Research Roundup #33: Cardio and Muscle Growth, Post-workout Baths, and Intermittent Fasting

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 cardio boosts muscle growth, whether post-workout baths enhance muscle growth and athletic performance, and if intermittent fasting hinders hypertrophy. 

Table of Contents

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• Cardio may boost muscle growth.
• Hot and cold post-workout baths don’t enhance muscle growth or athletic performance.
• Fasting probably doesn’t hinder your ability to build muscle. 

Cardio may boost muscle growth.

Source: “Short-term aerobic conditioning prior to resistance training augments muscle hypertrophy and satellite cell content in healthy young men and women” published on August 16, 2022 in The FASEB Journal.

Many weightlifters think cardio drains your gains.

That is, they think running, swimming, cycling, and so forth interfere with your ability to build muscle and get stronger.

Research has repeatedly shot holes in this shibboleth (when done properly, cardio doesn’t interfere with your gains), and according to this study conducted by scientists at McMaster University, cardio may even enhance muscle growth.

For 6 weeks, the researchers had 14 active young people do three 45-minute moderate-intensity cycling workouts per week using one of their legs. For the following 10 weeks, the participants completed a lower-body strength training program that included only bilateral exercises (exercises that trained both legs simultaneously), such as the squat, leg press, leg extension, leg curl, and calf raise.

The results showed that legs trained with cardio before the weightlifting program increased capillary density, type I and type II muscle fiber size, and satellite cell and myonuclear content significantly more than the legs that only got resistance training.

(For reference, satellite cells help repair damaged muscle fibers, and myonuclei carry the DNA that constructs new muscle proteins.)

The researchers also analyzed the 10 highest- and lowest-responding legs and found the following:

1. Muscle grew most in the legs that had the largest changes in satellite cell content and grew least in the legs that had the smallest changes in satellite cell content.
2. The legs that grew most had greater capillary density than those that grew the least.
3. The legs with the highest capillary density before weightlifting grew more than those with the lowest capillary density before weightlifting.
4. The legs that grew most increased satellite cell content more than the legs that grew least.

In other words, the results showed that doing cardio and weightlifting helped people build more muscle than weightlifting alone. 

While it’s not clear why this is the case, the authors of this study (and other similar research) think it’s most likely because cardio increases capillary density, which appears to spur muscle growth by improving how satellite cells and myonuclei respond to weightlifting. 

That said, other research shows that doing cardio and weightlifting produces more muscle growth than weightlifting alone, even when it doesn’t drastically increase capillary density, which suggests we need to learn more about how capillary density affects muscle growth before we can confidently comment on the mechanisms at play. 

Nonetheless, adding cardio to your weightlifting routine seems to benefit muscle growth. As such, if you want to include cardio and weightlifting in your routine, here’s the blueprint I recommend in my fitness books for men and women, Bigger Leaner Stronger and Thinner Leaner Stronger:

• Prioritize low-impact forms of cardio, such as cycling, rowing, skiing, and rucking.
• Do 2-to-3 low-to-moderate-intensity cardio workouts per week of 20-to-60 minutes each.
• Don’t do HIIT unless you enjoy it, and limit yourself to one HIIT workout per week (it’s no better than moderate-intensity cardio and harder to recover from).
• Don’t do more than 2-to-3 hours of cardio per week.
• Do your cardio and weightlifting on separate days if possible, and if you have to do them on the same day, lift weights first and try to separate the workouts by at least 6 hours.

And if you want a more in-depth explanation as to why this approach works, check out this article:

Concurrent Training: The Right Way to Combine Cardio and Strength Training

TL;DR: Adding cardio to your weightlifting routine may help you gain more muscle (when done properly).

Hot and cold post-workout baths don’t enhance muscle growth or athletic performance.

Source: “No effect of repeated post-resistance exercise cold or hot water immersion on in-season body composition and performance responses in academy rugby players: a randomised controlled cross-over design” published on October 25, 2022 in European Journal of Applied Physiology.

Post workout hot and cold baths have become tony among fitness tastemakers, largely because many pro athletes claim they boost recovery.

However, most studies show that bathing post training does little to help in this regard.

That said, these studies usually don’t involve high-level athletes. Thus, the reason the results are underwhelming might be that these methods only shine when someone with years of training experience employs them.

To test this theory, scientists at the Australian Institute of Sport had 31 professional (and thus experienced) rugby players follow 3 separate recovery protocols, each lasting 4 weeks.

