Everyone’s talking about running and rucking right now. How to decide which one to add to your fitness routine.

Meanwhile rucking, which involves walking or jogging with a weighted backpack, has become one of the buzziest trends in fitness. Guy Fieri said he lost 30 pounds over the last few years by hiking with a weighted vest uphill a few times a week (alongside eating a healthy diet), and it’s beloved by Silicon Valley elites and legendary CrossFitters.

Rucking and running are both great aerobic exercises

Aerobic exercise, which both activities count as, can increase cardiovascular fitness, endurance, reduce the risk of cardiovascular disease, and improve risk factors such as blood pressure, cholesterol, and insulin sensitivity, Dr. Allison Zielinski, a cardiologist and co-director of the sports cardiology program at Northwestern Memorial Hospital, told BI.

Higher levels of fitness are also linked to a longer lifespan, she said: Cardio lowers the risk of early death from any cause.

Luke Worthington, a top trainer and sports scientist whose clients include Naomi Campbell and Dakota Fanning, told BI that walking and running can improve body composition.

There are also some “hidden benefits,” he said. Rotating your pelvis and ribcage, which you automatically do when you walk or run, can help the digestive system run more smoothly and improve the movement of lymphatic fluids.

Walking outdoors can also boost your mood and calm the nervous system. The adjustment and re-adjustment that the eyes and brain continually make when walking or running has been shown to improve parasympathetic nervous system activity, Worthington said.

Carrying weight is good for bone health

The biggest difference between rucking and running is the weight-carrying component. Walking with an additional load requires more work from the cardiovascular system and some of your muscles than unloaded walking, Zielinski said. So it uses up more energy and burns more calories.

But the health benefit or quantity of fat you lose from rucking and running will depend on factors like speed, terrain, hill grade, duration, and how much load you carry, she said. You’re going to get more of a workout from sprinting up a hill than you would from strolling with five pounds on your back, for instance.

Regardless, carrying a load, which humans have done for centuries, can be beneficial for bone health, Worthington said. Applying force along the length of long bones, known as axial loading of the skeleton, is really great for maintaining bone and mass density, he said.

And because adding weight feels more difficult, it can create a higher sense of achievement, Worthington said.

Cardio won’t help you build muscle

Worthington said that neither exercise would have any noticeable impact on building muscle or weight loss.

Increasing muscle mass requires three things: A calorie surplus (because you can’t make something out of nothing), adequate protein, (the building blocks of muscle tissue), and gradually increasing workload over time, (whether through increased weight or mechanical tension), Worthington said.

Choose whatever you’re likely to do regularly

If you’re looking to establish an exercise regime, start slow and aim to be consistent, the experts recommended.

“I would say that for people who are not exercising, starting with walking, adding some grade, and then once they kind of get comfortable with that and feel like that’s not very hard anymore, then adding a little bit of weight can up the intensity,” Zielinski said. This might be easier on the joints than increasing speed.

Worthington said to pick whichever you enjoy more and are likely to do regularly. “Lots of different training modalities ‘work’ — the biggest issue people have is not doing them for long enough to actually allow them to,” he said.