Injury rate and patterns in group strength endurance training classes

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Injury Rate and Patterns in Group Strength-Endurance Training Classes Anna M. Batterson, DO; Raegan K. Froelich, BS; Cathy D. Schleck, BS; and Edward R. Laskowski, MD Abstract Objective: To identify the injury rate during high-intensity functional training. Participants and Methods: Adults (N¼100; 82 [82%] female) in group strength-endurance training at the Dan Abraham Healthy Living Center from January 9, 2017, through April 19, 2018, were recruited for the study. Participants were recruited before the class start date. Those who consented received a preclass survey and another survey 6 weeks after the class started to obtain data on demographic characteristics, baseline joint problems or pain, injuries in the preceding 6 weeks, class satisfaction, and exercise habits. Classes lasted 6 weeks and were led by a trainer for 60 minutes, once weekly. Participants were encouraged to perform similar exercise on 2 additional nonconsecutive days throughout the week. Injury was self-reported and defined as experiencing new pain or sustaining injury while exercising during the 6-week time frame. The primary outcome measure was the number of injuries per 1000 training hours. Results: The injury rate was 9.0 injuries per 1000 training hours (95% CI, 5.8-13.4 injuries per 1000 training hours) during the 6-week training and 5.0 injuries per 1000 training hours (95% CI, 2.8-8.2 injuries per 1000 training hours) during the 6 weeks preceding enrollment (P¼.08). Injury occurred in 18 (18%) of participants during the 6-week training, and 9 of 24 injuries (37.5%) occurred during a training class. The most commonly injured regions were knees (n¼7) and back (n¼6). Burpees and squats were the most common movements causing injury. Conclusion: The increased injury rate during the study was not statistically significant. It was higher than rates reported in previous retrospective studies of high-intensity functional training, weight lifting, or power lifting but comparable with rates reported in prospective studies of novice and recreational runners. ª 2019 Mayo Foundation for Medical Education and Research

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igh-intensity functional training (HIFT) programs use highly varied combinations of functional movements (movements that require motor recruitment patterns in multiple movement planes across multiple joints) done at a relatively high intensity for a short time. They are part of the broader category of high-intensity interval training (HIIT), which is primarily endurance or aerobic exercise without strengthening. High-intensity functional training tends to include more strength training and functional movement patterns. Strength training may include use of kettlebell movements, barbell movements, dumbbell movements, or calisthenic or body weight

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movements. It may also include ballistic movements, such as variations of push press, clean, jerk, and snatch. High-intensity functional training may also include plyometric exercises such as box jumps, jumping rope, and jumping squats. Examples of HIFT programs include “boot camp”estyle classes, some military training, CrossFit, or other variations of training that combine strength and endurance training. In a 2018 worldwide survey of fitness trends, HIIT and group training ranked first and second, respectively, as top fitness trends.1 Research has found that HIIT can improve cardiovascular fitness and reduce cardiometabolic risk factors.2 In a randomized study, participants in HIFT spent less time exercising

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From the Department of Physical Medicine and Rehabilitation (A.M.B., R.K.F., E.R.L.) and Division of Biomedical Statistics and Informatics (C.D.S.), Mayo Clinic, Rochester, MN.

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but had greater enjoyment of exercise and were more likely to continue with the program than did participants in moderateintensity aerobic and resistance training.3 A pilot study of cancer survivors suggested that HIFT participation improved emotional functioning, body composition, and functional movement.4 In addition, CrossFit members report higher social capital and community belongingness than do members of traditional gyms.5 Research is lacking about whether HIFT programs are associated with a higher risk of injury than are traditional exercise programs. A recent review article discussed the relative risk of injury with HIFT in military training and concluded that HIFT was an effective way of reducing training volumes, particularly running volume, while improving fitness and that it had a similar or lower risk of injury than did traditional training.6 Most research pertaining to injury rate in HIFT programs has been retrospective, and injury rates of 2.4 to 3.1 injuries per 1000 training hours have been reported.6-8 Several online surveys of CrossFit participants have reported an injury risk similar to or lower than that with other physical activities such as running, volleyball, basketball, soccer, and Olympic lifting.6 One prospective research study of HIFT in active military training participants reported no injuries, but injury was not the primary outcome measure.9 Research has found that HIFT-related injuries are more common in the shoulders, back, arms or elbows, hands, wrists, knees, and hips.8,10 To our knowledge, no prospective studies have investigated injuries in nonmilitary HIFT participants. PARTICIPANTS AND METHODS Recruitment The design was a prospective cohort study that took place from January 9, 2017, through April 19, 2018. Adult participants in a small-group training program at the Dan Abraham Healthy Living Center (DAHLC) at Mayo Clinic in Rochester, Minnesota, were recruited to obtain a convenience sample for the study. To be included in this study, participants had to be 2

