Dear Editor in Chief:

We read with great interest the recently published article by Tibana and de Sousa (1) titled “Are extreme conditioning programmes effective and safe? A narrative review of high intensity functional training methods research paradigms and findings.” We appreciate the opportunity to write this letter and hope to clarify some of the authors’ conclusions. Although the authors provide several examples of what they refer to as “extreme conditioning programs” we will focus mainly on the statements and evidence related to High Intensity Functional Training (HIFT), more commonly known as CrossFitTM training, as the authors’ review focuses primarily on this particular training program. We feel the authors have taken a biased position in describing this type of training and that their position is based on inaccurate and highly speculative interpretations of a fraction of the existing literature.

Research examining the acute and long-term responses to HIFT, as well as the incidence of injury, is quite limited. The observed responses predominantly describe changes from baseline and in the case of long-term adaptations, generally show a positive outcome. Further, the few studies that make comparisons to other exercise forms only show select differences. More importantly, by the authors’ own admission, research examining the risk of injury do not suggest HIFT/CrossFitTM to be different from other forms of recreational exercise. Yet, the authors describe this risk as “alarming” and “becoming more prevalent.” We certainly agree that additional research is needed on this popular form of exercise, but at this time, and based on the available evidence, any definitive conclusions are premature at best.

From their terminology to generalized conclusions, it is clear that Tibana and de Sousa (1) have taken a position on HIFT/CrossFitTM that fits the definition of “extreme”. However, the Merriam-Webster dictionary describes extreme as something “existing in a very high degree,” or “exceeding the ordinary, usual, or expected” or even “relating to, or being an activity or a form of a sport (such as skiing) that involves an unusually high degree of physical risk” (2). Yet, the scientific literature around HIFT/CrossFitTM does not support this notion or the use of “extreme” as an accurate descriptor. More importantly, the authors seem to have misrepresented some of the data. For example, alternating sets of push-ups and pull-ups on each minute for 48 minutes and totaling 400 push-ups and 200 pull-ups is equivalent to completing approximately ~16 push-ups and ~8 pull-ups per minute. Although this might appear to be a difficult task to some, it is not made clear by Tibana and de Sousa (or the original authors) how these tasks were performed (e.g., push-up starting position; assisted, butterfly, or kipping pull-ups) which would impact the severity of the exercise (3, 4). More importantly, the authors failed to mention that the individual played ice hockey on the following night and participated in another high-intensity workout the day after before seeing a medical professional. The authors also described additional protocols (e.g., 105 push-ups in 6 minutes [or 17 push-ups per minute]; 3 sets of chin-ups to failure over a 20-minute period) as being “extreme” when they were generally in line with the recommended prescription for endurance exercise (5). Another example of misrepresentation occurred with their description of the ‘Sissy Test’. This workout consists of 240 total repetitions evenly split between kettlebell swings and burpees, not 672 total repetitions. Likewise, the competition workout that included “unaccustomed exercise” (i.e., 60 GHD’s) is simply a version of the sit-up exercise performed with greater range of motion. In short, if we are to accurately describe HIFT and assess its merits and limitations, it is important to maintain perspective, describe protocols in detail, and avoid exaggerations.

Feito et al. (6) provided a working definition for HIFT: “a training style [or program] that incorporates a variety of functional movements, performed at high-intensity [relative to an individual’s ability], and designed to improve parameters of general physical fitness (e.g., cardiovascular endurance, strength, body composition, flexibility, etc.) and performance (e.g., agility, speed, power, strength, etc.).” Unlike “Extreme Conditioning Programs,” which would suggest that training is “abnormal” or “inherently dangerous,” this definition provides an objective description of workout modality and intensity, as well as its targeted outcomes. We acknowledge that this particular definition may not have been available at the time of the authors’ manuscript submission, but it is not drastically different than the definition provided by CrossFitTM itself: “constantly varied, high-intensity, functional movement” (7). Further, the term “HIFT” was first introduced in 2014, when Heinrich and colleagues (8) compared high-versus moderate-intensity training programs and has been used in more than a dozen other studies since. Tibani and de Sousa use ‘HIFT’ within their title, but the term is otherwise absent throughout their manuscript. We believe this omission, along with their exclusion of other works, represents a considerable amount of bias and limits the ability of the authors (and readers) to form accurate, objective conclusions on the impact of HIFT. Moreover, several studies have provided insight to the cardiovascular responses of HIFT training (9-11) and suggest these programs fit within the current exercise prescription recommendations provided by the American College of Sports Medicine (12).

