February 2015

High-intensity training: assessing injury risks

shutterstock_209568979By Cary Groner

As increasing numbers of fitness enthusiasts embrace high-intensity training, reports of injuries have sparked a debate among clinicians about the benefits and risks associated with these programs.

High-intensity training (HIT) programs are attracting increasing numbers of participants who want to improve their conditioning for other activities including sports and the military, but they’ve also become a kind of sport themselves. Such programs often include a mix of aerobic, core strengthening, weight training, gymnastic, and endurance elements, typically performed at high intensity with little recovery time between exercises. The growing list of HIT programs includes CrossFit, Insanity, TRX, and Tough Mudder and, although their approaches differ in the specifics, the general philosophy is to push people—alone or in groups, and sometimes competitively—through their perceived limitations in order to reach new levels of fitness.

Some HIT participants are reporting that they’ve pushed too far too fast and become injured, however. This has sparked a debate among clinicians about the benefits and risks associated with the programs.

“On the one hand, I see health and physical benefits,” said Yasmin Dhar, MD, an orthopedic surgeon in Harrison, NY, who is also chief of sports medicine at Montefiore New Rochelle Hospital. “On the other hand, there’s a definite amount of overuse, as well as significant training and supervision variability.”

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Dhar knows a little more about such programs than your average orthopedist because she’s a fitness enthusiast herself and has participated in CrossFit.

Assessing risk is difficult because HIT is a relatively new field, having emerged in the last 15 years, and research is scarce. Moreover, studies typically have relied on retrospective reporting by patients, which is notoriously unreliable.

“If someone is involved in a certain program and has had good results from it, they may be biased to downplay injuries,” Dhar said. “Conversely, if someone didn’t have good camaraderie with their group or didn’t feel it worked as well for them, they may play up injuries.”

David Geier, MD, an orthopedic surgeon and sports medicine specialist in Charleston, SC, agreed about the methodol­ogical challenges, but with a caveat.

“Surveys are unreliable because the people who fill them out may largely be the ones who got hurt, whereas the ones who never had a problem may not bother to respond,” Geier said. “Even so, surveys may help develop a sense of what injuries people are suffering so you can take steps to minimize them.”

2-cover-shutterstock_142293781-copyThe research

Prospective research does suggest that such programs can help users improve their fitness. In one 2013 study, 43 CrossFit participants (23 men) completed 10 weeks of supervised training, after which their maximal aerobic capacity (VO2max) increased while body fat percentage declined.1 The authors concluded the program significantly improved aerobic capacity and body composition in both sexes and at all fitness levels.

A recent retrospective study by UK researchers sought reports from CrossFit participants and found that 73.5% sustained an injury during training, 7% of which required surgical intervention.2 The authors noted these injury rates were similar to those reported for sports such as Olympic weightlifting and gymnastics, and were lower than those in competitive contact sports such as rugby.

One larger—though also retrospective—study tracked 386 CrossFit participants and found an overall injury rate of 19.4%, significantly lower than that reported by the UK study above. Men were injured more often than women, but there was no difference in risk associated with age, length of participation, or training days per week. The most common injuries were shoulder, lower back, and knee, and most were relatively mild.3 Such rates are at the low end of other athletic endeavors; for example, 20% to 80% of runners are injured annually.4

The military is interested in HIT approaches for improving the fitness of soldiers but has been concerned about injuries. In 2011, the Consortium for Health and Military Performance and the American College of Sports Medicine (ACSM) published a consensus paper on HIT programs and military personnel.5 They noted that “there is an apparent disproportionate musculoskeletal injury risk from these demanding programs, particularly for novice participants,” and concluded that “certain distinctive characteristics of [the programs] appear to violate recognized accepted standards for safely and appropriately developing muscular fitness.”

That sounds definitive but didn’t really settle anything. In a 2013 paper, for example, other researchers described a military training regimen called the Advanced Tactical Athlete Conditioning (ATAC) program, which consisted of high-intensity aquatic exercises, agility circuits, core conditioning, and interval speed training; some battalions also incorporated components of CrossFit.6 And although participants experienced an increase of 12% in overall injuries and 16% in overuse injuries after six months, soldiers who didn’t participate saw rises of 14% and 10%, respectively, in the same injury categories. This left the researchers unable to recommend for or against the HIT approach.

Clinical perspectives

Speak with clinicians, however, and a few themes appear that may help guide the discussion. One is that HIT regimens vary not only between programs, but within them.

“Every kind of extreme conditioning program is different, and every single CrossFit gym is different,” said Rami Hashish, PhD, DPT, a physical therapist at UrbanMed in Los Angeles.

Practitioners are concerned about the effect of rapid repetitions of exercises, without rest, on proprioception and kinematics, Hashish said.

