For optimal fitness, you know you should do both cardio workouts for aerobic health and strength training for muscle building, metabolic benefits, and bone density. But if you're a cardio bunny who maybe doesn't visit the weight room as often as you should, listen up: "Swimming, cycling, and running can all be used as mild to moderate ways of building strength,” says Lauren Jensen, head coach at Tri Faster, “as long as the amount of training stress is progressively increased over time.” In other words, as with weights, you have to increase the resistance or the number of reps as your muscles adapt.

While the cardio-strength combo suggestions below aren't a perfect substitute for lifting heavy, they'll definitely boost your strength, especially for the specific sport in question -- though don't be afraid to also cross-train by, for example, trying a swim workout if you’re a runner. Use one of these workouts in place of a shorter steady-state swim/ride/run (with a proper warm-up and cooldown), in addition to one more traditional weight-training workout per week for major muscle toning.

Swimming
Water by nature creates resistance on the body as you move through it. “In simply overcoming the resistance, an athlete gains strength,” says Jensen. She suggests swimmers increase the “load" further by isolating the upper or lower body by doing kick-only laps holding a kickboard or pull-only laps wearing hand paddles. You can also wear a “drag” swimsuit (i.e., one that’s intentionally baggy).

MORE: 4 Reasons Your Next Workout Should Be a Swim

Cycling
Anyone who's taken a challenging cycling class knows that by upping the resistance on the bike, your leg muscles get a real burn. Outdoors, you can get the same effect by tackling hills, riding into the wind on a gusty day, and standing up while pedaling to vary how the muscles get activated. If you’re an indoors-only cycler, Jensen suggests this workout: Pedal for one minute at a resistance that forces you to work hard to sustain 60 to 70 RPMs (revolutions per minute), then rest for a minute and repeat. Work up to five minutes of hard pedaling.

MORE: 7 Mistakes You Might be Making in Indoor Cycling Class

Running
As with cycling, running up hills provides a solid leg-strength workout. Pick a hill outdoors with an incline that takes you 30 to 45 seconds to get up at a pretty hard clip. (On the treadmill, try a 4 percent incline at a pace you can sustain for that amount of time.) Charge up your hill, using shorter strides and a faster step than you do on flat, then walk or lightly jog down (or reduce the treadmill's incline and speed for about triple your time on the "hill"), aiming to keep your timing the same for each uphill. Start with five reps and work your way up to 10. Another option? Run stairs or stadiums. For the harder core, try wearing a weight vest during an interval workout. One “strengthener" to skip: Holding weights in your hands while you run--it’s a shoulder injury waiting to happen.

MORE: 5 Cardio Myths You Need to Stop Believing

Amy Roberts is a certified personal trainer.

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