Abstract
With the increase in popularity of the CrossFit exercise program, occupational health nurses may be asked questions about the appropriateness of CrossFit training for workers. This systematic literature review was conducted to analyze the current research on CrossFit, and assess the benefits and risks of this exercise strategy. Thirteen studies (N = 2,326 participants) examined the use of CrossFit training among adults; CrossFit is comparable to other exercise programs with similar injury rates and health outcomes. Occupational health nurses should assess previous injuries prior to recommending this form of exercise. Ideal candidates for CrossFit are adults who seek high-intensity exercise with a wide variety of exercise components.
Keywords
Today, participation in high-intensity group exercise programs is gaining attention (Teetor, 2014). CrossFit, a form of high-intensity interval training (Milanović, Sporiš, & Weston, 2015), is one such exercise program that has grown rapidly since its inception in 2000. Although there is no official count of how many people participate in CrossFit, it has been estimated that the program is used at more than 2,000 facilities worldwide (Longe, 2012). CrossFit was originally designed to train individuals (e.g., police officers, military special forces) whose work requires physical fitness and muscle strength so these workers could transform from low to high levels of effort in seconds. Intended to improve movement efficiency, the program incorporates various functional movements to promote muscle strength and cardiorespiratory fitness (Weisenthal, Beck, Maloney, DeHaven, & Giordano, 2014).
CrossFit’s specific modes of exercise include power/Olympic lifting (i.e., squats, cleans, deadlifts, bench press, and presses), gymnastics (i.e., pull-ups, lunges, knees to elbows, handstand push-ups, push-ups, and sit-ups), and aerobic exercise/metabolic conditioning (i.e., swimming, running, and rowing; Longe, 2012; Weisenthal et al., 2014). These exercise movements are often performed for specific lengths of time, with little to no rest at high intensity (Weisenthal et al., 2014). CrossFit workouts are scalable, so the exercises can be performed safely and effectively, given an individual’s current level of fitness (Longe, 2012).
With the popularity of such programs, however, come concerns about possible injuries; occupational health nurses may be asked to answer questions about the appropriateness of CrossFit for workers. The purpose of this literature review was to assess benefits and risks associated with participation in CrossFit.
Method
A systematic review of the literature used the PRISMA protocol (Moher, Liberati, Tetzlaff, & Altman, 2009), searching PubMed, SPORTDiscus, and Combined Arms Research Library (CARL) databases (Figure 1). The search terms “CrossFit” and “high-intensity interval training” were chosen to yield the largest number of published articles. Inclusion criteria included full-text research articles exploring CrossFit with adult participants published in English. Studies of children or adolescents were excluded; case studies, review articles, and articles that did not present research or were opinion pieces were also excluded.
Study selection.
Results
A total of 13 studies examined the use of CrossFit for adults (Table 1). The total sample for all 13 studies was N = 2,326. Sample sizes for individual studies ranged from 10 to 1,393. The studies were conducted internationally, including the United States (n = 10), Canada (n = 1), Poland (n = 1), and an international online forum (n = 1). The studies fell into two groups: those that assessed the benefits of CrossFit and those that reported CrossFit injury rates. Although most of the studies included both men and women, one included only women (Heinrich, Patel, O’Neal, & Heinrich, 2014), and two included only men (Bellar, Hatchett, Judge, Breaux, & Marcus, 2015; Kliszczewicz et al., 2015). One study was conducted on a university campus with college students (Barfield, Channell, Pugh, Tuck, & Pendel, 2012), and three examined CrossFit as part of military training (Grier, Canham-Chervak, McNulty, & Jones, 2013; Knapik, 2015; Paine, Uptgraft, & Wylie, 2010). None of the studies were conducted in an occupational setting. All of the studies included healthy adults, but participants varied in levels of CrossFit experience; in one study, novices were compared with experienced CrossFit athletes (Bellar et al., 2015).
