What Happens to Your Body During a New Workout Routine

When it comes down to it, the human body is a pretty impressive, efficient machine. It adapts quickly to stress (at least, the physical kind), which is why you should mix up your training regularly if your aim is to get leaner, stronger, faster, and heart-healthier.

But what exactly changes in your body when you partake in a new activity? The answer: It kind of depends what you decide to do. We broke it down for you:

When You Start a Running Regimen (or Other Cardio Program)

It's called "cardio" for a reason: In just about three weeks of a new running program, your heart will be measurably more efficient. "It can pump out more blood in each heartbeat to send oxygen-rich nutrients to your muscle cells," says Michele Olson, Ph.D., a professor of exercise physiology at Alabama's Auburn University Montgomery. "This carries over to when you are not exercising, resulting in a lower resting heart rate." To see how much progress you make, take your pulse rate before you begin your training schedule some morning when you first wake up, then take it again four weeks in. If you're changing up your routine from, say, running to kickboxing, your cardio gains may not be as great, but you'd benefit from using your muscles in different, challenging movement patterns, for both strength and coordination gains.

When You Strength Train

You (shocker!) get stronger. But first, you break down a little bit. "You actually slightly damage your muscle fibers, because they are not used to being challenged with resistance—dumbbells, elastic resistance, and even bodyweight exercises like push-ups or standing lunges,"  says Olson. If you're going from bodyweight-only work like Pilates or yoga to using weights, you'll have a good base of control, particularly with your core, which will help you to go heavier—and get stronger—faster. Soreness can be expected in the first couple weeks, but if you feel totally debilitated, you’re going too hard. If that's the case, don't stop with your workouts, just take them down a notch. Studies show that repeating the same routine at a lesser intensity can ease adaptation and reduce the ouch factor. In three to four weeks, you’ll feel stronger and be able to lift more weight in your reps, though it may take longer before your results translate into visible muscle tone.

When You Begin Yoga (or Any Stretching Routine)

Lament that you can't touch your toes? The good news is, it won't take that much work to get there, at least compared to the weeks needed for cardio and strength gains. "Changes happen quickly, and even after just one session you'll be more flexible, able to extend farther with less resistance," says Olson. The downside: The effects won't last unless you keep it up.

Protect Yourself

New workouts also mean new potential injury risks. To be safe, not sorry, Olson suggests:

  • Don't jump into high-intensity workouts until you build a baseline of cardio and strength. You should walk before you run (literally), beginning with intervals of the two until your stamina permits uninterrupted jogging. For strength training, master the basic moves with bodyweight first before you load with dumbbells.
  • Learn proper form. Ask trainers for help, use mirrors, and work on feeling your body positioning. Find out how to check your form without a mirror.
  • Get the right shoes. That means running shoes for running, or lower profiles shoes if you are doing Zumba, for example.
  • Spread out your workouts to give yourself a day of rest between them. Your "off" days are a good time to do your stretching routine.
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