In a 2012 interview for “Faces of MN,” I discussed my answer to the not-infrequent question about what I do for exercise. When I answer that I lift weights and leave it at that, I’m often met with a hesitant follow-up question: “So…what do you do for cardio?”

My answer was, “I lift weights faster.”

No knock on those who prefer traditional cardio activities, but not everyone will do them.

After that interview, my answer to that question became a viral internet meme and even a greeting card, so I decided to, well, “run” with it (cardio pun!). Because I knew I wasn’t alone, I made shirts with this motto, and from there, I built an entire online workout library around this idea. (More on that at LiftWeightsFaster.com.)

There’s no question that metabolic-resistance training, circuit training, lifting weights faster or whatever else you want to call it — where you perform fast-paced, full-body workouts involving multi-joint movements (with or without free weights) — is a brilliant form of exercise for a multitude of reasons. It helps build strength, burn fat, boost metabolism, create more mitochondria and greater capillary density, and improve heart health and hormonal profiles, to name just a few — all while preserving your existing muscle mass (and potentially even building new!). It’s also an extremely time-efficient and enjoyable way to get fit, in my opinion.

But…is metabolic-resistance training, or lifting weights faster, indeed an apples-to-apples swap when it comes to cardiovascular benefits?

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Not exactly. And it’s time to clarify the issue.

Defining Terms

Historically, research has been tough to come by on this front, as oftentimes the resistance training that occurs during research studies is not exactly your hard-n-fast kettlebell workout. “It depends what you mean by ‘lifting weights,’” says Alex Koch, PhD, associate professor of exercise science at Lenoir Rhyne University in Hickory, N.C. “Most older studies on lifting looked at training on a machine circuit or doing traditional barbell exercises alone.” Yet even in those, he continues, there was evidence of statistically significant, though small, increases in VO2 max in previously untrained subjects who lifted weights.

“Popular routines to boost cardio with weights these days tend to consist of higher reps of ballistic exercises that engage more muscle mass — think snatch and swing versus bench and curl,” says Koch. “There is less published data about these routines. Two studies I coauthored looked at the kettlebell swing, and both found that performing high-rep swings increased VO2 to a level sufficient to improve VO2 max. Neither looked at long-term adaptation to kettlebell training, though.”

More recently, as the practice becomes even more popular, research has begun stacking up in favor of metabolic conditioning having aerobic carryover. For example, a 2013 study sponsored by the American Council on Exercise (ACE) found that a group of young, relatively fit individuals who trained with kettlebells for eight weeks experienced a 13.8 percent increase in aerobic capacity, along with the strength and balance benefits you might ￼￼￼￼also expect. “Using low resistance with lots of repetitions and short rest intervals seems to stimulate cardiovascular changes,” says Gary Miller, PhD, associate professor of health and exercise science at Wake Forest University in Winston-Salem, N.C.

￼￼￼￼Results among studies are still split, but the 10,000-foot view appears to lean toward the same conclusion. Suspecting something was amiss in less-than-glowing findings regarding the cardiovascular benefits of resistance training in older studies, researchers dug through a mountain of past research to investigate, and published their conclusions in the June 2012 issue of Journal of Exercise Physiology. The short version is that — again, no surprise here — the type of lifting you do matters. “When you look at research reviews that come to the conclusion that resistance training isn’t as effective at improving cardiovascular fitness as aerobic activities, they often don’t factor in effort exerted during resistance training,” wrote the researchers. That is, if your lift is leisurely, you probably aren’t going to boost your VO2 max (how well your body processes oxygen).

In studies where participants trained to momentary muscular failure, on the other hand — poof! — physiological adaptations that enhanced cardiovascular fitness materialized. The researchers called it a misnomer to label cardiovascular or aerobic exercise by modality alone, writing, “There is the very real likelihood that the distinction between [cardiovascular exercise and resistance training] is an oversimplification where such a distinct dichotomy does not, in fact, exist.”