Studies Using CrossFit
Note. CF = CrossFit; BMI = body mass index; HR = heart rate; RPE = rating of perceived exertion; MMHIIT = multimodal high-intensity interval training; MMCIR = multimodal circuit; BDNF = brain-derived neurotrophic factor; HIFT = high-intensity functional training; HIIT = high-intensity interval training; PFT = Physical Fitness Training; ECP = extreme conditioning program.
Injury Rates
Three studies reported injuries due to CrossFit, and one examined postexercise dysfunction. Injury rates among CrossFit participants were comparable to rates for other recreational or professional athletes (Chachula, Cameron, & Svoboda, 2016; Grier et al., 2013; Hak, Hodzovic, & Hickey, 2013; Weisenthal et al., 2014). Hak et al. (2013) reported CrossFit injury rates and patterns of injuries among 386 individuals; the overall injury rate was 19.4%, with males injured more frequently than females. The most common areas for injury were the shoulders, lower back, and knees. However, injury rates decreased with trainer involvement. Chachula et al. (2016) reported that participants with prior injuries were 3.75 times more susceptible to reinjury.
Grier et al. (2013) reviewed medical records of U.S. Army brigade combat team members spanning 6 months before and 6 months after the implementation of a new fitness program that incorporated CrossFit training and Ranger Athlete Warrior Program, both of which are considered extreme conditioning programs (ECPs), along with Advanced Tactical Athlete Conditioning (ATAC). Injury rates for participants in the ATAC/ECPs were consistent with rates for nonparticipants. Grier et al. found that injuries could be minimized with less long-distance running and more resistance training.
Hak et al. (2013) examined CrossFit-related injuries reported by 132 participants who responded to an online questionnaire; the most common injuries were shoulder injuries, spinal injuries (especially the lower back), and arm or elbow injuries. No incidence of rhabdomyolysis was reported. Injuries to the shoulder accounted for 25.8% of total injuries. Hak et al. suggested that CrossFit is safe for all athletes when activities are performed correctly in a safe environment with trainers; however, those athletes who report previous injuries should take precautions to avoid reinjury as in any sport.
Drum, Bellovary, Jensen, Moore, and Donath (2016) compared CrossFit training with American College of Sports Medicine (ACSM) training guidelines. In this study, the authors collected data on excessive muscle soreness postexercise, delayed onset muscle soreness, and shortness of breath via questionnaire; they also collected participant-reported ratings of perceived exertion (RPEs). CrossFit participants’ RPEs were higher, 7.3 ± 1.7, than those for ACSM, 5.5 ± 1.4 (p ≤ .001). Postexercise symptoms were also higher for CrossFit than for ACSM, respectively: excessive fatigue, 42 versus 8 (p < .001); muscle soreness, 96 versus 48 (p = .04); muscle swelling, 19 versus 4 (p = .048); shortness of breath, 13 versus 1 (p = .02); muscle painful to touch, 31 versus 4 (p = .001); and limited muscle movement during workouts, 37 versus 9 (p = .007). Thus, those individuals who train with CrossFit can expect greater postexercise pain than they might experience with other exercise routines. Drum et al. suggested that athletes should scale their training with planned rest cycles to avoid overuse and prevent injury.
Novices Versus Experts
Two of the studies compared outcomes for novice versus expert CrossFit participants. Butcher, Judd, Benko, Horvey, and Pshyk (2015) compared two different CrossFit-based multimodal workouts: multimodal circuit training (MMCIR) and multimodal high-intensity interval training (MMHIIT): In total, 57 participants completed the two workouts on different days. Butcher et al. examined heart rate (HR) and RPEs during both workouts, as well as the differences between novices and experts. Overall, mean HR was lower in the MMHIIT group (76% ± 7% predmax) than in the MMCIR group (88% ± 6% predmax); both groups had similar RPEs (17 ± 2 vs. 18 ± 1 on a scale of 20, respectively). Experienced participants in both groups had an overall higher mean HR but no differences in RPE. According to Butcher et al., the intensity of both types of CrossFit workouts was at the higher end of guidelines for health and exercise, and may increase cardiovascular fitness.