“Research shows that in people doing continuous barbell squatting exercises, there is a major change in kinematics with fatigue,” he said.

The paper he referred to was published last year, was conducted in 12 men and 13 women with at least six months of experience with resistance training, and involved measuring knee and hip angles throughout 55 repetitions of a barbell back squat.7 The authors concluded that “the technique changes that occur in high repetition sets do not favor optimal strength development and may increase the risk of injury.”

Hashish noted that runners are often unaware of their foot strike pattern,8 and that the same deficiencies in proprioception likely occur in HIT regimens as well.

“People doing a squat may get down to ninety degrees, but by the twelfth rep they’re getting fatigued and are trying to do it by any means necessary,” he said. “Their knees may fall in slightly, their feet may not be as externally rotated, they’re loading differently with a compensatory mechanism, their proprioception decreases, and their kinematics change.”

Hashish noted that osteoarthritis is one long-term risk of repetitive or high-impact loading, particularly if the knees tend toward valgus, but that HIT may put athletes at risk for different kinds of injuries.

“From a biomechanical perspective, something like a squat loads the glutes and the quads more than running, which tends to overload the hamstrings,” he said. “Running has a higher impact rate, but a squat has a higher total load. That load is more associated with energy absorption by the musculotendinous unit around the knee—the quadriceps or patellar tendons, or the quad muscle itself. I think there will be different types of injuries at the same joint—more like lateral meniscal pain, or to a lesser extent, medial meniscal pain or injuries.”

According to Barry Bowden, MD, an orthopedic surgeon with a subspecialty in sports medicine who practices at the Orthopedic Center in Rockville, MD, most of the HIT-associated problems he sees are overuse injuries.

“In the lower extremities we see things like patellofemoral pain and patellar tendinitis as well as Achilles tendinitis,” he said. “These are the kinds of problems you’d see with running and other sports, though I do see more shoulder injuries in HIT participants.”

Yasmin Dhar said that repetition without rest can lead to a deterioration in technique, which increases injury risk.

“One problem is that they do these exercises to [the point of] fatigue, and once people fatigue they lose their form and technique,” Dhar said. “They have to know when their body is telling them it’s too much; there’s a difference between the soreness you get from working your muscles and the pain that comes from harming your body. My feeling is that more experienced people will have fewer injuries because they find that balance and know what to do.”

According to Robert Gillanders, DPT, OCS, clinic director for Sports and Spinal Physical Therapy in Washington, DC, patients who are injured doing HIT tend to have underlying patterns of instability in their workout movements.

“Most of the time, they’re moving with such speed and intent that they’re not cognizant of those instabilities,” Gillanders said. “It’s valid to have concern about form when a person is fatigued; you can easily connect the dots in the kinetic chain toward these abnormal movements causing problems.”

Gillanders worries that if athletes proceed for months with poor techniques, small repetitive stresses will add up to bigger problems.

“It makes me wonder if these gyms are using adequate screening tools,” he said. “Are the instructors trained well enough to catch subtle instabilities? I’m all for people exercising, but you have to go slow and ramp up over months rather than weeks.”

Gillanders noted, tellingly, that instructors from one of the large HIT organizations have been among his injured patients.


When instructors are getting hurt, of course, it tends to raise red flags. Other clinicians expressed concern about the quality of instruction at HIT facilities, even while acknowledging that some instruction is better than no instruction. And indeed, some research has concluded that the presence of trainers at a CrossFit facility decreased the likelihood of injury.3

“It’s always a good idea to work with somebody to learn proper form, especially if you haven’t done these types of movements,” said David Geier. “If you do them incorrectly, you’re going to put stress where it shouldn’t go, and then you’re more likely to get hurt.”

According to Gillanders, good trainers ought to spot dynamic valgus occurring at the knees, for example, and intervene.

“Trainers should be cueing them about that,” he said. “They’re pushing them to do more, or more quickly, when clearly they’re topping out and their form is breaking down.”

Yuri Feito, PhD, MPH, an assistant professor of exercise science at Kennesaw State University in Kennesaw, GA, told LER that trainers at some HIT facilities may not be well trained themselves.

“It comes down to who is educating participants about these programs,” Feito said. “It’s easy to get a CrossFit certification and open your own gym, but that’s no different from the industry standard for personal trainers. The important part is that the instructors know what they’re talking about.”

Feito and his colleagues presented a paper at the 2014 meeting of the ACSM reporting that, of 737 CrossFit participants who responded to a survey, 373 (51%) reported some kind of injury in the previous year. However, most of those (84%) were considered minor, and almost all affected the upper body. The researchers did not correlate injury rates with working with a trainer. They did note, however, that there were three reported cases of rhabdomyolysis, a potentially fatal form of kidney failure that results from exercise so intense that it causes destruction of skeletal muscle.9 Greg Glassman, who developed the CrossFit workout, has been candid about the risk of rhabdomyolysis—there have been several documented cases—and has designed beginning classes to help avoid it that are offered by most gyms.10 Clinicians seem to consider the risk low as long as facilities provide reasonably competent supervision.

