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“If you think of the adrenaline system as something that mobilizes resources when you encounter, say, a bear on your way to work,
what we found is that the system also gets you ready for your next bear encounter.”

                  Michael D. Conkright, PhD, assistant professor, Center for Diabetes and Metabolic Diseases, The Scripps Research Institute, Jupiter, Florida (News Release, June 5, 2014)

Intensity Matters—at the Molecular Level

Muscle Adaptation Correlates with Workout Intensity

It was a staggering experience, and I do mean staggering. I was walking on a busy street near our home, in the middle of a cross street, when a car came speeding in the opposite direction and, with wheels screeching, careened in my direction. I was stunned. There was no time to get out of the way.  I thought I was going to be run down. Fortunately, the car made a U-turn and sped off in the other direction. I was left trembling in the middle of the cross street, barely able to move my legs. When I got home about 20 minutes later, my legs were still shaking. It may have been the single most intense moment of my life. (This happened about 30 years ago.)

I remember telling Carol that I had experienced a major “flight or fight” response. It’s nature’s way of preparing us to survive an attack or other emergency situation. Energy stores are released to run like hell or stand and fight. It happens automatically; it’s involuntary. Your heart accelerates and your blood vessels constrict, sending your blood pressure and alertness sky high. In my case, it left me quaking in the street. My adrenaline was flooding out with no job to do. My body was primed for action that was no longer necessary.

What I didn’t understand, and scientists are only now learning, is that the same response occurs during intense exercise. It drives specific adaptations in exercised muscle. It may also explain why short, intense workouts are proving so beneficial, providing greater overall fitness, better blood sugar and weight control, and more.

As usual, it’s complicated. We’ll hit the high points and focus on the take away message. Full details were published online March 27, 2014, in The EMBO Journal. A news release from The Scripps Research Institute, dated June 5, 2014, summarized the results. Gretchen Reynolds also wrote about the study in The New York Times.

Scientists from the Florida campus of The Scripps Research Institute (TSRI), led by senior researcher Michael D. Conkright, PhD, and first researcher Nelson E. Bruno, MD, used genetically modified mice to confirm the unique molecular nature of intense exercise. The mice were modified to express far more of a protein known as CRTC2 than other mice. The protein, found in both mice and people, proved to be the molecular bridge between exercise intensity and muscular response.

The scientists were able to show that following high-intensity exercise, which enlists the sympathetic nervous system’s “fight or flight” response, CRTC2 integrates signals from two different pathways—the adrenaline pathway and the calcium pathway—to direct muscle adaptation and growth only in contracting muscle.

“The sympathetic nervous system gets turned on during intense exercise, but many had believed it wasn’t specific enough to drive adaptations in exercised muscles,” Dr. Conkright explained. “Our findings show that not only does it target those specific muscles, but it improves them—the long-term benefits correlate with the intensity of the workout.”

The difference in the response to intense exercise in the modified and normal mice was amazing. When the mice began a program of intense treadmill running, the genetically altered animals improved 103 percent, compared to an 8.5 percent improvement in normal animals. The modified mice also developed tighter, larger muscles than the other mice, and their bodies became far more efficient at releasing fat and sugar from muscles for use as fuel.

In the genetically altered animals, this resulted in a muscle size increase of approximately 14 percent. Metabolic parameters, indicating the amount of fuel available to the muscles, also increased substantially—triglycerides (muscle fat) went up 48 percent, while glycogen (muscle sugar) supplies rose by a startling 121 percent.

Dr. Conkright offered a metaphor to illustrate how stress—intensity—prepares us to meet future challenges. “If you think of the adrenaline system as something that mobilizes resources when you encounter, say, a bear on your way to work, what we found is that the system also gets you ready for your next bear encounter.” Conkright said.

Gretchen Reynolds, in her piece on the TSRI study, emphasized that intensity is personal. That’s important, because a challenge for one person may be a walk in the park for another. “The point is to get out of your body’s comfort zone,” Dr. Conkright told Reynolds, “because it does look like there are unique consequences when you do.”

Importantly, you can’t rest on your laurels. Your “fight or flight” system demands action. You must keep challenging your limits.

As indicated earlier, scientists are proving the value of brief, high effort exercise in study after study.

