CrossFit[R] is a relatively new sport's modality of training and competition that has recently exponentially expanded worldwide. Its exercises cover many movement patterns and are conducted at high intensity (Glasmann, 2007). Training is organized as daily sessions called "workouts of the day" or WODs. These WODs are executed with short or no rest periods, and combine exercises and movements in the form of a circuit (Glasmann, 2007). The objective of some of these exercises is to achieve the best time possible, while for others the goal is to complete as many rounds as possible over periods of 10 to 20 minutes (Smith et al., 2013). According to the contents of the WOD, there are three session modalities: gymnastics (G), in which the work involves the body itself (pull-ups, rope climb, pushups, ring row exercises, air squats, burpees, etc.); metabolic conditioning (M), including cardiovascular exercises such as running, rowing, or skip rope; and finally weightlifting (W), consisting of Olympic lifts (snatch, clean and jerk), deadlifts, squats, or overhead press lifts using, for example, kettlebells, sandbags, or medballs (Mate-Munoz et al., 2017). Thus, although the different CrossFit[R] sessions vary widely in their exercises and movement patterns, they share the feature that training is performed at high intensity with little or no rest periods. However, the workloads used in each exercise are not controlled and preestablished workloads can be excessive for some individuals (Weinsenthal et al., 2014). Further, few studies have examined physiological responses to these WODs. In a recent study, Fernandez-Fernandez et al. (2015) determined in a group of subjects with CrossFit[R] experience, acute physiological responses (V[O.sub.2], oxygen consumption; HR, heart rate, [lactate], blood lactate concentration and RPE, rate of perceived exertion) to a W and G type WOD known respectively as "Fran" (thrusters + pull ups) and "Cindy" (pullups, push-ups, air squats). According to these responses, both WODs were described as high intensity ([HR.sub.mean] = 9095 % [HR.sub.max]; [lactate] > 14 [mmol.sup.-1]; RPE > 8). When cardio-responses to the "Cindy" WOD were examined in another study in participants with little experience with CrossFit[R] (Kliszczewicz et al., 2014), [HR.sub.mean] = 91 [+ or -] 4.2 % [HR.sub.max], a rate equivalent to vigorous exercise according to the American College of Sports Medicine (ACSM) (Garber et al., 2011), and V[O.sub.2] = 63.8 [+ or -] 12.3 % V[O.sub.2max], also considered to indicate vigorous exercise (Kliszczewicz et al., 2014). Butcher et al., (2015) examined HR and RPE responses to two G type WODs. One included rest intervals (total 21 min of exercise, 6 sets of 60 s of 8 bench press + 10 kipping pull-ups or ring rows and box jumps for the remainder of the 60 s, and 3 min of rest) and the other WOD was 20 min of "Cindy" without rest. Results indicated that although both WODs gave rise to high HRs and similar RPEs, significant differences were produced in [HR.sub.mean] (87.6 [+ or -] 5.6 for Cindy vs. 76.4 [+ or -] 7.3% [HR.sub.max], P = 0.01).
Hence, although CrossFit[R] sessions can vary widely, very few work intensities have been quantified for the different WODs, with "Cindy" being the most widely analyzed.
Quantifying the intensity of exercise of different CrossFit[R] sessions will provide information about adequate training loads. As the training load has been linked to a risk of injury and/or disease (Drew and Finch, 2016), prescribing adequate training loads will lead to beneficial physical and physiological adaptations, reducing the risks of injury and thus increasing the probability of competition success (Fox et al., 2018).
Only one study has compared CrossFit[R] WODs with and without rest intervals. In a recent study by Mate-Munoz et al. (2017), it was noted that high intensity exercise without intervals, like the "Cindy" WOD or 5 min of power cleans, generated muscular fatigue, whereas muscular fatigue produced in response to a high-intensity interval exercise (double skip rope jumps), disappeared after 3 min of rest.
Muscular fatigue has been defined as any exercise-induced reduction in the maximal voluntary force or power produced by a muscle or muscle group (Bigland-Ritchie and Woods, 1984, Gandevia, 2001). This fatigue has a significant negative impact on performance (Meeusen et al., 2013) and has been related to a risk of injury due to biomechanical modification of the movement (Weisenthal et al., 2014). Hence, it is essential to quantify the muscular fatigue and intensity of Crossfit[R] WODs for training prescription and to elicit optimal adaptations, reducing the risk of injury.
Accordingly, the objectives of this study were: 1) to quantify the intensity of exercise and measure muscular fatigue during 3 CrossFit[R] WODs involving different movement patterns with varying work volumes and rest periods, and 2) to compare the different physiological and mechanical responses to the 3 WODs.
