Page 114 - JSOM Winter 2021

An Ongoing Series

Injuries During High-Intensity Functional Training

Joseph J. Knapik, ScD

ABSTRACT
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High-intensity functional training (HIFT) involves high- volume squats. Rather than “extreme exercise programs,” these pro-
and high-intensity physical activities with short rest periods grams might be better described as “high intensity functional
between movements and the use of multiple-joint exercises. training” (HIFT). HIFT can be defined as “a training style (or
This paper analyzes narrative and systematic reviews covering program) that incorporates a variety of functional movements,
studies of injuries sustained during HIFT. Two narrative and performed at high-intensity (relative to an individual’s ability)
six systematic reviews on injuries during HIFT were identified. and is designed to improve parameters of general physical fit-
Seven reviews concluded that the injury incidences or injury ness (e.g., cardiorespiratory endurance, strength, body compo-
rates during HIFT were similar to those of comparable sports sition, flexibility) and performance (e.g., agility, speed, power,
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and exercise programs. The most often injured anatomic lo- strength).” Programs of this type may be more appropriate
cations were shoulders, backs, and knees. The most compre- for military personnel because of the range of activities per-
hensive and recent review involved 21 retrospective and three formed by military personnel (e.g., lifting, carrying, running,
prospective studies. In this review, mean ± standard deviation jumping, rolling, crawling, sprinting, load carriage) and the
(SD) injury prevalence was 35% ± 15%, the injury rate was fitness domains (e.g., aerobic, strength, mobility) required for
approximately 3 ± 5 injuries/1,000 hours of training, and the carrying out military tasks. 3,4
prevalence of injuries requiring surgery was 6% ± 5%. Most
injuries were associated with weightlifting exercises, especially An example of a HIFT program is CrossFit, which has been
deadlifts, snatches, clean and jerks, and overhead presses. described by founder Greg Glassman as a “constantly varied,
Other risk factors included participation time in HIFT, par- high-intensity functional movement . . . [involving] universal
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ticipation in competition, prior injuries, weekly training fre- motor-recruitment patterns.” A typical CrossFit workout in-
quency, male sex, older age, and alternating training loads. volves a warm-up, a strength- or skill-development exercise,
Although most studies included in these reviews were of lower and then a Workout of the Day (WOD). The WOD varies with
methodologic quality, current evidence suggests that injury each training session but typically involves a 5- to 20- minute
rates in HIFT are similar to those of other exercise activities. high-intensity activity containing exercises that might in-
More high-quality prospective studies are needed to fully eval- clude weightlifting (including barbells, kettle bells, and other
uate HIFT safety. devices), gymnastics, running, rowing, and/or plyometrics.
Workouts are often conducted in groups, thereby injecting a
Keywords: CrossFit; weightlifting; exercise; systematic reviews; competitive element. WODs are available online for those who
narrative reviews desire to train on their own, but CrossFit-affiliated gyms are
also available across the country. In 2020, there were >15,000
CrossFit-affiliated gyms across the world in an estimated 120
countries. 6
Introduction
In a previous edition of this journal, an article examined the In 2015, when the previous article on the risks and benefits
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risks and benefits of extreme conditioning programs. Programs of HIFT was published in this journal, there were few studies

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of this type usually involve physical activities of high volume that had examined the safety of these programs. In fact, there
and high intensity with short rest periods between movements were only two observational studies and nine case studies,
and the use of multiple-joint exercises. These programs use the latter involving exertional rhabdomyolysis, carotid artery
a variety of exercise modes, including aerobic activities such dissections, and retinal detachment. The article concluded
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as running and rowing, resistance exercises such as deadlifts that more studies were need before the safety of HIFT could
and snatches, and body-weight exercises such as push-ups and be determined and general guidelines for the safe execution
Correspondence to joseph.j.knapik.civ@mail.mil
MAJ (Ret) Knapik served over 50 years with the US military as a wheel vehicle mechanic, medic, Medical Service Corps officer, contractor, and
Department of Defense civilian. He is currently a research physiologist at the US Army Research Institute of Environmental Medicine (Natick,
MA) and an adjunct professor at Uniformed Services University (Bethesda, MD) and Bond University (Robina, Australia).

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