Is Your Cardio Actually Making You Weaker? 7 Signs You’re Doing It Wrong

You’re grinding in the weight room, chasing new personal records, but something feels off. Your lifts are stalling, you’re constantly tired, and that hard-earned muscle seems to be getting softer.

The old gym myth that “cardio kills gains” keeps echoing in your head, and you’re starting to believe it. It’s not just a myth; doing cardio the wrong way can absolutely sabotage your progress. The real fight isn’t between you and the treadmill, but a war happening inside your muscle cells between “build” and “burn” signals.

This guide cuts through the noise. You’ll learn the seven critical warning signs that your cardio is making you weaker and get a simple, science-backed playbook to turn it into your greatest ally for getting stronger.

Why Your Body Gets Mixed Signals From Lifting and Cardio

To see why the wrong kind of cardio can hurt your strength goals, you have to look at what’s happening inside your muscles. Your muscle cells have to decide whether to build new, stronger tissue or to get better at saving energy. When you lift weights and do cardio, you’re sending two different messages.

The Construction Site Analogy: mTOR vs. AMPK

Think of your muscle cell as a construction site.

• mTOR (The Builder): Lifting heavy weights and eating protein turns on mTOR. It’s the construction foreman. Its main job is to start building bigger and stronger muscle fibers. When mTOR is active, construction is in full swing.
• AMPK (The Energy Auditor): Cardio, which uses up a lot of energy, turns on AMPK. It’s the emergency energy auditor. When AMPK shows up, it yells, “Stop all construction! We have an energy crisis.” It stops processes that use a lot of energy, like building muscle, and tells the cell to focus on saving energy.

The problem happens when the energy auditor’s alarm (AMPK) is so loud that it drowns out the foreman’s orders (mTOR). When that happens, muscle-building stops.

It’s Not Just a Theory: What the Research Says

This conflict inside your cells has real effects. That first 1980 study found that a group doing both lifting and cardio gained much less leg strength than a group that only lifted weights.

But today’s science shows that this “interference” depends on how you train. One recent study found that the negative effect on strength was small. It only happened when people did cardio and lifting in the same session, less than 20 minutes apart.

A big review of many studies also found that the type of cardio matters a lot. Running caused problems for both muscle size and strength, but cycling did not. Why? Because running causes a lot of muscle damage and requires more recovery. Cycling is much easier on your muscles.

The interference isn’t an on/off switch. It’s more like a dimmer switch. Long cardio sessions (over 75 minutes) use up your muscle’s energy stores. This creates a strong “save energy” signal that gets in the way of muscle growth. So, the science doesn’t say to avoid cardio. It says you need to be smart about how you do it.

Sign 1: Your Lifts Are Stuck or Going Backward

• What it feels like: You’ve stopped making progress in the gym. Your squat, bench press, and deadlift haven’t gone up in weeks or even months. Even worse, weights you used to handle now feel incredibly heavy, and you might have to lift less.
• The Science: This is the clearest sign you’re overtrained. Your body is breaking down more muscle than it’s building. Your nervous system is tired, so it can’t tell your muscles to lift hard.
• Action Step: Take a “deload” week. This means you cut your training volume and intensity in half for both lifting and cardio. This break gives your nervous system a chance to recover so you can start making progress again.

Sign 2: You Have “Heavy Legs” and Are Always Tired

• What it feels like: This is more than just normal muscle soreness. Your legs feel slow and heavy all the time, even on days you don’t train them. You also feel tired all day. You wake up feeling exhausted after a full night’s sleep and have no motivation to train.
• The Science: Your muscles aren’t recovering, and their energy stores are always low. Your muscles use a fuel called glycogen for heavy lifting. If you do hard cardio too close to your lifting sessions, your muscles don’t have time to refuel.
• Action Step: Put more time between your hard workouts. It’s best to separate cardio and strength sessions by at least 3-6 hours. An even better idea is to give yourself a full 24 hours between them, especially for leg workouts. Don’t do hard cardio the day before or the day of a heavy squat session.