(While using high-level athletes as participants for this study was a strength, a glaring weakness was that the researchers didn’t also recruit a group of recreational athletes to whom they could’ve compared the pros.)

In the first, the players followed their 2 weekly weightlifting workouts with a 15-minute cold bath (~59 degrees). In the second, they followed their weight training with a 15-minute hot bath (~102 degrees). And in the third, they followed their weight training with 15 minutes of static stretching. 

The results showed that none of the protocols increased muscle growth or fat loss and that stretching and having a cold bath negligibly improved jump performance more than taking a hot bath.

Just like previous studies on amateur athletes,  “thermal therapies” didn’t do much of anything for these pro athletes, either. 

Thus, even if you’re an “advanced” weightlifter (training for 5+ years), hot and cold post-workout baths don’t offer much of an ROI. Likewise, if you’re just getting started with weightlifting, there’s no need to take a hot or cold plunge after your workouts. 

Instead, stick to more proven recovery strategies, such as getting more sleep (or napping when sleeping 7-to-9 hours at night isn’t possible) and using science-backed supplements, such as creatine.

And if you want a recovery supplement that contains a clinically effective dose of micronized creatine monohydrate and two other ingredients designed to boost recovery and minimize soreness, try Recharge.

(Or if you aren’t sure if Recharge is right for you or if another supplement might be a better fit for your budget, circumstances, and goals, then take the Legion Supplement Finder Quiz! In less than a minute, it’ll tell you exactly what supplements are right for you. Click here to check it out.)

TL;DR: Taking a hot or cold bath after your workout probably won’t boost recovery, muscle growth, or performance.

Fasting probably doesn’t hinder your ability to build muscle. 

Source: “Eight-hour time-restricted eating does not lower daily myofibrillar protein synthesis rates: A randomized control trial” published on December 22, 2022 in Obesity (Silver Spring). 

One common knock against intermittent fasting is that it may reduce muscle growth.

The reason people believe this is simple: Intermittent fasting involves forgoing food for 16 hours per day (depending on which protocol you follow), during which time muscle protein synthesis (MPS) rates sag. This means that someone who follows an intermittent fasting diet spends much of the day not building muscle.

In contrast, someone who follows a regular diet can keep MPS rates elevated by consuming protein every few hours and therefore keep their muscle-building machinery purring throughout the day.

The theory seems solid, but does it play out in practice?

To investigate, researchers at Australian Catholic University split 18 overweight or obese men into 2 groups: a regular-diet group and an intermittent-fasting group.

For 10-days, the regular-diet group ate breakfast at 8 a.m., lunch at 2 p.m., and dinner at 8 p.m. (12-hour feeding window), and the intermittent-fasting group ate breakfast at 10 a.m., lunch at 2 p.m., and dinner at 6 p.m. (8-hour feeding window).

Otherwise, the diets were the same: everyone ate in a calorie deficit and got 56% of their daily calories from carbs, 30% from fat, and 14% from protein.

The results showed that MPS rates were similar for both groups throughout the entire study.

However, the researchers also found that while both the intermittent-fasting and regular-diet groups lost about the same amount of total weight (~3.5 lb and ~2.4 lb, respectively), the intermittent-fasting group lost significantly more muscle (~2.2 lb vs. ~0.4 lb).

Which sounds like a big rusty nail in the coffin of intermittent fasting for muscle growth. 

Keep in mind, though, that none of these people were lifting weights or eating an optimal amount of protein for preserving muscle mass, and the absolute amount of muscle lost was fairly small (a few pounds). 

What I think is more interesting is that there was no difference between muscle protein synthesis between the two groups, which suggests—very tentatively—that intermittent fasting probably wouldn’t hinder muscle growth long-term. 

This theory also aligns with other research showing that intermittent fasting combined with strength training and adequate protein doesn’t hurt muscle retention while dieting (slightly different than muscle growth, but the mechanisms involved are similar). 

(And if reading this has got you wondering whether intermittent fasting is the right diet for your circumstances and goals, then take the Legion Diet Quiz! In less than a minute, you’ll know exactly what diet is right for you. Click here to check it out.)

TL;DR: Muscle protein synthesis rates are similar whether you eat over an 8- or 12-hour period each day, so intermittent fasting-style dieting probably won’t hinder your ability to build muscle.