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18 years or older, provide consent, and be signed up for a 6-week DAHLC strengthendurance training program. The only exclusion criterion was refusal to participate. Participants could participate in the study more than 1 time for nonconcurrent sessions. This study was approved by the Mayo Clinic Institutional Review Board (IRB no. 16007501). Approximately 1 week before their training start date, participants in strengthendurance training were notified in a welcome e-mail from the DAHLC of an upcoming e-mail that would include details about the research study. The e-mail containing informed consent information and research study details was then sent individually to each potential participant within 3 days of the start date. Interested participants provided consent by e-mail and were then sent a link for the online preclass survey. An online postclass survey was sent by e-mail 6 weeks after the start date. Both the preclass and postclass surveys included questions about the preceding 6 weeks. Both surveys were linked by using a unique participant identification number and the date submitted. Training Details The DAHLC is a wellness facility serving Mayo Clinic employees, retirees, students, volunteers, and spouses on the Rochester, Minnesota, campus. Members have the option to sign up for small-group training as part of their membership. Members are asked to complete a 30-minute group training placement consultation before participating in small-group training. The purpose of the consultation is to place members in the appropriate training on the basis of their goals, interests, and current fitness level. The consultation consists of a brief discussion on health and fitness history and goals and a functional movement screen. After completion, members may sign up for the recommended group-training type. Strength-endurance is 1 of 7 training types. It focuses on improving and maintaining the ability to produce force over repeated efforts (muscular endurance). This class is an example of an HIFT class. This goal is XXX 2019;nn(n):1-8

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accomplished by increasing the number of repetitions and reducing the amount of recovery time between sets and exercises. Various techniques are used, such as minimizing rest time between sets and circuit training, to increase the efficiency of the workouts. The workout structure has 4 key sections, referred to as training components: workout preparation, muscular system development, energy system development, and recovery. Exercises focus on multijoint movements to improve coordination between multiple muscle groups. The class may use a combination of barbell, dumbbell, medicine ball, resistance band, kettlebell, and body weight movements. The class has a maximum of 4 participants, meets once weekly for 6 weeks, and lasts 60 minutes. In addition to the weekly trainer-led class, participants are encouraged to independently complete up to 2 additional exercise sessions using a similar format on nonconsecutive days throughout the week. Primary Outcome Measure The primary outcome measure for this study was the injury rate, calculated as the total number of injuries incurred per 1000 training hours during the 6-week training. Injury was self-reported and defined as sustaining an injury or experiencing new pain while exercising. Training time was calculated by asking participants to estimate the mean number of hours per week spent exercising during the preceding 6 weeks, rounded to the nearest 0.5 hour. Training hours were calculated per person by multiplying the mean number of hours spent exercising per week by 6. The total number of injuries and training hours reported were then used to calculate the injury rate per 1000 training hours. Secondary Outcome Measures Secondary outcome measures included demographic information, previous participation in strength-endurance training class, baseline joint problems or pain, exercise movement type, class attendance, class satisfaction, pretraining injury rate per 1000 training hours, exercise frequency and duration, preferred exercise setting, injury rate as a percentage of injured participants out of total participants, Mayo Clin Proc. n XXX 2019;nn(n):1-8 www.mayoclinicproceedings.org

