By peoplestaff225
Updated October 13, 2014 12:00 PM
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As a former U.S. Marine, Erin Oprea is dedicated to fitness. Now a Nashville-based personal trainer with clients like Carrie Underwood, she uses her expertise to help those with busy lives stay fit. Visit her site Oprea Personal Fitness or follow her on Twitter @OpreaFitness. Then come back to PEOPLE.com every Monday to master a new workout move.

Don’t let all the hard work you did this summer crafting those tank-top-ready arms go to waste.

Just because the weather is getting cooler and you’re bundling up in your favorite sweater, doesn’t mean that you want to stop toning your arms.

Keep your upper body solid with this great twist on two classic moves: the push-up, which helps sculpt your chest, arms and shoulders, and the side plank, which hits your whole core.

The T Push-Up

1. Perform a push-up, keeping your abs tight with a little space between your feet.
2. As you come up from the move, pivot your feet and raise your left arm straight into the air. Hold for two counts, keeping your core engaged.
3. Return to the starting position, perform a push-up and repeat step 2 on your opposite side.
4. Keep alternating the side planks as you count to 10–20 repetitions.

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