In order to assess the prevalence of occupational disorders experienced by FI and SI, a retrospective cross-sectional self-reported observational study was conducted. Subsequently, in order to investigate physical fitness and daily workload during a typical working day in FI and SI a prospective cross-sectional observational study was performed.
Participants were recruited from various fitness centers companies (Get Fit, 20 Hours, Virgin Active). These companies employed both FI (e.g. dance aerobics, step aerobics, spinning, Pilates, yoga, low back pain exercise classes, strength training, boxing/kickboxing) and SI (e.g. water aerobics, swimming courses, mother/baby swimming courses). The inclusion criteria were being a FI or SI instructor and teaching a minimum of one class a week. The exclusion criterion was being unable to fill out the questionnaire. Potential participants e-mail addresses were provided by the head of each center. The responders were contacted by email in which they were fully informed about study procedures, benefits and risks associated with participation. Written informed consent was e-mailed and participant have to sand it back signed. At this point the online survey was e-mailed to the participant who agreed to participate and met above inclusion and exclusion criteria. Participant who agreed also to participate to the second part of the study were phone called in order to organize laboratory testing and daily workload monitoring.
The online survey
The online survey was created according to the guidelines provided by Artino et al. . Subjects were requested to complete an online survey during a 2 weeks period. The survey requested information regarding personal physical data, frequency, duration and time (early morning, morning, afternoon, or evening) of class participation. For the purposes of this study, all self-reported occupational disorders related to their work during their entire career were asked by answering the following question: “Have you experienced any occupational disorder as FI or SI in the last 2 years of your career?” . If positive response, participants have to specify each injury, type of injury (acute/overuse), in accordance with the definitions provided by the consensus statement regarding disorder registration . Only those participants who saw a physician for their disorders were asked to report a diagnosis. Participants who did not see a physician were asked only to report the location of the disorder. The extent of the disorder was examined through contingency questions regarding the limitation that the injury placed on activity. The survey took 20-25 minutes to complete. The administered questionnaire, after English translation, is available as supplementary material.
Physical Fitness Assessment
Participants’ physical fitness was assessed by maximal oxygen consumption assessment (O2max). Oxygen consumption (O2), carbon dioxide production (CO2), and pulmonary ventilation (E) were measured using a metabolic device on a breath-by-breath basis (Quarkb2 Cosmed, Rome, Italy) during a graded ramp cycle ergometer test (Monark Ergomedic mod. 839E, Monark, Vansbro, Sweden). All tests were carried out in a well-ventilated laboratory at a temperature of 20-22°C under standardized constant ambient conditions (i.e. a temperature of 22±2°C and humidity of <70%). The protocol consisted of 3 minutes at 50W/min (warm up and familiarization), followed by an increase of 20W every minute until exhaustion. Achievement of O2max was considered as the attainment of at least two of the following criteria: 1) a plateau in O2 levels despite increasing speed; 2) a respiratory exchange ratio above 1.1; and 3) a HR of ±10 bpm of age-predicted maximal HR (i.e. 220 - age) . Heart Rate (HR) was recorded during the entire test using a HR monitor (Polar RS800, Polar Electro 2011, Kempele, Finland). Maximal HR at exhaustion was consider as HRmax.
Daily Workload Monitoring
Each participant was equipped with a HR monitor (Polar RS800, Polar Electro 2011, Kempele, Finland) and instructed to wear it during their typical workday for one week. In particular, FI and SI were instructed to wear it before the start of their classes and to remove it at the end of the last lesson of the day. For the subsequent analysis, the central 3 hours of this workday has been considered to standardize results, by selecting an equal time frame for all participants, and excluding preparation and conclusion periods, that can differ among activities. HR recordings were expressed in the percentage of the maximum value (%HRmax) reached during the maximal oxygen consumption assessment. All the HRs obtained were then compared to the American College of Sports Medicine recommendations  for the development of aerobic fitness, which define the relationship between work HR ranges and work intensity. Participants were asked to continue their normal daily working routine and to maintain their usual diets during the monitoring period.
Rating of Perceived Exertion Assessment
The Borg CR100 scale  was selected to rate per perceived exertion of a typical lesson. A verbal-anchored was provided to the participants who were instructed to use it 30-min after the end of the last lesson of their working day. Each participant was familiarized with the Borg CR100 scale, including anchoring procedures.
Respondents with missing data were excluded from the analysis. Descriptive statistics (mean ± standard deviation, m ± SD) for the outcome measures were calculated. The normality of the distribution was checked using graphical measures and the Kolmogorov-Smirnov test. Since all anthropometric variables were normally distributed, differences between male and female FI and SI were checked using an unpaired Student’s t-test. A Chi-square test was used to compare the questionnaire’s variables of educational level, professional information, and job characteristics between FI and SI groups. Differences between FI and SI were studied using the Student’s paired t-test when analyzing the perceived exertion after the maximally fatiguing workday, the perceived exertion 30 minutes after the end of their lessons, and the perceived exertion and fatigue upon waking up in the morning. Intra- and intergroup differences (gender ´ instructor type) between in daily workload for FI and SI for O2max, HRmean, and HRmax were checked using two-way analysis of variance (ANOVA) with Bonferroni’s multiple comparisons test. The level of statistical significance was set at p<0.05. Statistical analysis was performed using the software STATISTICA (version 7.1, StatSoft, Tulsa, OK, USA).