More Effective, More Efficient, More Enjoyable

High-intensity interval training (HIIT) refers to relatively short bursts of high-intensity activity, interspersed with periods of rest or low-intensity exercise for recovery. Many studies have compared the benefits of this type of training and moderate-intensity, continuous exercise. Several of the most recent examinations make a powerful case for high intensity exercise.

Professor Martin M. Gibala, a pioneer in the study of low volume HIIT, teamed up with Andrew M. Jones to author a paper in the Nestle Nutrition Institute Workshop Series (July 25, 2013) regarding the physiological and performance adaptation to HIIT. They concluded by suggesting an optimal balance of low- and high-intensity training.

They remind us that as little as 6 sessions of HIIT (30 seconds of “all-out” effort separated by a few minutes of recovery, repeated 4-6 times) spread over 14 days, 15 minutes of maximum effort and a total time commitment of ~2.5 hours, is “sufficient to dramatically improve exercise capacity and enhance muscle oxidation capacity.” Studies directly comparing the response to such training and much higher volume of traditional endurance training have revealed similar improvements, “despite large differences in weekly training volume and time commitment [~ 2.5 hours versus 20 hours].”

While most studies involve recreationally active individuals, Gibala and Jones refocused their attention on HIIT for highly trained individuals. While research is sparse, they say there is evidence to suggest that inserting short periods of HIIT into the already high training volume of well-trained athletes can further enhance performance. HIIT stimulus has varied from repeated intervals lasting from 5 minutes to 30 seconds of all-out effort. Bursts would likely vary depending on training goals; long distance cyclists would probably perform long bursts, while sprinters would prefer short bursts. Most athletes would probably fall somewhere in between.

“While the mechanisms responsible remain to be fully elucidated,” Gibala and Jones wrote, “it has been proposed that a polarized approach to training in which ~75% of total training volume be performed at low intensities, with 10-15% performed at very high intensities, may be the optimal training intensity distribution for elite athletes who compete in intense endurance events,” they reported. Again, it would depend on training goals. (Note that little or no time is devoted to steady-state moderate exercise.)

Considerable attention has been focused on HIIT for weight control and blood sugar stabilization. Researchers led by Lauren Skelly and Martin Gibala reported in the journal Applied Physiology, Nutrition, and Metabolism (vol. 29, 2014) that high-intensity interval exercise induces 24-hour energy expenditure similar to traditional endurance exercise despite reduced time commitment. “HIIT may represent a practical exercise option to improve body composition in healthy and clinical populations in a time-efficient manner,” they concluded.

Jonathan P. Little and colleagues compared after-meal blood sugar spikes following HIIT (10 x I min at 90% of peak heart rate with 1-min recovery) and matched work with continuous moderate-intensity (CMI) exercise. “We conclude that a single session of HIIT has greater and more lasting effects on reducing postprandial [after eating] glucose when compared with CMI,” they reported November 20, 2014, in the same journal. Improved blood glucose control after HIIT extended out to the next day, continuing for about 24 hours. That’s very good news for people with Type 2 diabetes.

These findings are giant steps forward in the search for time-efficient and effective ways to stay fit and healthy. An important bonus is that most people find interval training more interesting and enjoyable than steady state exercise.

Researchers from the School of Health and Exercise Sciences at the University of British Columbia in Canada, led Mary E. Jung and Jonathan P. Little, compared the emotional response of people to HIIT and continuous intensity exercise. Their report was published December 8, 2014, in PLoS ONE.

The report is impressive and complex: “Participants reported greater enjoyment of HIIT as compared to continuous moderate intensity exercise (CMI) and continuous vigorous exercise (CVI), with over 50% of participants reporting a preference to engage in HIIT as opposed to either CMI or CVI.” Jung et al wrote.

The researchers observed that intervals bolstered self-efficacy in a way not seen in continuous exercise. Participants liked going hard for a short time, resting, and then going hard again. Rest periods gave them confidence that they can keep going, providing a sense accomplishment not seen in continuous exercise.

The bottom line is that we are more likely to do exercise that we enjoy and find rewarding. Few people are going to keep doing exercise that bores them or saps their confidence.

High-intensity intervals are hard, but effective and time efficient. Consider sub-max intervals as an alternative that will appeal to most people; see “Sub-Max Intervals Everyone Can Use” http://www.cbass.com/SubMaxIntervals.htm  

Any exercise is better than none, but intensity matters—for fitness, health, and emotional satisfaction.

As always, check with your health care provider before making major changes in your exercise regimen.

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