Research Roundup #38: High-Intensity Exercise and Heart Disease, Capsaicinoids and Performance, and Stretching for Hypertrophy

It’s estimated that there are over 2+ million scientific papers published each year, and this firehose only seems to intensify.

Even if you narrow your focus to fitness research, it would take several lifetimes to unravel the hairball of studies on nutrition, training, supplementation, and related fields.

This is why my team and I spend thousands of hours each year dissecting and describing scientific studies in articles, podcasts, and books and using the results to formulate our 100% all-natural sports supplements and inform our coaching services. 

And while the principles of proper eating and exercising are simple and somewhat immutable, reviewing new research can reinforce or reshape how we eat, train, and live for the better. 

Thus, each week, I’m going to share three scientific studies on diet, exercise, supplementation, mindset, and lifestyle that will help you gain muscle and strength, lose fat, perform and feel better, live longer, and get and stay healthier. 

This week, you’ll learn if high-intensity exercise causes heart disease, whether capsaicinoids boost strength and endurance, and if stretching causes muscle growth. 

Table of Contents

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• High-intensity exercise probably doesn’t cause heart disease. 
• Capsaicinoids boost strength endurance and lower RPE.
• Stretching alone can cause muscle growth.

High-intensity exercise probably doesn’t cause heart disease. 

Source: “Exercise Volume Versus Intensity and the Progression of Coronary Atherosclerosis in Middle-Aged and Older Athletes: Findings From the MARC-2 Study” published on Jan 4, 2023 in Circulation.

Several recent studies have made headlines after suggesting that perceived healthful behaviors are actually harmful.

For example, one study conducted by scientists at Radboud University Medical Center investigated how exercise intensity affects coronary artery calcification (the buildup of calcium in the arteries, regrettably referred to as CAC).

CAC levels are a good barometer of cardiovascular health. Thus, unraveling how exercise intensity affects CAC will help us understand which types of exercise may minimize your risk of cardiovascular issues in the future. 

The researchers measured 289 middle-aged and older men’s CAC levels between 2012 and 2014, then again ~6 years later between 2019 and 2020.

At the same time points, the researchers collected data about the men’s exercise habits using a questionnaire, including information about the types of exercise the men did and how much and intensely they exercised.

The researchers then tried to identify patterns between the types of exercise the men did and how their CAC levels changed across the study duration.

The results showed that men who regularly exercised at a very-high intensity had higher CAC levels than those who exercised at moderate-to-high intensities.

This led many news outlets and social media health “gurus” to conclude that very-high-intensity exercise increases your risk of heart disease. And while contrarian conclusions like this might earn you views and shares, it’s not an accurate interpretation of the results for two reasons.

First, this study was an observational study, without any “control” or “intervention.” 

The researchers didn’t recruit a group of people with similar CAC levels and have some do no exercise (the control group) while others did low-, moderate-, or high-intensity exercise (the intervention groups), then analyze how each type of exercise affected CAC levels compared to those who remained sedentary. 

Instead, the researchers measured men’s CAC levels and exercise habits at two points and tried to spot links between the amount and type of exercise the men did and their CAC levels.

In other words, it only showed that very-high-intensity exercise and increased CAC levels correlate, not that very-high-intensity exercise increases CAC or heart disease risk.

Second, this study didn’t look at the number of men who got heart disease; it only looked at the prevalence of a mechanism that might contribute to heart disease (high CAC levels).

While this tells us something about the relationship between exercise and cardiovascular health, there are better ways to understand how exercise intensity affects heart disease risk. 

For instance, you could look at studies that actually investigate how exercise intensity affects heart disease risk. And when you do, you find that most evidence shows that higher-intensity exercise (like weightlifting) is more beneficial to your cardiovascular health than lower-intensity exercise (like walking).

In studies that measure how high-to-very-high-intensity exercise affects heart disease risk (not just a marker of cardiovascular health that may or may not develop into heart disease, such as CAC), results show that higher-intensity exercise is as good or better for heart health than lower-intensity exercise.

Thus, there’s no good reason to avoid high-intensity exercise. It’s a perfectly healthy way to train that offers equal or even superior benefits in terms of heart health to low-intensity exercise.

You may, however, want to avoid articles in the lay press with titillating health-related headlines. Remember: they’re written to draw eyeballs and clicks and bounce around on social media, but they’re rarely scientifically sound or worthy of your attention.  

TL;DR: High-intensity exercise doesn’t cause heart disease, regardless of what [insert corporate media organization] says. 

Capsaicinoids boost strength endurance and lower RPE.

Source: “Effects of Capsaicin and Capsiate on Endurance Performance: A Meta-Analysis” published on October 28, 2022 in Nutrients.

Most people know capsaicinoids as the compounds that give chili peppers their spice.

Recently, they’ve also caught the attention of scientists as a way to boost athletic performance, with some studies showing they boost strength and strength endurance and help you do more volume in your weightlifting workouts. 

Many experts remain skeptical about their effectiveness, though. This is because it’s difficult to draw firm conclusions about a molecule from isolated studies. 

The results of a recent meta-analysis conducted by scientists at Victoria University adds a few missing pieces to this puzzle.. 