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and postetraining-specific injury characteristics. Demographic data collected included age and sex. The preclass survey also asked whether participants had previously participated in a DAHLC small-group strengthendurance training class. Participants were instructed to document exercise types during the 6 weeks preceding group training as a percentage of time spent (totaling 100% for all movement types). Exercise movement types were categorized as ballistic weight lifting, endurance, plyometrics, nonballistic weight lifting with free weights, weight machines, calisthenics or body weight movements, and mind-body exercises. Participants ranked motivators for exercise from highest to lowest on a 1 to 4 scale in both the preclass and postclass surveys. Preclass and postclass surveys asked participants to self-report injury and, if present, in how many areas. If participants reported any injuries in the postclass survey, they were asked to specify injured locations, the movements in which injuries occurred (if known), and how many of these injuries or pain episodes occurred in small-group training, outside small-group training, or at an uncertain time because of gradual onset. They were also asked to report whether they missed a small-group session because of injury and whether they had seen a physician, physician assistant, or nurse practitioner for the injury or pain. Chiropractors were excluded because of the possibility of participants seeing a chiropractor routinely for maintenance care. Class attendance was measured on the postclass survey by asking participants how many classes (of 6) that they had attended. A visual analog scale was used on the postclass survey to measure satisfaction: 0 (not at all satisfied) to 100 (extremely satisfied). Satisfaction was further measured on the postclass survey by asking participants whether they intended to sign up for another DAHLC strength-endurance training class; if not, they were asked whether they planned to continue with a similar style of training. Statistical Methods A sample size of 100 was chosen because of the feasibility of recruitment within the

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planned time to complete the study and number of usual participants historically in the class. Data are reported using standard summary statistics, including mean  SD for continuous variables and count (percentage) for categorical variables, unless otherwise noted. Rate ratios were calculated to measure the difference in injury rate during the class relative to the injury rate before the class. The association of demographic factors and exercise data with the injury rate was evaluated using Poisson regression. Changes from the preclass survey to the postclass survey were assessed using Wilcoxon signed-rank tests for continuous outcomes and McNemar tests for categorical binary outcomes. All statistical tests were 2-sided, and P values less than .05 were considered significant. All analyses were performed using SAS version 9.4 (SAS Institute, Inc.). RESULTS Participant Characteristics A total of 109 participants were recruited for the study, and 100 (91.7%) completed both the preclass and postclass surveys and were included in the final data analysis. The mean age was 40.111.6 years (range, 22-64 years), and most participants were female (Table 1). Most participants had previously participated in a DAHLC strength-endurance training class. Nearly one-third of participants reported baseline joint problems or pain before the start of the class. Exercise Behaviors and Motivation During the 6-week training, participants exercised for a mean of 270 min/wk and 56.4 min/ exercise session; these values were similar to the 6-week time frame before the class start (mean, 276 min/wk and 54.8 min/exercise session). From the preclass survey to the postclass survey, the percentage of exercise time dedicated to calisthenic or body weight movements significantly increased (P¼.03) and that of endurance and mind-body movements significantly decreased (P¼.02) (Table 2). Increases in ballistic weight lifting, nonballistic weight lifting, and plyometric movements were not 4

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TABLE 1. Demographic Characteristics and Baseline Data for 100 Participants in Group Strength-Endurance Training Classesa,b Characteristic Age (y)

Value 40.111.6

Sex Male Female

18 (18) 82 (82)

Attended a previous DAHLC strength-endurance classc No Yes

28 (29) 70 (71)

Preferred exercise setting Small-group setting With personal trainer Independently With friend Large-group setting

74 9 7 6 4

(74) (9) (7) (6) (4)

