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An Ongoing Series

Injuries During High-Intensity Functional Training
Systematic Review and Meta-Analysis

Joseph J. Knapik, ScD

ABSTRACT like running and rowing, resistance exercises like deadlifts
and snatches, body weight exercises like push-ups and squats,
High-intensity functional training (HIFT) involves high-vol-
ume and high-intensity physical activities with short rest pe- and gymnastic movements involving exercises on the rings and
3
riods between movements and use of multiple joint exercises. floor. HIFT may be more appropriate for military personnel
This article reports the results of a systematic review and me- because the range of activities performed by military personnel
ta-analysis of studies involving injuries during HIFT. Individ- (lifting, carrying, running, jumping, rolling, crawling, sprint-
ual studies were selected for review if they involved individuals ing, load carriage) and the fitness domains (aerobic, strength,
4,5
≥18 years of age and provided quantitative data on injury prev- mobility) required for carrying out military tasks.
alence and/or injury rates during HIFT. Twenty-eight studies CrossFit is an example of a HIFT program. A CrossFit workout
involving 11,089 participants met the inclusion criteria. There involves a warm-up, a strength or skill development exercise,
was considerable variability in individual studies with injury and then a “Workout of the Day” (WOD). The WOD varies
prevalences ranging from 12% to 74% and injury rates from each training session but typically involves a 5- to 20-minute
0.04 to 18.90 injuries/1000 h of training. Meta-analyses indi- high-intensity activities that might include weightlifting (in-
cated that the overall injury prevalence was 36% (95% con- cluding barbells, kettle bells, and other devices), gymnastics,
fidence interval [95% CI] = 32–41%) and overall injury rate running, rowing, and/or plyometrics. Workouts are often con-
4.3 injuries/1000 h (95% CI = 3.35–5.23). Injury rates among ducted in groups injecting a competitive element. WOD are
the five available prospective cohort studies was considerably available online for those who desire to train on their own, but
higher, 9.9 injuries/1000 h (95% CI = 3.3–16.4). The most CrossFit-affiliated gyms are also available across the country
commonly injured anatomical locations (with % of total inju- in which individuals can train in a group setting with other
ries) were the shoulder (26%), back/spine (26%), knee (14%), individuals of comparable fitness. In 2020 there were >15,000
wrist/hands/fingers (12%), arm/elbow (10%), and ankle/foot CrossFit-affiliated gyms across the world in an estimated 120
(6%). Given the higher injury rates among prospective studies countries. 6
that likely more effectively tracked injuries over time, more
prospectively designed studies are required before the injury There have been a number of reviews examining injuries in
7–15
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rate during HIFT can be appropriately quantified. HIFT, and these have been summarized ; however, these re-
views have limitations. First, the earlier reviews 7,9–11 and even
some later ones 12,14 included few studies either because these
3–9
Keywords: high-intensity functional training; training; injury
prevalence; injury rates; movements; physical training were all that were available at the time or because particular
studies that were available were not located by the processes
used by the reviewers. The most recent review included 21
15
retrospective cross-sectional and three prospective studies.
15
Introduction However, this review included only injuries experienced
during CrossFit training and did not consider other forms of
Previous articles in this journal investigated the risks and bene-
fits of high-intensity functional training (HIFT) and examined HIFT. Further, meta-analysis was not conducted on the avail-
1
15
reviews on injury prevalence and injury rates during HIFT. able data, and since this review was published, several new
2
HIFT can be defined as physical training that involves a variety studies have become available.
of functional movements designed to improve general physi- This article reports on a systematic review and meta-analysis
cal fitness and task performance. Exercises performed during of injuries experienced during all types of HIFT. Injury prev-
3
HIFT involve physical activities of high volume and high inten- alence, injury rates, and anatomical locations of injuries are
sity with short rest periods between movements and use of mul- examined and all studies available up to November 2021 are
tiple joint exercises. Exercise modes including aerobic activities included.
Correspondence to joseph.j.knapik.ctr@mail.mil
MAJ (Ret) Joseph J. Knapik served over 50 years with the US military as a wheel vehicle mechanic, medic, Medical Service Corps officer, con-
tractor, and Department of Defense civilian. He is currently a research physiologist at the US Army Research Institute of Environmental Medicine
and an adjunct professor at Uniformed Services University (Bethesda, MD) and Bond University (Robina, Australia).
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