Strength Training Vs. Cardio: What Happens When You Do Cardio and Don’t Strength Train?

Strength Training Vs. Cardio: What Happens When You Do Cardio and Don’t Strength Train?

Some people think a good workout is a jog on the treadmill or a spin on an exercise bike. Although committed to exercising, they do the same workout day in and day out with little variation, never picking up a weight or a pair of resistance bands. Ever known someone like that? You don’t see them change very much over time.

When you ask them why they don’t strength train, they’ll tell you they want to lose weight not “bulk up.” Problem is they don’t do either. Oh, these well-meaning individuals may lose a few pounds in the beginning, but ultimately their body adapts to the repetitive nature of doing the same workout and they reach a frustrating weight-loss plateau. They end up “stuck” – not to mention bored with their routine.

You have to give these individuals credit for doing something. At least, they’re moving their body and getting some cardiovascular benefits. What they aren’t doing is MAXIMIZING the benefits they get from their workout time. Neither are they doing a balanced workout routine. A balanced routine is one that targets all components of physical fitness including strength, flexibility, and balance as well as endurance. One of the most important parts of any fitness program is strength training – and that’s the part some women de-emphasize.

Strength training helps preserve lean body mass and prevent sarcopenia or age-related loss of muscle tissue. Workouts that involve resistance become MORE important with age. Muscle strength and good balance skills are what keep us functional and able to do the things we enjoying. Just as importantly, strength training helps preserve bone mass and prevent osteoporosis, a health concern for all women and some men too. So what happens if you do aerobic exercise and no strength training?

Cardio without Strength Training, Cortisol and the Catabolic State

When you do aerobic exercise for a sustained period of time, an hour or more, your cortisol level slowly begins to rise. This rise is more substantial when you’re in a fuel-depleted state. That’s one of the drawbacks to fasted cardio.

What’s so bad about cortisol? It’s cortisol’s job to ensure you have enough glucose in your bloodstream to fuel your muscles and prevent a drop in blood sugar. When you’re under-fueled and running low on muscle glycogen and glucose, cortisol signals your body to use protein as an alternative fuel source during exercise. Amino acids from protein can be converted to glucose by your liver through a process called gluconeogenesis. Amino acids from protein serve as a “backup” source of fuel during exercise.

As a result, aerobic exercise, especially when you’re not eating enough carbs, can lead to muscle breakdown. You lose a certain amount of muscle mass as you age and aerobic exercise without strength training can aggravate this problem. Heavy or prolonged strength training also stimulates cortisol release but to a lesser degree, especially if you eat a snack and are well fueled beforehand.

There’s another difference. Strength training increases the production of testosterone and growth hormone. Growth hormone and testosterone help counteract the negative effects cortisol has on muscle breakdown. In addition, strength training activates anabolic pathways, including one called mTOR that’s important for muscle growth. In contrast, long periods of cardio have a catabolic effect by blocking these pathways.

Is High-Intensity Cardio Better?

High-intensity interval training, because it’s shorter, doesn’t activate cortisol to the same degree as long periods of moderate-intensity cardio. Research shows endurance athletes, including long-distance runners, have higher levels of cortisol, as measured by the amount in their hair, compared to the average person. Having high cortisol has other health implications as well.

High cortisol levels are associated with fat redistribution, movement of fat from the hips, thighs, and buttocks to the abdominal region. Here comes the belly fat! Plus, cortisol suppresses your body’s immune response and puts you at greater risk for colds and viral infections. When your cortisol level is high you may experience sleep problems and early morning awakenings. If your adrenal gland continues to pump out cortisol, you can experience symptoms of adrenal exhaustion like fatigue and increased susceptibility to infection.

Like strength training, high-intensity exercise activates anabolic hormones like growth hormone and testosterone that help preserve lean body mass and counteract the effects of cortisol. Other ways to counteract the effects of cortisol are to:

.   Get enough sleep

.   Avoid overtraining

.   Don’t restrict calories excessively

.   Refuel with a carbohydrate/protein snack within 30 minutes after a workout

.   Spend at least one day a week resting or doing a light workout like yoga or stretching

To preserve lean body mass, it’s important to minimize the effects of cortisol. People who spend hours doing steady-state cardio, especially if they’re restricting calories to lose weight, end up in a catabolic state and lose muscle mass.

Steady-State Cardio Alone Isn’t Very Effective for Weight Loss

One reason people focus so much on cardio is the misconception that it alone will help them lose weight. Is there truth to this? A 2011 review looked at 14 trials relating to aerobic exercise and weight loss. They showed aerobic exercise programs lasting 6 to 12 months result in only modest weight loss – on average around 4 pounds. Although participants in these studies lost some waist circumference, aerobic exercise didn’t significantly change the composition of their bodies. That’s precisely where strength training has benefits. Even if you lose weight doing steady-state cardio, you won’t necessarily be firmer and you won’t be stronger.

 Strike a Balance

Don’t turn your workouts into a “cardio fest.” To balance things out, devote as much or MORE time to strength training to preserve muscle mass and improve your body composition. Everyone has to battle the age-related decline in muscle mass at some point. Strength training is the best way to do that.



Am J Med. 2011 Aug;124(8):747-55. doi: 10.1016/j.amjmed.2011.02.037.

Nutr Metab. 2012;9(83)

J Endocrinol Invest. 2008 Jul;31(7):587-91.

Christiane Northrup M.D. “Adrenal Exhaustion”


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