DAHLC ¼ Dan Abraham Healthy Living Center. Data are presented as mean  SD or as No. (percentage). c Two participants did not submit a response to this question. a

b

statistically significant, and the decrease in weight machine movements was not statistically significant. Among the responses pertaining to rank of exercise motivation, 57 were completed correctly and able to be used for analysis. From preclass to postclass, losing weight or getting more “toned” decreased in frequency as the top motivator and all other motivators increased (Figure). Injuries The total number of injuries was 24 in 18 participants during the 6-week training period and 15 in 13 participants during the 6 weeks before the training. The injury rate was 9.0 injuries per 1000 training hours (95% CI, 5.813.4 injuries per 1000 training hours) during the 6-week training period and 5.0 injuries per 1000 training hours (95% CI, 2.8-8.2 injuries per 1000 training hours) during the 6 weeks before the training. Although the rate ratio of this increase was 1.8 (95% CI, 0.93.5), it was not statistically significant (P¼.08). The difference in injury rate between men and women was not statistically significant (rate ratio, 0.8; 95% CI, 0.3-2.1; P¼.60). In addition, when the evaluation of the training XXX 2019;nn(n):1-8

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TABLE 2. Exercise Movement Types Before and During Strength-Endurance Training Classes Percentage of the total exercise time spent Movement type Ballistic weight lifting

Examples

6 wk before the training

During 6 wk of the training

P value

47.9

610.8

.07

3925.7

3220.6

.02

68.9

78.3

.13

2218.0

2515.3

.06

69.0

46.6

.15

1610.8

1910.8

.03

913.4

713.5

.02

Olympic lifts (clean, jerk, and snatch), push press, kettlebell swing, kettlebell clean, and kettlebell snatch

Endurance

Running, walking, cycling, rowing, and swimming

Plyometrics

Box jumps, jumping squat, jumping lunge, jump rope, and long or broad jump

Nonballistic weight lifting with free weights

Back squat, front squat, overhead squat, shoulder press, bench press, Turkish get-up, and bicep curls

Weight machines

Leg press, lateral pull-down, leg extensions, calf press, and abdominal crunch machine

Calisthenics or body weight movements

Push-up, pull-up, dip, squat, lunge, sit-up, burpee

Mind-body exercises

Yoga, Pilates, tai chi, qigong, and Feldenkrais method

Data are presented as mean  SD.

period vs the pretraining period was adjusted for sex, the injury rate ratio remained 1.8 (95% CI, 0.9-3.6). The increases in injury rate for participants who reported more than 20% calisthenic or body weight movements and for those who were repeat participants in the class were not statistically significant (Table 3). Of the 24 injuries, 9 (37.5%) occurred during a small-group class, 13 (54.2%) occurred while exercising outside the small-group class, and 2 (8.3%) were of gradual onset. Of the 18 participants who reported injury, 4 (22.2%) missed a small-group class as a result and 3 (16.7%) saw a medical provider. The most common locations injured were knees (n¼7) and back (n¼6). Other injured areas were hips (n¼3), elbows (n¼2), shoulders (n¼2), fingers (n¼1), and other (n¼3). Back or knee injury or pain was more common with nonballistic weight lifting (free weights), plyometrics, and calisthenics, and burpees and squats were the most commonly cited movements causing injury. Of the 24 injured joints or regions, 4 (16.7%) were locations that participants reported as having pain or problems before the class. Furthermore, 3 of 6 back injuries (50%) occurred in participants with baseline back pain or problem and 3 of 7 participants (42.9%) reporting baseline back Mayo Clin Proc. n XXX 2019;nn(n):1-8 www.mayoclinicproceedings.org

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pain or problems reported a new injury or pain in the back during the study. Class Satisfaction and Adherence Most of the 100 participants attended at least 5 of the 6 classes (n¼82) or at least 4 of the 6 classes (n¼91). All 6 classes were attended by 38 participants. Class satisfaction was high; the mean visual analog scale score was 88.7. At the end of the study, 89 participants reported plans to participate in another DAHLC small-group strength-endurance

42%

Lose weight or get more “toned”

25% 32%

Improve strength or endurance

40% 23%

Improve overall health and wellness

30% 4%

Personal enjoyment

5% 0%

Preclass Postclass

10% 20% 30% 40% Participant ranking, %

50%

FIGURE. Highest motivators for exercise, as ranked by participants in a group strength-endurance training class.