As is so often the case in fitness, there are shades of grey. When we look at all types of running and say “cardio” and all types of lifting and say “strength,” we miss the overlap. “Truthfully, movement is movement. Whether it’s a squat, a run, a pedal push, a deadlift, a kettlebell swing, the body doesn’t know whether it’s supposed to be doing cardio or resistance training — all it sees is a challenge and a demand,” says Alex Viada, MS, CSCS, founder of coaching company Complete Human Performance. “The modality matters less than the intensity and total volume.”

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A circuit of heavy squats, heavy deadlifts, and 25-meter sprints or 50-meter max-effort row intervals, he continues, would be a strength-focused circuit with few cardiovascular benefits, but significant strength benefits. A circuit of bodyweight squats, high-rep PVC pipe deadlifts, and half-mile jogs would be just the opposite — outstanding for conditioning, but less so for strength. Think of intensity and what the limiting factor is (whether it be muscle failure, lactate buildup, or “breathing/lungs”) when it comes to each circuit you put together, and you’ll know what you’re working, Viada advises.

Physiological Logic

The positive adaptations of metabolic-resistance workouts are similar to those of traditional strength training, with a few additional benefits.

“Any activity that requires the heart to pump more blood will strengthen the heart muscle itself, particularly the left ventricle (the portion of the heart that pumps blood out),” says Viada. “Keep in mind, this increase in left ventricle size is sometimes associated with disease states (for example, high blood pressure can also cause this), but it’s only an indicator of a disease. An enlarged heart is only a problem if the larger heart is the result of poor health!”

On the contrary, resistance training has quite the positive chain reaction on health. “The heavy usage of muscle glycogen and lactate buildup during resistance training forces the skeletal muscles to adapt by creating more mitochondria, improving the muscle’s ability to use fat for fuel. And, this increased ability to utilize fat and oxygen deficit from exercise initiates the creation of more blood vessels in and around the working muscles, improving the muscle’s ability to clear waste products and get more oxygen to the working tissue,” says Viada. “This improved blood flow and work capacity also increases the muscle’s ability to utilize glucose in the bloodstream, which for individuals who are prediabetic or consume a lot of carbohydrates is tremendously beneficial.”

Here’s where a lot of confusion stems from when it comes to circuit training and cardiovascular benefits: During resistance training or HIIT, a combination of muscle occlusion and the Valsava maneuver (holding your breath to increase intra-abdominal pressure) increases blood pressure but occludes venous return. “In other words, since everything is tensed during every repetition or short interval, no blood is flowing during muscular contraction, and less is flowing into the heart,” explains Viada. “When an individual is running or biking, this isn’t the case — venous return is actually increased.” Essentially, with weight training, though the heart rate is increasing, it’s not necessarily pumping more blood. With cardiovascular training, it is. “Since strengthening the entire heart is a major goal of aerobic conditioning, the fact that high-intensity training does not improve this to the same extent as traditional cardiovascular activity is important to note,” adds Viada. “This is why going too heavy on circuits can provide less of an aerobic benefit.”

As before, however, the type of lifting you’re doing matters greatly. “[Restricted venous return] is really most pertinent to powerlifting exercises, like heavy squats and deadlifts, where a very high intra-abdominal pressure is maintained for a fairly long time,” says Koch. “Weights can be lifted in such a way where venous return is not restricted for an extended period, and Olympic-type lifts, such as the power snatch and power clean, or other ballistic exercises, such as swings, where the lifter get to enjoy a period of unloading after a brief, high effort, should not restrict venous return nearly so much.” In other words, you may not be able to squat big weight faster for cardio, but the idea certainly applies to dynamic lifts without a long “grind” period.

Koch is not alone in his assessment. “With a low resistance and high repetitions, you do not create the same tension in the vessels [as you do with high resistance] and you get adequate venous return to the heart,” says Gary Miller, PhD, associate professor of health and exercise science at Wake Forest University in Winston-Salem, N.C.