Regarding injuries related to HIFT, the authors acknowledge that the incidence of injury (approximately 2 – 4 injuries per 1000 hours) is similar to that of weightlifting and other recreational activities and lower than several contact sports. Though limited by the cross-sectional nature and small sample sizes, these rates are consistent with what others have reported (13-19). As such, we agree with the authors that additional studies are needed to elaborate on the potential risk of this training modality, particularly among different population subgroups (i.e. youth, older adults, etc.). However, we do diverge from the authors’ presentation of this information throughout the manuscript. The authors introduce three areas of concern that are related to participant’s safety: “(1) use of fast movements with a high number of repetitions and insufficient rest intervals; (2) participants with a weak motor repertoire to perform complex movements and (3) a possible lack of educated, certified and experienced ECPs professionals, especially in the integration of all training methods.” Unfortunately, the authors do not provide evidence to validate these concerns.

Tibana and de Sousa cite a pair of studies that indicate a greater occurrence of overuse injuries (20) and a higher prevalence of shoulder injuries (14) following HIFT, but neither of these studies report the programming characteristics (i.e., training volume, relative intensity loads, or exercise frequency) employed by their participants. To make generalizable conclusions such as these, these authors (14, 20), as well as Tibani and de Sousa, must assume that higher training volumes, high training intensities, and complex movements are consistently employed across training. If we are to assume that HIFT/CrossFitTM is being appropriately employed, then this cannot be the case. By definition (6, 7), HIFT is constantly varied and scalable (i.e., intensity is relative to the individual), and would suggest that participants do not regularly employ any specific volume, intensity or modality with sufficient regularity to evoke overuse injuries. Nevertheless, without an accurate programming record, it is impossible to conclude that HIFT/CrossFitTM participants are regularly exposed to inappropriate complex movements that lead to injury. Likewise, it is not suitable to assume HIFT participants possess a “weak motor repertoire to perform complex movements” or that coaches lack education, certification, or experience. Although not provided by the authors, evidence refuting these claims have been provided by Tafuri et al. (21) and Waryasz et al (22), respectively.

Another important topic related to injury discussed by the authors is that of exertional rhabdomyolysis (ER). Regardless of the activity, developing ER is an extreme case that is detrimental to physical performance improvements and may lead to death. Nevertheless, the authors provide a very narrow view of how ER can develop and their conclusion that it is “becoming increasingly more prevalent” in HIFT is unsupported. The evidence provided within the manuscript consists of nine case studies that detail the occurrence of ER in 15 individuals. When put into perspective, the incidence of ER in relation to HIFT is quite small. Compared to the number of CrossFitTM affiliates existing worldwide (approximately 13,000), this would equate to 0.12% of participants if each location had a single member develop ER. Compared to the number of participants in the 2018 Open competition (N = 429,157 competitors), it would equate to less than 0.004%. A closer look at the literature related to ER demonstrates a similar number of case studies that support the development of ER in other activities (23-25), such as stationary cycling (26-28), ultimate frisbee (29) and even hiking (30).