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TABLE 3. Injury Rates by Demographic and Exercise Factors No. of injuries

No. of training hours

Injury rate per 1000 training hours (95% CI)

Rate ratio (95% CI)

P value

Time of reporting Preclass Postclass

15 24

3000.0 2662.2

5.0 (2.8-8.2) 9.0 (5.8-13.4)

1.8 (0.9-3.5)

.08

Sex Female Male

20 4

2104.2 558.0

9.5 (5.8-14.7) 7.2 (2.0-18.4)

0.8 (0.3-2.1)

.60

Age 40 y >40 y

9 15

1234.2 1428.0

7.3 (3.3-13.8) 10.5 (5.9-17.3)

1.4 (0.5-4.0)

.48

Exercise experience New to class type Previous participation

4 19

816.0 1780.2

4.9 (1.3-12.6) 10.7 (6.4-16.7)

2.2 (0.7-6.5)

.16

Pain or joint problem at baseline Not present Present

13 11

1848.0 814.2

7.0 (3.7-12.0) 13.5 (6.7-24.2)

1.9 (0.7-5.2)

.20

Injury in the preceding 6 wk No Yes

19 5

2374.2 288.0

8.0 (4.8-12.5) 17.4 (5.6-40.5)

2.2 (0.6-7.8)

.23

19 2

2071.2 312.0

9.2 (5.5-14.3) 6.4 (0.8-23.2)

0.7 (0.2-2.8)

.61

16 5

1822.2 561.0

8.8 (5.0-14.3) 8.9 (2.9-20.8)

1.0 (0.4-2.7)

.98

19 2

1966.2 417.0

9.7 (5.8-15.1) 4.8 (0.6-17.3)

0.5 (0.1-3.5)

.48

13 8

1306.2 1077.0

10.0 (5.3-17.0) 7.4 (3.2-14.6)

0.7 (0.3-1.9)

.55

18 3

2116.2 267.0

8.5 (5.0-13.4) 11.2 (2.3-32.8)

1.3 (0.3-5.8)

.71

12 9

1680.0 703.2

7.1 (3.7-12.5) 12.8 (5.9-24.3)

1.8 (0.7-4.6)

.22

16 5

1945.2 438.0

8.2 (4.7-13.4) 11.4 (3.7-26.6)

1.4 (0.4-4.7)

.60

Factor

Movement type Ballistic 10% >10% Endurance 50% >50% Plyometrics 10% >10% Free weights 20% >20% Machines 10% >10% Calisthenics 20% >20% Mind-body 10% >10%

training class. Of 10 participants who did not, 7 (70%) reported plans to continue with similar training. Overall, 96 participants planned to continue performing HIFT at the end of the study. DISCUSSION The injury rate in our HIFT population was higher than previously published rates of 6

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injury during HIFT. The injury rate of 9.0 injuries per 1000 training hours in this study was approximately triple the 2.4 to 3.1 injuries reported in the literature for HIFT.6-8 Unlike in previous studies, upper limb injuries were not common in our population. This difference may be attributed to the low use of ballistic weight-lifting movements during the class, which participants reported XXX 2019;nn(n):1-8

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using for a mean of only 6.0% of their total exercise movements. This percentage is likely lower than that of many other HIFT participants, such as those who participate in CrossFit. Because HIFT is increasingly popular for maintaining fitness among the general population of adults, it may be helpful to compare injury risk in other adult fitness activities, such as weight lifting (clean, jerk, and snatch), power lifting (squat, bench press, and dead lift), or running. We are not aware of any published injury rates for HIIT that do not include HIFT. Injury rates reported in the literature range from 2.4 to 3.3 injuries per 1000 training hours for weight lifting and from 1.0 to 5.8 injuries per 1000 training hours for power lifting, and all but one of the studies are retrospective.11 Published injury rates have ranged from 8.9 to 33.0 injuries per 1000 training hours in novice runners and from 5.2 to 12.1 injuries per 1000 training hours in recreational runners; these rates were from prospective studies.12 The injury rate in our study was higher than the rate reported for power lifting or weight lifting and was comparable with those reported in novice and recreational runners. The data from our study further support previous findings of high exercise adherence and satisfaction with HIFT. Most participants missed only 1 class, and the mean participant hours per week of exercise was 4.5, which is higher than the current US Department of Health and Human Services physical activity guidelines of at least 150 minutes of moderate-intensity or at least 75 minutes of vigorous-intensity exercise per week.13 The mean satisfaction reported by participants was also high, and most participants planned to continue doing HIFT at the end of the 6-week session. In addition, participants were more likely to be motivated to exercise by personal enjoyment at the end of the study, a factor that could increase exercise adherence. This study had several strengths. Unlike previous studies evaluating HIFT injury rate, this study was prospective, a factor that allows for a more accurate estimation of injury rate than do retrospective studies because of the Mayo Clin Proc. n XXX 2019;nn(n):1-8 www.mayoclinicproceedings.org

