Most fitness fanatics define themselves as either runners or lifters. If you categorize yourself in the latter camp, then you're probably most comfortable clanking weights, slamming through supersets, and perfecting your Olympic lifts. Cardio? It's not for you—so you think.

We talked to Joe Holder, S10 performance trainer, Nike trainer/run coach, and founder of The Ocho System, to show you how to incorporate cardio in a way that enhances your training without stripping away any of your hard-earned muscle. 

First, stop thinking of it as cardio; it's conditioning

"Conditioning is the tool to improve your cardiovascular system, and it's viable to do this without losing muscle," Holder says. 'Cardio,' for many, is uninspired elliptical workouts, long slogging runs, or tired treadmill workouts. But that's not getting muscleheads—or runners for that matter—anywhere. That's also an incomplete view. Typically, regular cardio works just your aerobic system—and not even to maximum efficiency. Conditioning, on the other hand, takes a systematic approach to priming, working, and pushing all your energy systems* which improves your overall cardiovascular fitness.

Remember: Cardiovascular fitness is the end result; conditioning is the means by which you improve it. And you can do this with sprint and weightlifting workouts, and tempo and recovery runs, so long as you're programming them correctly.  

"Actually, if you have a good enough base shape, you can gain muscle," says Holder. When you work out, your muscles are constantly being broken down and turned over, micro-tearing and growing larger (a.k.a. hypertrophy). To complement hypertrophy, you need to have proper nutrition so you're eating enough calories to promote muscle gain while keeping fat at a minimum. 

"To gain muscle with cardio, you need to stress proficiency in your anaerobic and aerobic systems," he adds. 

*So, what are these systems, what do they do, and how do you train them? Keep reading. But if you just want the quick and dirty, scroll down.

Identify your energy systems

You have three basic energy systems, all of which are key to improving your cardiovascular fitness and efficiency: 

1. Anaerobic alactic: This system provides massive energy spurts in short periods of time (about 20 seconds) to increase maximal strength, speed, and/or power. It doesn’t use up oxygen or create lactic acid; once the 20 seconds are up, however, the anaerobic lactic system kicks in.
2. Anaerobic lactic: This system provides energy for activities lasting up to a minute. It doesn’t require oxygen, but it does produce lactic acid. 
3. Aerobic: This system provides energy for longer bouts of activity by breaking down carbs, amino acids, and fatty acids. It requires oxygen and can create lactic acid depending on the intensity. 

"Every type has its purpose, but dependent upon your goals, some should be stressed more than others," Holder says. And in order to tap into and train each while maintaining the most muscle possible, you need a different approach, or stimuli, which means proper work, rest protocols, and volume, Holder says. 

The workouts

If you want to hang on to as much muscle as possible, do two workouts a week—max three. 

If you're doing excessive amounts of low-intensity cardio (3+ days a week), you're probably not engaging in any strength activities that preserve what muscle you already have and promote the growth of new muscle. You're probably also neglecting conditioning activities that improve your energy system fitness, too. You can work your aerobic energy system and still increase your muscle, but you need to work at the right intensities.

Three days worth of conditioning, on the other hand, won't deplete all your muscle mass, though 4+ days a week likely will. Your body will drop muscle with constant low-intensity exercise to optimize your size for the task at hand (a.k.a. you'll develop the stringy body of a long-distance runner).

So, if you take this three cardio workouts a week approach, two of them should tax the anaerobic alactic and lactic systems through sprints. "Utilizing sprints is basically a 'cardio' workout that can help preserve muscle mass," Holder says. Look at elite sprinters—they're jacked for a reason. "Sprint workouts will primarily work the alactic and lactic energy systems, improving your recovery, work capacity during workouts, fuel utilization, and energy production in the gym," Holder explains.

The other workout should be aerobic-focused (i.e. running, spinning, swimming, hiking, boxing/kickboxing), as a tempo or recovery pace.

Check out the example workouts below. 

Don't forget to warm up properly before each of these workouts.

Example sprint run
2 rounds: 5x20-40 meter sprints w/full recovery in between each (2 minutes) and 4 minutes rest between sets. "Effort is key here," Holder says. "Progressively increase the amount of sprints and distance you do as you move through weeks." If you can find a hill to do this on, even better. You can also do this on a stationary bike, Holder says. "A good circuit is 8-second bike sprint, 12-second recovery for 20 minutes, 3x a week; this has been proven to promote fat loss and muscle gain," he adds.

Example tempo run
3x800 meters
3x400 meters
1x1600 meters

This can be done at a track or the treadmill. Take 60-90 sec rest in between each interval. "On a scale of 1-10, the runs should feel about a 7-8."

Example recovery run
Go for a strong, brisk walk on an incline on a treadmill, a light run, or bike for 20-30 minutes. Follow with a mobility-focused workout

BONUS:
Add cardio into your strength routine by doing sprints after your warmup before you hit the weights. Try 5-second sprints with about 30-60 second recovery in between each for 5 rounds. Give yourself full recovery (4 minutes) before your start your full strength workout.