Bellar et al. (2015) measured VO2 max and anaerobic power in 32 male participants who were either naïve to CrossFit or highly experienced, and found that their history of CrossFit participation was associated with higher performance in CrossFit workouts (F = 35.72, p ≤ .001). Participants with greater experience had better aerobic capacity and anaerobic power.
Comparison Studies
In three of the studies, CrossFit was compared with other high-intensity functional training (HIFT) programs. Heinrich et al. (2014) studied 23 participants, examined exercise enjoyment, and compared standard aerobic and resistance training (ART) with group-based HIFT using CrossFit. The ART group reported lower exercise enjoyment than the CrossFit HIFT group. High intensity functional training participants reported higher exercise enjoyment (p = .049) than participants in the aerobic exercise control group.
Kliszczewicz et al. (2015), who studied 10 participants, compared a high-intensity treadmill workout with a CrossFit workout, focusing on acute oxidative stress. Blood plasma was tested preexercise, immediately following workout, 1 hour after exercise, and finally 2 hours after exercise for oxidative damage from high-intensity exercise and antioxidant capacity. The CrossFit workout demonstrated oxidative stress comparable to the oxidative stress following the high-intensity workout. Oxidative stress is particularly sensitive to the level of exercise intensity.
In a semester-long study of 87 college students, Barfield et al. (2012) compared three exercise groups: an instructor-led traditional class, an independent class (the instructor provided a training program, but students followed it on their own), and CrossFit training with a coach. The authors measured body composition and muscular strength, endurance, and power at the beginning and end of the semester. The instructor-led traditional group had significant improvement in comparison with the other two groups for both muscle power and strength (p = .008). However, the CrossFit resistance training participants showed fitness gains at an average of 17%. This study did not report statistical power; it is possible that it lacked sufficient power because only 20 matched participants were included per group.
Physiological Benefits of CrossFit
Smith, Sommer, Starkoff, and Devor (2013) measured changes in VO2 max, body composition, and aerobic capacity in a 10-week study of 43 men and women participating in CrossFit-based high-intensity power training (HIPT). These participants, who were also following a Paleolithic diet, presented all levels of fitness, body composition (measured with the BOD POD, an air displacement plethysmography device), and VO2 max (measured by a max treadmill test using Bruce protocol). The maximum volume of oxygen consumption (VO2 max) serves as a proxy for fitness. Over the 10 weeks, body fat percentages dropped 3.7% (p = .00008) and VO2 max improved from 11.8% to 13.6% (p = .001). Smith et al. concluded that CrossFit training and HIPT can improve aerobic capacity and body composition among individuals with varying levels of fitness; the changes in body composition might have been due to diet or a combination of diet and exercise.
In a study of younger participants using CrossFit, Murawska-Cialowicz, Wojna, and Zuwala-Jagiello (2015) assessed changes in brain-derived neurotrophic factor (BDNF), a protein that enhances the production of neurons, irisin, physical performance, body mass/composition, and muscle circumference during a 3-month training program. The exercise-induced hormone, irisin, increases the expenditure of energy; in some animal models, irisin decreases obesity and insulin resistance (Sanchis-Gomar, Lippi, Mayero, Perez-Quilis, & García-Giménez, 2012), increases aerobic capacity, increases VO2 max (p = .02), reduces adipose tissue percentage (p = .02) in women, and increases lean body mass (p = .004; Murawska-Cialowicz et al., 2015) in all participants. Brain-derived neurotrophic factor levels showed marked increases in all participants; irisin levels showed no change.
Discussion
Overall, the research on the effectiveness of CrossFit in improving physical fitness among adults is not extensive. The literature search located only 13 studies that examined CrossFit in various ways, including consideration of its safety and benefits.