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reduced risk of recall bias. Other strengths are a relatively large sample size of 100 and a high study completion rate of 91.7%. To our knowledge, this is the largest prospective study of HIFT documenting injury rate in any population and the first prospective study of HIFT injury rate in the adult general population. This study also has limitations. It cannot be generalizable to all HIFT programs because of the considerable variability between different HIFT programs and within HIFT programs. For example, most boot camp programs and CrossFit differ from each other, and within the broader category of CrossFit, the training and environment of different affiliates are variable. The class size was limited to 4 participants, which is likely smaller than many group HIFT classes. Thus, the typical trainer to participant ratio could be lower in the general HIFT population. In addition, a limited amount of time during our study was spent on clean, jerk, or snatch movements, which may be more similar to boot camp programs and less similar to CrossFit. This difference could account for the lower shoulder injury rate than that in previous studies of HIFT injury in CrossFit participants. In addition, the female predominance among participants in our study may or may not be generalizable to other facilities and programs. The survey used during the study was not a validated survey, a factor that additionally limits generalizability. Furthermore, given the manner in which participants were recruited, there was potential for selfselection or nonresponse bias, and this was a nonprobability sample, which also reduces generalizability. Other limitations of this study include lack of a control group and self-report of injury by participants. The study also used a broad definition of injury. CONCLUSION The United States, and the world in general, remains in the midst of an epidemic of obesity and sedentary lifestyle. Programs and classes that promote physical activity and exercise can help to mitigate the effects of this epidemic and provide the motivation

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needed to get people moving. High-intensity functional training has become popular in recent years, yet few studies have assessed injury risk in the general HIFT population. Our study found a trend toward an increase in injury during the course of a typical HIFT class. Most of the injuries were related to movement patterns that are essential to be performed with optimal technique to decrease injury risk. Ensuring optimal technique for participants performing these movement patterns, notifying the instructor of any preexisting injuries or medical conditions, monitoring for fatigue, and modifying or eliminating these movement patterns and exercises entirely in persons not capable of performing them may help to modify the risk of these injuries. Participants made many positive comments about HIFT, and many choose this type of activity as their preferred training method. Larger prospective studies should be performed to evaluate injury rate and risk factors. Our data may provide a stimulus for programmatic interventions to reduce the risk of injury in HIFT participants even further. ACKNOWLEDGMENTS We acknowledge the Dan Abraham Healthy Living Center and its staff. Mayo Clinic does not endorse any products or services mentioned in this article. Abbreviations and Acronyms: DAHLC = Dan Abraham Healthy Living Center; HIFT = high-intensity functional training; HIIT = high-intensity interval training

Grant Support: Funds from the Department of Physical Medicine and Rehabilitation Small Grants Program, Mayo Clinic, were used for statistical analyses. The funding source had no involvement in any aspect of the study. This work was supported by Clinical and Translational Science Award Grant Number UL1 TR000135 from the National Center for Advancing Translational Sciences (NCATS). Its contents

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are solely the responsibility of the authors and do not necessarily represent the official views of the National Institutes of Health. Potential Competing Interests: The authors report no competing interests. Correspondence: Address to Edward R. Laskowski, MD, Department of Physical Medicine and Rehabilitation, Mayo Clinic, 200 First St SW, Rochester, MN 55905 (laskowski. [email protected]).

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