In the most current and comprehensive study of HIFT related injuries, Feito et al. (19) demonstrated the overall low incidence of injuries associated with HIFT programs. According to that study, which included over 3,000 participants over a 4-year period, only 0.6% of participants reported the occurrence of ER (19). Even though the study by Feito and colleagues (19) is also limited in its ability to depict incidence of ER, it certainly provides a more appropriate measure of prevalence than any case study alone. Moreover, Knafl and colleagues (31) reported an increased number of ER cases being admitted to an emergency room in Brisbane, Australia and alluded to high-intensity workouts (e.g., CrossFitTM training) as a potential cause. However, this may have been merely speculation by the authors, as the underlying causes were divided into seven specific categories: 1) gym, 2) marathon/running, 3) manual labor, 4) military/police training, 5) team sports, 6) walking/heat, and 7) other (31). Therefore, we certainly agree that additional studies are warranted to accurately portray the incidence of ER, but we cannot yet conclude that HIFT presents any greater danger than other exercise forms.

In regards to the physiological responses to HIFT, Tibana and de Sousa refer to a number of studies documenting a larger inflammatory response compared to traditional exercise protocols. They summarize and interpret the literature as follows: “These results indicate that ECPs elicited a higher metabolic, cardiovascular, hormonal and inflammation response. Therefore, strength and conditioning professionals need to be aware of the level of stress imposed on individuals when performing metabolic workouts of ECPs… it is recommended that the incorporation of lower intensity sessions (e.g., through the rating of perceived exertion or HR) and/or resting days would help to minimize this exacerbated physiological response.” Although the authors partly summarize the more pronounced physiological responses, the recommendation to reduce intensity is not warranted, nor is the subjective description of the response as “exacerbated.” It is well documented that neuroendocrine and inflammatory responses play a critical role in exercise-induced signals for health-related benefits, including cardiovascular and metabolic adaptations (32, 33). It is further documented that these responses are directly correlated to the intensity of the exercise bout, and the degree of adaptations to exercise are, accordingly, primarily linked to the intensity of the exercise stimulus (34, 35). Therefore, we express the concern that the recommendation put forth by Tibana and de Sousa to reduce exercise intensity is tantamount to recommending a suboptimal exercise stimulus that would produce smaller magnitude health benefits compared to HIFT. Indeed, the authors failed to mention a very important recent study demonstrating that HIFT reduces fat mass, increases insulin sensitivity, increases fat oxidation, reduces blood concentrations of the pro-inflammatory cytokine resistin and increases levels of the anti-inflammatory cytokine adiponectin in people with type 2 diabetes (36).

With respect to the chronic adaptations associated with HIFT, Tibana and de Sousa reach the conclusion that the “scientific literature showed few or no chronic effects on body composition and improvements in physical fitness and psychological parameters; however further studies are important.” For two reasons, we fail to understand how the authors reached this particular conclusion. First, the evidence provided by the authors seems to contradict their conclusions. Namely, the majority (i.e., 6 out of 11 studies) of these studies demonstrate positive effects of HIFT interventions on both body composition and fitness outcomes. Second, the authors fail to include two recent (but available prior to the date of publication) investigations that further demonstrate these positive effects (37, 38). Thus, we would like to offer an alternative interpretation of the authors’ presented evidence with respect to the chronic adaptations to HIFT. We would argue that not only does the majority of the evidence support the efficacy of HIFT for multiple domains associated with health and fitness, it also demonstrates efficacy across multiple population subgroups (e.g., teens, cancer survivors, people with Type 2 Diabetes, etc.). Further, the considerable heterogeneity across these studies with respect to intervention dosing (e.g., included interventions ranged from five weeks to six months in duration) suggests the likelihood of a dose-response relationship between HIFT and its desired outcomes. Current evidence suggests that at least 15 HIFT sessions, over a 3 – 5-week period, are needed to elicit changes in fitness outcomes, but the minimum effective dose specific to body composition changes is not well established and could potentially be significantly higher than it is for fitness-related outcomes.

Overall, we commend Tibana and de Sousa’s for their work and their attempt to review the current available literature related to HIFT. However, based on our interpretation of the evidence presented herein, we urge caution on the part of the BMJ readership in regards to its presentation and applying the recommendations put forth in their article.

 
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