Sign 3: Your Sleep is Awful & Your Resting Heart Rate is High

• What it feels like: Even though you’re exhausted, you can’t fall asleep. You toss and turn all night and wake up feeling like you haven’t rested at all. If you use a fitness tracker, you might see two red flags: your resting heart rate is 5-10 beats higher than normal, and your heart rate variability (HRV) is much lower.
• The Science: Too much exercise puts your body in “fight or flight” mode. This raises your stress hormones, like cortisol, which messes with your sleep. A high resting heart rate and low HRV are clear signs that your body isn’t recovering properly.
• Action Step: Make sleep your top priority. Get 7-9 hours of quality sleep each night. Go to bed and wake up at the same time every day. Avoid caffeine and alcohol before bed, and turn off screens an hour before you sleep. Use your heart rate data as a guide. If it shows you’re not recovered, swap a hard workout for a light walk.

Sign 4: You Have Nagging Aches and Get Sick All the Time

• What it feels like: Small aches in your knees, shoulders, or lower back never go away. You also find yourself catching every cold that goes around.
• The Science: Overtraining attacks your body in two ways. First, the combination of high-impact cardio like running and heavy lifting can cause overuse injuries in your joints. Second, high levels of the stress hormone cortisol weaken your immune system, making you more likely to get sick.
• Action Step: Switch to low-impact cardio. Instead of running, try cycling, rowing, or pushing a sled. These activities cause less muscle damage and are easier on your joints.

Sign 5: You’re Irritable, Anxious, and Have No Motivation

• What it feels like: Your mood is all over the place. You might feel irritable or anxious for no reason. The fun you used to get from training is gone, and now it feels like a chore. You might even feel confused or depressed.
• The Science: Overtraining affects your brain. The constant stress messes with the part of your brain that controls your stress response. This can throw off the chemicals that regulate your mood.
• Action Step: Schedule at least one full rest day each week. Do not do any intense exercise on this day. Instead, do things that help your body relax, like walking in nature, meditating, or gentle stretching.

Sign 6: Your Hormones Are a Mess (The Invisible Sign)

• What it feels like: You might not notice this without a blood test, but there can be small signs. For men, it might be a lower sex drive. For women, the biggest sign is a change in their menstrual cycle, like irregular periods or no periods at all.
• The Science: This is one of the most serious effects of overtraining. The constant stress raises your cortisol levels. High cortisol breaks down tissue, including muscle. It can also lower key muscle-building hormones like testosterone. In women, high training stress combined with not eating enough can shut down the reproductive system.
• Action Step: If you think you have a hormone issue, especially with your period, see a doctor. You also need to make sure you’re eating enough calories. Big calorie deficits are a major trigger for these problems.

Sign 7: You’re Becoming “Skinny-Fat”

• What it feels like: You’re doing hours of cardio and the number on the scale might be going down, but you look “softer” in the mirror. You’re losing muscle definition but holding onto body fat, especially around your stomach.
• The Science: This is the worst-case result of a bad training plan. It happens for a few reasons. First, the bad hormone environment (high cortisol) causes you to lose muscle. Second, high cortisol tells your body to store fat around your organs. Third, being so tired from cardio means you can’t lift hard enough to keep your muscle.
• Action Step: Make lifting weights your main focus. It’s the most important thing for keeping and building muscle. If you do cardio and weights in the same workout, always lift first. This way, you have the most energy for the workout that builds muscle.

How to Make Cardio Work for You: The 2025 Playbook

Seeing the warning signs is the first step. The next is to build a smart cardio plan that turns it from a problem into a benefit. Good cardio doesn’t hurt your strength. It builds a stronger heart, which helps you recover faster between sets and workouts. This playbook gives you a simple plan for how much, what kind, and when to do your cardio.

How Much Cardio Should You Do?

There’s no perfect amount for everyone, but science gives us a good place to start.

• For General Health: Health organizations recommend at least 150 minutes of moderate cardio or 75 minutes of intense cardio per week. For most people who lift weights, this amount won’t cause any problems.
• The Lifter’s Rule of Thumb: If your main goal is to get stronger, a good rule is to keep your total cardio time to no more than half the time you spend lifting weights. For example, if you lift for five hours a week, do two and a half hours of cardio or less.
• Make It Your Own: The right amount of cardio depends on your goals (losing fat vs. gaining muscle), how well you recover, and what you eat.

What Kind of Cardio Is Best?

The type and intensity of your cardio are the most important things for protecting your muscle gains.

