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Is Too Much Cardio Killing Your Muscle Gains?

Here are four telltale signs you are losing muscle and tips on how you can rebuild.

Man sitting on treadmill with palm to his forehead.

You’re working out, doing cardio several times a week, and eating in a calorie deficit. And while you started losing weight, you’ve now plateaued, and you’re also feeling weak and low-energy. These may be signs that you are losing muscle. 

Tonal coach and certified personal trainer Joe Rodonis sees this all the time with clients who go heavy on cardio and dieting while neglecting their strength training. While they might lose weight, a substantial portion of that could be coming from muscle. 

“You’re tired all the time, and you’re fatigued,” he says. “That’s because you’re not fueling your body, you’re just running it into the ground.” 

Make sure you don’t fall into this trap by learning the signs you are losing muscle, and how to safely lose fat while maintaining your muscle mass.

4 Signs You’re Losing Muscle

1. Shifting Body Composition

While the number on the scale can tell you if your overall weight is going up or down, it can’t tell you if you’re losing muscle or fat. To do that, you’ll need to assess your body composition—the ratio of lean body mass (which includes muscles, bones, and organs) to fat mass.

“If you are trying to lose weight, you want it to come from the body fat percentage bucket, rather than the lean muscle mass bucket,” says Christian Hartford, Senior Performance Manager of Applied Sports Science at Tonal. 

There are a few different ways to measure your body composition including DEXA (dual x-ray absorptiometry) scans, body-fat scales, skinfold caliper testing, and even mobile apps that use your phone’s camera. While these methods have varying accuracy rates, if you regularly use the same form of measurement and notice your body fat is going up and your lean mass is going down, you’re likely losing muscle.

2. Decreased Performance 

Your lifts may also suffer and start to plateau if you’re losing muscle, explains Rodonis. 

“If you’re lifting, and you’re not making any weight changes or the weights are going down, that’s usually a pretty obvious [sign],” he says. While other factors, such as a bad night’s sleep or an oncoming cold could leave you feeling weak in one workout, if this trend continues over several weeks it could be a sign you’re losing muscle. 

3. Loss of Muscle Definition

Another telltale sign: Your shape starts to soften where you used to feel chiseled. Hartford says you might notice less muscle definition visually if you’re losing muscle, even if your weight is the same or going down. This side effect may even impact how your clothes fit.

4. Sweat That Smells

Here’s a pretty common sign you might not have heard of: After working out, you notice your sweat has a strong ammonia odor. When your body doesn’t have enough carbohydrates or fat to burn for fuel, it’ll resort to creating energy by breaking down protein. Protein breaks down into amino acids and ammonia is a byproduct of this process. The excess ammonia is then excreted through sweat. That smell is a sign you’re not taking in enough carbs to power your workouts, and your body has to break down muscle instead.

So What Happens When You Start Losing Muscle?

Maintaining muscle isn’t just about aesthetics. When you lose muscle, you also increase your risk of injury and illness and miss out on the health benefits of a muscular build. 

“Having more muscle mass and more muscular strength is associated with a better quality of life,” says Hartford. In a meta-analysis that included nearly 2 million participants, higher levels of muscular strength were associated with a lower risk of death from all causes. Another meta-analysis showed that increasing strength through resistance training was linked to injury reduction. When you lose muscle, you lose out on these mobility and joint-strengthening benefits. 

Maintaining muscle mass is also important because it improves bone density, which is especially critical for women who are at increased risk of osteoporosis as they age. Sarcopenia, or age-related muscle loss, is closely linked to reduced bone density and a higher rate of bone fractures. 

Even if weight loss is your primary goal, you’ll be more successful if you build or maintain muscle mass. A six-year study with more than 11,000 participants found that regular resistance training reduced rates of obesity, and a review of 58 studies concluded that resistance training reduced body fat mass and body fat percentage. Lean muscle is also a major calorie burner—the more lean muscle mass you have, the more calories you’ll burn at rest. Lose muscle, and you’ll become less efficient at burning calories, possibly increasing your body fat.

5 Steps to Stop Muscle Loss 

If you’re noticing any of the signs you are losing muscle or not making strength gains and want to reverse course, you’ll need to reevaluate your diet and your training. Try these steps to start rebuilding:

1. Up Your Protein Intake

Protein is always the priority. No matter what you’re doing in your training, you need to hit your protein goals,” Rodonis says. Protein is required for muscle growth and tissue repair. If you’re not getting enough, your muscles won’t have the foundational building blocks they need to grow. 

“When people cut calories and they’re just trying to lose weight, a lot of times, protein is the first thing to go,” says Hartford. As a starting point, he recommends eating 0.8 to 1 gram of protein per kilogram of body weight, which is aligned with the recommended daily allowance (RDA) for adults. A meta-analysis of studies found that 1.6 grams per kilogram may be even more beneficial if you’re training regularly and trying to put on muscle. 

2. Choose Quality Calories 

If you’re still trying to lose weight, you can eat in a calorie deficit as long as you’re getting enough protein. You’ll also need adequate carbohydrates to fuel your training. To keep your calories in check while getting enough protein and carbs, Rodonis recommends cutting out less nutritious calorie sources such as sugary drinks and fried foods. You can eat in a small calorie surplus if you’re prioritizing muscle growth, but make sure your diet is still full of nutrient-rich foods. 

3. Add Resistance Training

Once you’re taking in enough protein to fuel your muscles, it’s time to put them to work in your training. “You have to keep resistance training in your routine,” says Hartford. “You have to give some stimulus to those muscles.” 

To shift your body composition and gain lean muscle, Hartford suggests a minimum of two days of resistance training per week with a focus on hypertrophy—lifting 70 to 80 percent of your one-rep max in each exercise for 6 to 12 reps. 

Tonal Coach Joe Rodonis resistance training on Tonal

4. Try Supersets 

Additionally, Rodonis recommends adding supersets to your workouts, alternating between upper- and lower-body exercises with very little rest “You’re never really resting, but you’re letting the muscles rest in between,” he says. “The rest periods are low, but you’re still able to build muscle because you’re allowing that body part to rest while you work another.” 

These types of workouts develop lean muscle, which increases your metabolic rate so your body burns more fat and calories even at rest. 

5. Keep Up With Some Cardio

This isn’t to say you should give up cardio entirely. A balanced exercise routine includes both cardio and resistance training, as cardio benefits your heart and lungs. Hartford explains that adding strength training to your routine can actually boost your performance in activities such as running and cycling, and make you more resilient against injuries by strengthening your joints and connective tissue. Even light to moderate cardio like power walking can be beneficial on a quest to shift body composition without losing muscle.

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