Experimental approach to the problem
To compare exercise intensity and muscular fatigue across the three different CrossFit[R] modalities, 4 exercise sessions were completed in 4 consecutive weeks in the order: Session 1--WOD 1 or G WOD, consisting of "Cindy"; Session 2--WOD 2 or M WOD, consisting of double skip rope jumps; Session 3--incremental power clean test (Olympic lifts) to calculate the maximum lifting strength of the individual; and Session 4--WOD 3 or W WOD, consisting of power cleans.
All sessions were completed on the same week day within the same three hour time window. The rest period between each session was one week. Ambient conditions for all sessions were the same (temperature: 21-25[degrees]C, atmospheric pressure: 715-730 mm Hg, and relative humidity: 40-50%). Exercises were executed in the CrossFit[R] Box of the Universidad de Alfonso X El Sabio, Madrid, Spain (see Figure 1 for experimental design).
The subjects selected for this study were 32 healthy men who were students of the degree course in Physical activity and Sport Sciences. Mean participant age was 21.75 [+ or -] 2.54 years, weight 76.85 [+ or -] 7.26 kg, height 1.79 [+ or -] 0.06 m and body mass index (BMI) 23.99 [+ or -] 1.70 kg x [m.sup.-2]. Participants' experience was more than 6 months of strength training, including free weight and Olympic lifts in their training routines. No subject consumed any type of medication or performance-enhancing drugs during the study. Further exclusion criteria were cardiovascular, metabolic, neurologic, or lung disease, or any orthopedic condition that could limit performance of the exercises. None of the participants had experience with CrossFit[R] WODs. Elite athletes were also excluded. In the 48 hours before each exercise session, it was required that subjects refrained from physical exercise, smoking or the intake of caffeine or alcohol.
After receiving an explanation of the nature of the study, written informed consent was obtained from each participant. The study design was in line with the tenets of the Declaration of Helsinki and received approval from the ethics committee of the University.
Power clean incremental load test: One week before WOD 3 (W type), an incremental load power clean test was conducted to determine each individual's maximum strength or 1RM. This test has been described in detail elsewhere (Mate-Munoz et al., 2017). One week before the study onset, the subjects practiced the power clean with the help of a qualified weightlifting trainer (Figure 1).
Warm up: Before each WOD or the incremental test, a warm-up was performed consisting of 5 min of low intensity running followed by 5 min of joint mobility and dynamic stretching exercises.
WOD 1 (G): "Cindy" The gymnastics WOD was the "Cindy" workout (Kliszczewicz et al., 2014, Kliszczewicz et al., 2015). This WOD consists of as many rounds possible of 5 pull-ups, 10 push-ups and 15 air squats in 20 min. Each round had to be properly executed according to preestablished minimum standards to continue onto the next round. One of the authors was responsible for counting rounds using a hand held counter. The techniques used for each exercise have been described in detail elsewhere (Mate-Munoz et al., 2017). For the pull-ups, butterfly or kipping variations were avoided as the subjects did not have sufficient experience with these movements (Figure 1).
WOD 2 (M): CrossFit[R] skip rope double unders The metabolic conditioning WOD consisted of double skip rope jumps (CrossFit[R] double unders) conducted as high-intensity interval training (HIIT). For the intermittent training protocol (Tabata et al., 1996), subjects completed as many double unders as possible in 8 sets of 20 s with 10 s of rest between sets. Test duration was 4 minutes. The number of double unders completed per set was counted by an observer while another observer guided the time periods of work and rest (Figure 1).
WOD 3 (W): power cleans The weight lifting WOD consisted of the maximum number of power cleans possible in 5 min lifting a load equivalent to 40% of the individual's 1RM determined 1 week previously. An observer counted the total number of power cleans completed (Figure 1).
Heart rate: Before each of the WOD sessions, subjects were fitted with a HR monitor (Polar RS-800CX; Polar Electro OY, Kempele, Finland). Heart rate data were stored and subsequently extracted using the software Polar ProTrainer 5. During each WOD, [HR.sub.mean] values were recorded as follows: WOD 1, for the whole trial and for minutes 110 and 10-20; WOD 2, for the whole trial and for sets 1 and 2 (S2), 3 and 4 (S4), 5 and 6 (S6) and 7 and 8 (S8); and WOD 3 for the whole trial and for minutes 1-2.5 and 2.5-5 (Figure 1). The equation used to calculate [HR.sub.max] for each participant, with which the HR data obtained for each WOD were compared, was 208-0.7 x age...