Every form of physical activity holds a potential risk for injury (Oh, 2013). Rates for CrossFit training injuries are consistent with those rates for injuries that occupational health nurses routinely encounter for workers who engage in other fitness routines. Approximately 74% of all runners, for example, experience a moderate or severe injury each year (Daoud et al., 2012), which is much higher than the injury rate of 19.4% among CrossFit participants (Weisenthal et al., 2014). A history of previous injury predisposes those who train with CrossFit to reinjury (Chachula et al., 2016), which is also consistent with the findings of other sport-related injury studies (e.g., Hespanhol, Pena Costa, & Lopes, 2013). Chachula et al.’s (2016) study examined 12 elite soccer players and injury rates; players who had previous injuries had almost three times the risk of reinjury. Occupational health nurses should consider individuals’ histories of injury and injury patterns (Oh, 2013) before CrossFit is recommended.
As the present literature review suggests, individuals experienced in CrossFit perform better and have higher gains in aerobic capacity and anaerobic power than do CrossFit beginners (Bellar et al., 2015; Butcher et al., 2015). When a CrossFit athlete is familiar with the program’s movements, exercises, and expectations, CrossFit’s effectiveness increases as well. To benefit most from CrossFit, it is best to find a CrossFit gym with an “On-Ramp” program, which provides instruction on fundamental CrossFit movements as well as CrossFit-certified coaches (Oh, 2013). Novices may experience less physical change during their initial sessions, so they should be encouraged to continue to gain the exercise regimen’s full benefits.
Barfield et al.’s (2012) comparison study found that traditional exercise provided greater muscular fitness gains than did CrossFit workouts. It may be that the mode of exercise did not matter as much as the exercise intensity, which is difficult to measure. Kliszczewicz et al. (2015) have shown that the intensity of routines and movements provides the most benefit to participants. In addition, the sample size in Barfield et al.’s study may have been inadequate, and Barfield et al. did not measure exercise enjoyment. Heinrich et al. (2014), on the contrary, showed that participants in a CrossFit HIFT group could maintain exercise enjoyment and were more likely to continue in the program. Occupational health nurses who care for workers should select exercise routines that stress the importance of exercise intensity and enjoyment to optimize outcomes.
Nursing Implications
With more than 13,000 licensed CrossFit affiliates worldwide (CrossFit, 2016) and throughout the United States, occupational health nurses have general knowledge of CrossFit exercise programs because they may encounter clients engaged in or planning to engage in such activities. With an understanding of CrossFit’s benefits and risks as well as exercise safeguards, occupational health nurses can safely consult with clients who are interested in CrossFit training. Nurses should assess previous injuries and possible limitations prior to recommending CrossFit; nurses can recommend their clients find gyms with On-Ramp programs or classes of basic CrossFit movements used in the workouts (Oh, 2013). When treating injuries from CrossFit, nurses should be aware of CrossFit-certified trainers’ ability to individualize and scale workouts for those injured or recovering. The effective scaling of workouts should address the unique needs of each athlete adequately (CrossFit, 2016).
Conclusion
In summary, CrossFit is comparable to other high-intensity exercise regimens in terms of both injury rates and health outcomes. CrossFit, just as any other high-intensity training, increases VO2 max, strength, musculature, and endurance, and decreases lean body mass. With proper training and incremental increases in intensity, CrossFit can be an effective form of exercise for healthy adults looking for a diverse workout routine.
Applying Research to Practice
CrossFit is a form of high-intensity interval training. It is comparable to other high-intensity exercise regimens in terms of both injury rates and health outcomes. CrossFit, just as any other high-intensity training, increases VO2 max, strength, musculature, and endurance, and decreases lean body mass. With proper training and incremental increases in intensity, CrossFit can be an effective form of exercise for healthy adults looking for a diverse workout routine.
Footnotes
Conflict of Interest
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
Author Biographies
Jena Meyer recently completed her MSN at the University of Texas at Austin School of Nursing, where she studied public health nursing. She is currently serving in the U.S. Air Force.
Janet Morrison is a recent graduate of The University of Texas at Austin School of Nursing doctoral program. Her area of interest is physical fitness, cognitive function and disability.
Julie Zuniga is an assistant professor at The University of Texas at Austin School of Nursing. Her research focus is self-management of chronic conditions.