I’ve been in the fitness industry long enough to see what workout trends stick, and I declare that high intensity workouts are in for the long run. High intensity workouts yield higher calorie expenditure by requiring participants to maintain a heart rate of roughly 80-90% of the maximum effort in sessions that are typically less than an hour. 

High intensity workouts come in different flavors. In high intensity interval training, you might hold an 80% effort for one to three minutes at a time before a period of active recovery to bring the heart rate down. In steady state high intensity training, the goal may be to hold a heart rate of 80% of the maximum for 20 to 30 consecutive minutes. The point is that high intensity training is exactly what the name implies: it’s intense.

While the combo of a time efficient, high-calorie burning workout is appealing to many gymgoers, one thing that concerns me is that many people still lack an understanding of the potential consequences of this style of training if performed incorrectly, as well as how calorie expenditure changes as you grow and adapt to high intensity workouts (I’m saving calorie burn for next month’s article). 

Participants in my classes frequently hear me refer to high intensity workouts as “Goldilocks” training. Continuously under-exert yourself, and you won’t reap the cardiovascular benefits of a high intensity workout. Over-exert yourself one or more sessions in a row, and you’re at serious risk for injury and burnout. The intensity and frequency of high intensity training must be just right to make progress using this method of training. 

Pro tip: Always err on the side of under-exertion when you start a new workout program. 

Far too often, many high intensity exercise participants adopt a “go big or go home” mentality that is fueled by a workout environment that discourages attendees from taking necessary breaks while simultaneously encouraging them to push beyond their physical limits. This mantra of fitness is especially dangerous for newbies, who in such an intense atmosphere may find themselves listening less to their body and ultimately pushing too hard. 

Nobody, including your fitness instructor, knows your body like you do. If you pace yourself well and respect your body’s boundaries, trying a high intensity workout class should not put your health at risk. Ignoring your body’s signals because an instructor is pushing you to DEFCON level 1, or because you’re bent on beating your calorie burn from your last high intensity session, is a recipe for disaster.

 

How Can a Workout Class Be So Dangerous?

On a Friday afternoon in 2018, my friend Rosemary signed up for her first spin class. She was looking forward to trying something new and was seated in the front row. The lights were dim and the music blared as she pedaled her way through intervals of high speeds for an hour. 

When the class was over, she knew something was wrong. As an experienced gymgoer, she was familiar with the feeling of “jello legs” after a hard workout, but this was different. She almost collapsed to the floor because her quads could not work properly to help her walk. 

By Saturday, her legs were red and so swollen that she could not bend her knees. By Sunday night, she was in the emergency room.

The diagnosis? Rhabdomyolysis (rhabdo for short). 

 

What is Rhabdo?

Experiencing soreness after a workout is due to a light degree of muscle cell breakdown. You may experience discomfort for a few days, but it typically dissipates as your muscles start to heal. However, over-exertion during exercise can lead to muscle cell death. When muscle cells die, they release their contents into your bloodstream. The degree of muscle cell death can be so high that the kidneys cannot filter out the cellular debris fast enough. Because kidneys are vital organs, this extra stress placed on them can lead to kidney failure and ultimately death.

  

Who Is at Risk?

While rhabdo is a rare complication of over-exertion during exercise, the consequences are life-threatening. You are more susceptible if you:

 

What Are the Symptoms?

Rosemary had all the classic symptoms of rhabdo, some showing up two days after her workout. Remember, it’s common to have muscle soreness a few days after doing new high intensity exercises. However, you should call a doctor if you develop any of the following additional symptoms: 

  • Tea-colored urine. I know it sounds gross, but this is the most concerning symptom of rhabdo. Myoglobin is the protein that gives muscle its red color, and when muscle cells die, some of that myoglobin (or a lot, in the case of rhabdo) is released into the bloodstream. This wreaks havoc on your kidneys to the point that some of that myoglobin winds up exiting the body through urination. If your urine is dark brown after a workout, do not wait another second to call a doctor. Your kidneys are in distress. 

  • Muscle swelling and tight, red skin around the affected muscles

  • Decreased urination

 

Education Rocks

The gravity of rhabdo is grossly under-recognized within the fitness community. Many instructors have not heard of the condition. 

“When I attended a new gym and was asked to give pertinent medical history, I explained that I’ve had exercise-induced rhabdo before. The staff member had no idea what I was talking about.” 

Considering that once a person has been diagnosed with rhabdo they’re more susceptible to getting it again, the lack of knowledge of this condition within fitness leaders (coupled with the popularity of high intensity training) is concerning. 

 

Chase Progress, Not Fatigue

I develop high intensity training methods for a living and I love it. But part of loving it means respecting it. High intensity training is only one training tool (of many) to help your heart and lungs become stronger, not beat you to a pulp. If you keep approaching your high intensity workouts like each one is going to be a personal best for calorie burn, you’re going to crash. I call that the “chasing fatigue” mindset, which leads to overtraining and halts progress.

 Any of us can fling our bodies around with absolutely zero control for the sake of burning calories and getting sweaty. And I get it. High intensity training can be really fun. But treat it like a rollercoaster: It feels thrilling for a while, until you start to feel sick. That’s the point at which progress stops and your risk for injury and conditions like rhabdo increase. 

If you’re a high intensity workout addict, consider the amount of time you spend per week engaging in high intensity exercise, especially if you find yourself going into workouts feeling fatigued from prior ones and/or your progress has halted. If you are convinced that high intensity training is the best and fastest way to achieve your fitness goals, remember that Olympic athletes, who represent the peak of their respective fitness specialties, do not approach every practice session with the mindset of setting a personal record. They train methodically all year long (part of this training includes deconditioning) so they can hopefully PR a few times the next season. This mindset is a great mindset for us to adopt to keep ourselves in a state of progression while minimizing risk. Chase progress, not fatigue.

 

Lee Ann Jolly, Ph.D., is the co-founder of Jolly Bodies Fitness where she leads the design and development of fitness programs based on the core concepts of creativity, education, imagination and efficiency.