• Cardio Types (From Best to Worst): Some types of cardio are much better than others.
  1. Best: Low-impact exercises that don’t cause much muscle damage are the top choice. Cycling and pushing or dragging a Sled are great options. They don’t interfere much with muscle growth.
  2. Good: Other low-impact options like the Rowing Machine, Stair Climber, and Incline Walking are also good choices that are easy on your joints.
  3. Use with Caution: Running has the biggest potential to cause problems. It’s high-impact and causes a lot of muscle damage, which means you need more time to recover.
• Cardio Intensity (Mix It Up): A good cardio plan uses different levels of intensity.
  • Zone 2 Cardio (The Foundation): This is easy, steady cardio where you can still hold a conversation. It’s usually done at 60-70% of your max heart rate. Zone 2 cardio is perfect for lifters. It builds your aerobic fitness and helps you recover without causing too much stress. Aim for 150-180 minutes of Zone 2 cardio per week.
  • HIIT (The Finisher): High-Intensity Interval Training means short bursts of all-out effort followed by rest. HIIT can be good because the workouts are short. It also uses the same type of muscle fibers as lifting. Do one or two short HIIT sessions (10-20 minutes) per week.

When Should You Do Cardio?

When you schedule your cardio is a key part of a smart plan.

• The Golden Rule: Lift First. If you have to do cardio and weights in the same workout, always lift first. This makes sure you have the most energy for building muscle. A 2023 review of studies confirmed that lifting first is better for building lower-body strength.
• The Separation Strategy: The best way to avoid interference is to do your workouts on different days or at different times. Waiting at least 6 hours between a lifting session and a cardio session can get rid of most of the negative effects. An even better plan is to do them on separate days.
• Sample Weekly Schedules:
  • Ideal (Split Sessions):
    • Day 1: Upper Body Lifting (Morning), Zone 2 Cardio (Evening)
    • Day 2: Lower Body Lifting (Morning), Rest (Evening)
  • Good (Same Session):
    • Day 1: Full Body Lifting, then 20-30 minutes of Zone 2 cardio on a bike.
  • Good (Separate Days):
    • Monday: Upper Body Lifting
    • Tuesday: 45 minutes Zone 2 Cardio
    • Wednesday: Lower Body Lifting
    • Thursday: 15 minutes HIIT
    • Friday: Full Body Lifting
    • Saturday: 45-60 minutes Zone 2 Cardio
    • Sunday: Rest

How to Eat to Build Muscle and Do Cardio

Food isn’t just fuel. It’s a tool you can use to manage the interference from cardio. Eating the right things at the right times can help you reduce the muscle-breakdown signals from cardio and boost the muscle-building signals from lifting.

Rule 1: Eat Enough Food

Trying to do a lot of lifting and cardio while on a very low-calorie diet is a recipe for muscle loss. When you don’t eat enough, your body starts breaking down muscle for energy. This makes it almost impossible to build or even keep muscle.

• Actionable Advice: If you want to build muscle, you need to eat slightly more calories than you burn. If you want to lose fat while keeping muscle, your calorie deficit should be small (about 300-500 calories below maintenance). You also need to eat a lot of protein.

Rule 2: Get Enough Protein

Protein gives your body the building blocks it needs for new muscle. Certain parts of protein also act as powerful signals.

• How Much: If you lift weights seriously, you should eat between 1.6 and 2.2 grams of protein per kilogram of your body weight each day. This amount is proven to get the best muscle-building response from training.
• The Leucine Trigger: An amino acid called leucine is the main switch for muscle growth. Eating something rich in leucine is like flipping that switch on.
• Timing: To keep your body in a muscle-building state, spread your protein out over three to five meals a day. The time right after your workout is very important. Having 20-40 grams of a fast-digesting protein like whey within an hour or two after you train sends a strong signal to your muscles to grow.
• Food Sources: Good sources of leucine include whey protein, eggs, chicken breast, lean beef, salmon, and Greek yogurt.

Rule 3: Use Carbs for Fuel

Carbs are not your enemy. They are a key tool for performance. They give you the fuel you need for hard workouts and help refill your muscle’s energy stores after you’re done.

• Before Your Workout: Eating complex carbs (like oats or brown rice) 2-3 hours before you train will make sure your energy tanks are full.
• After Your Workout: Eating fast-digesting carbs (like fruit or white rice) with your protein right after a workout is a great way to fight interference. It helps refill your energy stores, which turns off the “save energy” signal. It also causes an insulin response, which helps push nutrients into your muscle cells. A 3:1 or 4:1 ratio of carbs to protein in your post-workout meal is a good target.