but rates varied across discipline subgroups. Aerial with ground elements, including Chinese pole and pole dance, had the highest injury rate (5.93/1000 session exposures). In a professional training programme, injuries from Chinese pole were most prevalent among aerial disciplines but less frequent than ground acrobatics.4 Injury rates for recreational pole dance were higher (7.65/1000 hours training)20 than in circus arts (0.3–3.3/1000 hours).3 5 7 Although study methods and population differences limit comparison between studies, aerial with ground elements may have an elevated injury risk.

Ground acrobatics-related injury rates were highest for the balance/control subgroup (3.66/1000 sessions), which includes contortion, hand-balancing and partner acrobatics. In contrast, Munro4 found ground acrobatics was the leading cause of injury with a similar proportion in balance/control and human propulsion disciplines. Exposure was not quantified, but the tumbling practice was a sizeable part of the school curriculum. Similarly, floor exercise is associated with the highest proportion of gymnastics injuries.21 22 Future studies should account for discipline exposure in injury rates to improve our understanding of injury risk across disciplines and to avoid misrepresenting injury risk by just reporting frequency.

Age

Age is an intrinsic risk factor. Like the pilot study,12 adults had a higher injury rate than adolescents (3.98 vs 2.42/1000 session exposures), although incidence decreased with age. Adolescents had more ground than aerial-related injuries (60%;14%) but also higher exposure to the ground than aerial (3.98; 1.48 sessions/week). Adults showed the opposite pattern with lower injury frequency (37%;44%) and exposure (2.78; 3.23) in ground than aerial disciplines, highlighting the importance of interpreting injury risk relative to exposure. Adolescents had significantly more NTL injuries (77%) than adults (47%), suggesting that adults have more severe injuries. However, across participants with TL injuries (excluding dropouts), adolescents had greater average TL over one year (65.8; SD=50.62; 30.8; SD=59.93). Accounting for injury management might be useful to understand this contrast as high school circus arts students were less likely than basketball players to report the injury to the athletic trainer.23 Adults may also be better at implementing rest or accessing healthcare resources.

Preprofessional versus professional

A high absolute workload can positively or negatively impact injury risk depending on ramp-up and internal factors such as stress, anxiety and fatigue.24 Fatigue is a known injury risk factor in sport25 and dance,26 27 and is affected by training load in circus students, despite sufficient sleep.28 Professional circus artists reported less fatigue than students at a circus college,29 which could explain why despite higher exposure (9.27 vs 6.87 sessions/week), professional circus artists experienced a lower injury rate than preprofessionals (3.49 vs 4.08/1000 sessions).

Injury rate differences may also relate to greater professional experience (9.27 years) than preprofessionals (5.71 years). Third-year students at a circus college experienced lower injury rates than first-year students suggesting that experience may be protective.7 This could be attributed to training adaptations, mental performance or focus on artistry and consistency for a marketable act rather than technical development and building a skill repertoire. Professional circus artists have also reported lower social isolation, better social skills, fewer problems in shows/evaluations and greater performance satisfaction compared with circus students,29 reinforcing the need for a more holistic consideration of factors related to an injury.

Sex

Sex differences included a higher injury rate (5.69 vs 3.5/1000 exposures) and a higher proportion of acute than repetitive injuries for AMAB than AFAB. A study in collegiate athletes also found AMAB had more acute injuries.30 However, differences were no longer apparent when analyses were limited to 10 sports with similar movement skills for both sexes.30 In gymnastics, AFAB had more overuse injuries than AMAB,30 possibly due to apparatus differences. These studies suggest that gender biases in training are more likely responsible for this difference than anatomical or physiological sex differences.

In circus arts, sex differences in overall injury rates are uncommon,2 4 6 7 but become apparent for injury location. In our study, lumbosacral (14%; 9%) and hip/groin (9%; 2%) injuries were higher in AFAB than in AMAB. In other studies, AFAB had more hip/groin injuries2 4 and AMAB more wrist, forearm and ankle injuries.4 These differences may arise from gender biases in circus education, where women are often encouraged to train in disciplines exhibiting grace and flexibility and men in more dynamic disciplines.31 Understanding the impact of gender identity is also important, including the implications for for non-binary (3.9% of our study population) and transgender/gender diverse artists (4.6% in another study).32

Medical history

Overall self-reported eating disorder prevalence (18%) was lower than in another study of circus artists (36%),33 however, 35.71% of AFAB adolescents in our study had a history of amenorrhoea despite none reporting an eating disorder. Amenorrhoea could indicate the physiological effects of underfuelling and an undetected eating disorder. Similarly, only 1.5% of participants reported having a connective tissue disorder despite 34.5% meeting the GJH criteria. Lack of healthcare access may have contributed to the underdiagnosis of eating and connective tissue disorders and impacted injury burden and should be further examined in future research. Similar to other studies,34 35 eating disorders were associated with higher injury risk, but unlike other sports research,36 37 amenorrhoea and history of connective tissue disorders were not. Baseline physical characteristics such as strength, flexibility and balance did not influence injury incidence. However, these factors are often the focus of injury prevention, suggesting that prevention strategies must shift to address our emerging understanding of risk factors.

Strengths

Despite the required weekly training log, the study had a high completion rate (77.11%). Including NTL injuries versus only TL or medical attention injuries better represents the breadth of circus injuries. Injury assessment by a PT enhances diagnostic accuracy for the injury cause and type compared with self-report.14 Using the circus-specific extension framework, including the circus discipline subgroups to report our data, will allow for valid comparisons to future research.

Limitations

Based on challenges during the pilot study,12 exposure by hours was not tracked, limiting the comparison to some research. Future studies should include total circus exposure in hours.9 Recruitment challenges limited adolescent participation (n=16), which could be increased with greater parent and coach engagement. There were also limited AMAB participants (n=29/201), which seems to reflect the circus demographics in the USA.38