They aimed to investigate the effects of capsaicin and capsiate (two types of capsaicinoid) on aerobic endurance (e.g., time trials or time-to-exhaustion tests), muscular endurance (e.g., how many reps you can do before reaching failure), and rating of perceived exertion (RPE) by parsing the results of 14 studies. 

The results showed that capsaicinoids had little effect on aerobic endurance performance and tended not to make endurance exercise feel easier. 

Capsaicin and capsiate did significantly improve muscular endurance over three sets of exercises such as the bench press, squat, and leg extension. People who took capsaicinoids before weightlifting also reported finding their workouts significantly easier.

We don’t need to get into effect sizes (the measurement used in statistics to represent the magnitude of an observed change), but according to studies on similar performance-enhancing supplements, capsaicinoids seem to boost muscle endurance about as much as caffeine—which is not to be sniffed at. 

Why aren’t more weightlifters taking capsaicinoids, then?

A big reason is that supplement companies haven’t yet caught on. This is the opposite of what typically happens—supplement sellers are usually grasping at any molecule that seems vaguely promising so they can peddle it as a “new, natural, safe steroid.”

That said, if studies continue to pile up, I doubt it’ll be long before the market is awash with capsaicinoid supplements. And on current evidence, using one might be worthwhile.  

(If you feel confused about what supplements you should take to reach your health and fitness goals, take the Legion Supplement Finder Quiz, and in less than a minute, you’ll know exactly what supplements are right for you. Click here to check it out.)

TL;DR: Capsaicinoids likely boost strength endurance and make weightlifting workouts feel easier but don’t improve aerobic endurance performance.

Stretching alone can cause muscle growth.

Source: “Influence of Long-Lasting Static Stretching on Maximal Strength, Muscle Thickness and Flexibility” published on May 25, 2022 in Frontiers in Physiology.

“Stretching doesn’t reduce injury risk and it makes you weak.”

This has been most evidence-based fitness folks’ stance on stretching in recent years.

And while these points are correct in certain circumstances, they’re not always right.

For example, research shows that intense stretching immediately before training hinders muscle growth, but light stretching during rest periods between sets may actually increase muscle gain.

Likewise, studies show that long, deep stretching before a workout stymies strength and power, but stretching away from training may boost strength over time. 

The recent excitement surrounding stretch-mediated hypertrophy (the idea that training your muscle in a stretched position has unique muscle-building benefits) has renewed the evidence-based fitness fraternity’s interest in stretching. 

Specifically, many wonder whether stretching alone can cause muscle growth. 

To test this concept, researchers at Leuphana University split 52 people into 2 groups: a stretching group and a non-stretching group.

Those in the stretching group used an adjustable foot brace to place one of their feet into dorsiflexion (raising their toes toward their shin) for an hour daily, adjusting the mechanism until they felt a stretch in their calf that they rated as an 8 out of 10 for discomfort (this machine didn’t involve any resistance training—just stretching). 

As their ankle mobility improved on their stretched leg, the people tightened the mechanism so that the stretch always felt like an 8 out of 10 on the pain scale. 

After 6 weeks, the results showed that calf isometric strength (the strength of the calf while pressing as hard as possible against an immovable object) increased by ~17% in the stretched legs. In contrast, calf isometric strength in the non-stretching group decreased by ~1.5%. 

The leg that the stretching group didn’t stretch gained a little strength, too (+1.4%). This is likely due to the cross-education phenomenon, a bizarre neurophysiological process where an increase in strength in one limb helps to increase the strength of the other, even when you don’t train it.

The results for dynamic exercises (exercises that involve lifting and lowering a weight) were similar, too: calf raise one-rep max increased by ~25% in the stretched leg and ~11% in the unstretched leg of the stretching group and declined by up to ~4% in the non-stretching group. 

Range of motion also increased significantly more in the stretched legs than the unstretched legs (+13.2% vs. -0.8-to-2.4%).

The results regarding muscle growth were the most important., however.

Gastrocnemius thickness increased by ~15% in the stretched leg and ~2% in the unstretched leg of the stretching group. The researchers didn’t measure muscle growth in the non-stretching group. Still, we can assume there would’ve been little difference compared to the beginning of the study since they didn’t train.

Thus, this study demonstrates that stretching alone can cause muscle growth and strength gain, provided the stretching is long and deep enough.

Interesting stuff . . . but how can it help us get more jacked?

I wouldn’t recommend trying to replicate the exact methods of this study—sitting for an hour daily in considerable discomfort. That’s not what the study is for, though. This study is simply a “proof of concept;” it shows that stretching alone can help you gain muscle and strength under the right conditions.

Based on these results, scientists will investigate how much and hard we need to stretch to increase hypertrophy and strength and what mechanisms are at play. There certainly seems to be a connection between stretching and muscle growth.

That said, one thing we don’t know is whether or not you can get similar benefits with less aggressive protocols, or possibly superior benefits by altering your technique during various weightlifting exercises. Time will tell.

TL;DR: People who deeply stretched their calf muscles for an hour per day for 6 weeks increased their calf size and strength.