What Lifting Weights Faster Won’t Do

“Circuit training, if implemented properly, has tremendous value in developing specific work capacity in certain movements, training individuals to perform while fatigued, building and developing ‘pacing’ ability — and yes, even eliciting several positive cardiovascular adaptations,” says Viada.

The adaptations that do not happen, however, are also important to note. “You will not notice as steady an improvement in VO2 max with higher intensity (i.e., heavier) lifting. With high-intensity training, improvements peak very quickly in most individuals, with rapid gains for six to eight weeks before the gains diminish,” says Viada. “Slower, steady-state activity or lower-intensity activity is needed to keep things progressing on that front. I think of steady-state activity as growing or building of the foundation, and higher-intensity circuits and intervals as optimizing what you’ve got.”

And, elite athletes have progressed to the point where they are so proficient in certain movements that the modality does indeed matter. “An elite cyclist may get nearly zero benefit from running, for example,” says Viada. “She is so proficient in cycling that running may simply be awkward and uncomfortable — thus discomfort and odd aches and pains will limit performance long before any aerobic threshold is attained, because her peak there is so incredibly high. “ The more elite the athlete, the more they’ll need to focus on sport movements for resistance training, and practiced movement for cardiovascular adaptations.

How to Boost Your Cardio Benefit

1) Use lightish weights. “Many individuals don’t like using appropriately submaximal weights [during circuit workouts],” says Viada. “If a load is so heavy that it interrupts the flow of a circuit, changes pacing, forces the use of the Valsava maneuver (or otherwise occludes bloodflow), the [cardio] point is being missed entirely.” That is, while the workout may have plenty of benefits in terms of building strength or strength endurance, it is less cardio-centric.

If done properly, a circuit or complex intended to develop general work capacity should start taxing the heart and lungs around the same time as the muscles, says Viada. “Ego is the enemy here; people use 75 to 80 percent of their maximum loads when they should be using 25 to 30 percent!” (He later added 40 percent as an option, too.)

“The body itself is a load — moving a weight in a complex is no different than moving the body in a run, the difference is only when individuals decide that if they’re not groaning under the weight, they’re not working their muscles,” he continues. “If individuals can reign in that tendency, circuits can be extremely useful for conditioning, developing both general and specific work capacity, and otherwise building both strength and endurance in general populations.”

2) Be dynamic. You’re looking to engage more muscle mass, and speed is the name of the game here. Ballistic movements give blood a chance to flow into your heart.

“Weights can be lifted in such a way where venous return is not restricted for an extended period, and Olympic-type lifts, such as the power snatch and power clean, or other ballistic exercises, such as swings, where the lifter get to enjoy a period of unloading after a brief, high effort, should not restrict venous return nearly so much,” says Koch. In other words, you may not be able to squat big weight faster for cardio, but the idea certainly applies to dynamic lifts without a long “grind” period.

“I believe it is possible to improve your VO2 max with high-rep, dynamic, weighted exercises like the snatch, clean, or swing,” says Koch.

3) Perform high volume. Amp up the reps! As with more traditional cardio endeavors, you need to work for a while for the aerobic energy system to really kick in.

“Workouts should consist of a high level of effort and a duration of effort that is constant, or intermittent with short rest breaks,” says Koch. “But that is not nearly as pithy as ‘lift weights faster.’”

There you have it: You can get a cardio effect by lifting weights, as long as your intensity is low, work rate is high and volume is there. (In case the huffing and puffing after you finish a set of kettlebell swings or snatches hadn’t convinced you of this already.

“For folks who just want a healthier heart, to be strong and capable, to improve their overall cholesterol levels, and otherwise feel capable of handling most anything that can be thrown at them, they don’t need a treadmill,” says Viada. “They need something they enjoy — something they can progress in and want to keep doing. Adherence, mental engagement, and avoiding stagnation matter infinitely more than any ‘optimization’ they could get from an 8.5-mile run at 72 percent of their maximum heart rate run every Sunday.”

Note: None of the sources in this article are associated with the Lift Weights Faster program.