Jim Thornton, MA, ATC, is concerned about competency, however. Thornton, president of the National Athletic Trainers Association (NATA), said that CrossFit coaches may study for a few days or weeks; by comparison, NATA-certified athletic trainers have a minimum of a four-year degree. (According to the CrossFit website, several levels of certification are available, from entry-level certificates to more advanced credentials such as certified CrossFit trainer or coach. A visit to the TRX website reveals an array of training videos described as eight-hour courses.)

“An athletic trainer can reduce injury because first of all we assess whether participants are prepared to do that type of work,” Thornton said. “Second, if we start to see a breakdown in the kinetic chain, we can identify that and adjust the individual’s workout. A person who’s just been through a course is not going to have the knowledge of the kinetic chain and physiology, in general, compared to a certified athletic trainer.”

Group dynamics

Because some HIT programs emphasize a group dynamic of encouragement and competition, experts agree that another important job for coaches is management of the degree to which group members push each other.

“They create a community that helps people adhere to the program, and in some ways that works,” said Yasmin Dhar. “For example, a lot of women may not feel as comfortable going to the weight room at their gym as they are in a class with other people, where they have a sense that they belong. The risk, I feel, has to do with fatigue and with the trainer. You’re trying to push people, and as a sports physician I understand that mentality. But, in that group dynamic, some people feel they need to push because everybody else is doing it, and they may push too far. I think there has to be increased focus on maintaining proper technique throughout the workout, and not as much concern for the competitive side of it. As a physician, I always tell people that pain is your body’s way of telling you something’s not right. You should push, but you should also rest when you need to, because not everybody is going to be able to do all those exercises to the same extent.”

David Geier agreed.

“In a lot of the group programs, there is team motivation to push, and I think people get hurt trying to do too much too soon,” he said. “I’d caution them to listen to their bodies, not push through pain, vary their activities, and take rest days when they need them.”

Barry Bowden added that customization of workout regimens is important for avoiding injury.

“When you exercise as a group, and people are starting at different levels, they can’t all do the same things,” he said. “You really need to individualize the workout; overuse injuries occur when you try to increase your activity level too quickly. It’s critical to have someone with a good physical education background assess these individuals and see what they should be starting with.”


Most of the clinicians LER spoke with said that, once an athlete is injured, treatment protocols are similar to those in any sport, particularly the emphasis on evaluating the source of the problem so it doesn’t repeat itself.

“More often than not, I’m having them just do some foundational things—put them in front of a mirror and break down the movements, so they can see when there’s femoral internal rotation or adduction,” said Robert Gillanders. “The people I’ve worked with, as overzealous as their training can be, are usually really good about focusing on form once they’re banged up.”

Barry Bowden said that clinicians need to pay attention to patient age, as well.

“You have to do age-appropriate exercise,” he said. “As you move into your forties and fifties, you want to transition from high-impact exercises to more low-impact ones—elliptical training, biking, swimming, light weight training—to lower your injury risk.”

For Hashish, initial rehab is similar regardless of whether the injury resulted from HIT or another sport.

“Once you’re past that acute phase, however, the primary emphasis has to be neuromuscular reeducation to ensure the optimal movement pattern,” Hashish said.


Although a few shoe manufacturers are responding to the popularity of HIT, the field is new enough that designs are still evolving. The weightlifting aspect of the sport has affected trends, but it’s difficult to find a shoe that does everything well in such a varied regimen, and personal preferences also tend to vary.

“Weightlifting shoes are probably better than running shoes for the lifting parts of the workout, because there’s less trunk lean and greater reliance on the legs instead of loading the back,” said Rami Hashish. “I also like them because they prevent foot pronation, which helps keep the knees from falling inward and overloading the menisci.”

Robert Gillanders said that minimalist footwear may find a place in HIT training because it allows lightness and flexibility in more active parts of the workout while providing a platform similar to lifting shoes for hefting weights.

“Plyometric work—squat and scissor jumps, those kinds of things—are going to be fundamentally different in a lighter shoe that isn’t built up,” he said. “The person is also going to be lower to the ground, which provides a more stable base; you don’t want to be teetering on something when you’re lifting weights.”


Any athletic endeavor entails some possibility of injury, of course, and the particular risks associated with new forms of exercise tend to become clearer over time. The extent to which HIT programs gain popularity or fade from view in the next few years will likely have much to do with how well the programs train their staff and prepare their participants—not just to get healthy, but to stay that way.

Cary Groner is a freelance writer based in the San Francisco Bay Area.

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