For over ten years, high-intensity exercise programs are gaining attention. Many studies have demonstrated the benefits of high intensity efforts for health and athletic performance. Among these activities, cross-training (that is, mixing different physical activities and different types of effort in one session) has become very popular thanks to the appearance of CrossFit in 2000's.
With more than 13,000 boxes affiliated with the brand, CrossFit is now a sport in its own right. It involves weightlifting, gymnastics, cardiovascular endurance, and many more. All these activities are mixed in one session (the "WOD", Workout Of the Day) and are performed at high intensity with little or no rest period. The arrival of this brand in the world of fitness, and sports in general, has generated strong antagonisms and CrossFit has often been accused of being a dangerous and inappropriate sport. But is CrossFit really so dangerous in comparison to other sports ?
To answer this question, a team of American researchers conducted a systematic review of the benefits and risks of CrossFit as a high-intensity activity. For this purpose, the researchers consulted the medical databases by searching for the keywords "CrossFit" and "High-Intensity Interval Training" (HIIT). Only CrossFit studies and adult practitioners were considered. Out of 251 studies, only 13 studies were selected with samples ranging from 10 to 1393 individuals. These studies fell into two main categories: those describing the benefits of CrossFit and those reporting injury rates.
IMPORTANT : For these studies, an injury is "any injury sustained during training that prevented the participant from training, working, or competing in any way and for any period of time".
Four studies studied the CrossFit injury rate. Out of 386 individuals, one study reported an injury rate of 19.4%, with a higher incidence of injury for men than for women. Injuries were mainly located in the shoulders, lower back and knees. When a coach was involved, the injury rate was decreased. The same authors conducted an online questionnaire and obtained 132 responses, the most common injuries were located in the same locations. Injury in the shoulder accounted for 25% of total injuries. In addition, another study showed that a previous injury multiplied the chances of recurrence by 3.75. Lastly, one study observed the injury rate among US military personnel over a period of 6 months before to 6 months after the introduction of a new training program (CrossFit and Ranger Athlete Warrior). The injury rate was similar to that observed in the non-active group. To reduce the rate of injuries, the authors of this study suggested that running distances should be reduced and resistance training should be increased.
Finally, a recently published study (2018), conducted by Dutch researchers, collected data on injuries during CrossFit practice in Holland. More than 550 people responded to the questionnaire, and 449 questionnaires were retained for analysis. All questionnaires collected corresponded to an adult practitioner (age ≥ 18 years) enrolled in an affiliated box. These questionnaires revealed that 56.1% of the athletes (252) were injured during the 12 previous months, and 15.2% (68 athletes) sustained more than 2 injuries. Again, injuries were mainly located in the shoulders (28.7%), lower back (15.8%) and knees (8.3%). Of all these injuries, 77.4% were diagnosed by a medical professional, and 58% of total injuries were due to overuse. The statistical model derived from the data collected showed that the duration of practice could influence the risk of injury. Thus, practitioners with 0 to 6 months of practice would be more likely to get hurt than more experienced practitioners. This can perhaps be explained by a too rapid rise in volume and/or intensity for a beginner.
Three studies compared different training protocols with CrossFit training. One study observed the enjoyment of practicing by comparing a group practicing a combination of endurance and strength training with a group practicing CrossFit. The pleasure of practicing was significantly higher for CrossFit practitioners. Probably due to the diversity of the sessions, the notion of "challenge" and the group effect. Another study observed similar oxidative stress (a proxy to measure the intensity of effort) during a high-intensity treadmill session in comparison to a CrossFit workout.
Finally, during a university semester, researchers wanted to compare three physical fitness training programs (strength, endurance, muscular power and body composition) in 60 students: one group practiced resistance training under the supervision of a coach, another group also practiced resistance training provided by a coach but without any supervision or follow-up, and the third group practiced CrossFit training. At the end of the semester, the researchers observed a significant improvement in strength and muscle power for the group with coach compared to the other two groups. The CrossFit group showed an improvement in the different variables measured in comparison to the unsupervised group, but no higher gains compare to the resistance training group under supervision. The presence and the follow-up of a coach play a lot in the evolution of the performance. In reality, this monitoring is systematic in a box CrossFit, which is not the case for most gyms, except for individuals paying for personal monitoring.
To determine the physiological improvements that CrossFit training could provide to practitioners of all levels, in 2013, a team of researchers observed the evolution of V02MAX and body composition over 10 weeks of CrossFit practice in 43 practitioners. In 10 weeks, body fat decreased by 3.7% and V02MAX improved by 11.8-13.6%. It should also be noted that all practitioners followed a "Paleolithic" diet (Read the full summary of this study).
Like all physical activity and sports, CrossFit presents risks for the practitioner. But the fact that it is done at high intensity does not imply higher risks than another activity (the risks seems to be similar). As always, it is imperative that the practitioner be properly supervised as this reduces the risk of injury. And in the case of CrossFit, it is imperative for a practitioner to learn and practice in an affiliated box, where professional coaching can help to adapt the exercises and intensity at the level of the practitioner. Regarding the benefits of CrossFit, based on the observed studies, they seem similar to those of a high-intensity activity for V02MAX improvements, muscle strength, cardiovascular endurance, and lean body mass.
Despite this, studies on CrossFit are still too few and the scientific quality of these is still too low. The practice is in essence very complex (each session is different, no standard competition, permanent implementation of new exercises), studies on CrossFit must be precise and exhaustive, and conducted separately with novice practitioners and with high-level practitioners, so that the learning effect does not alter the observed results. In addition, these studies must be comparative in order to compare the effects of different high-intensity physical activities with those obtained with CrossFit.
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