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When it comes to arm day, it's easy to fall into a rut -- or not even know where to begin. This 21-day dumbbell arm challenge, created by NASM-certified personal trainer Désirée Triolo, is designed to help you mix things up, pretty much guaranteeing that you'll feel stronger by the end of the three weeks. Don't worry if you've never lifted a dumbbell in your life; this upper-body arm plan was crafted with everyone in mind, whether it's your first time starting to exercise or if you work out regularly.

This dumbbell arm challenge leverages a simple arm workout that consists of five basic dumbbell exercises. The moves target your biceps, triceps, back, and shoulders to build overall upper-body strength. Over the course of the challenge, you'll increase the number of reps you're doing of each exercise, eventually working up to three sets of 15 reps of each.

Désirée Triolo, is a personal trainer certified through NASM and the founder of the "Get Stronger with ItsDesiFit" fitness app.

Ahead, you can find a detailed explanation of how to do each of the five arm exercises, followed by the three-week plan. The first step is choosing the appropriate weight -- Triolo says you want a dumbbell that fatigues your muscles by the end of the last two reps. She suggests beginners start with five-pound dumbbells (we like the PS Neoprene Dumbbell ($10), sold as a single) and more advanced lifters go heavier at 15 to 25 pounds. If the challenge ever feels too easy (because you're gaining strength!), that's your cue to increase the weight of the dumbbells. Likewise, if you find your muscles are fatiguing too early or your form becomes compromised, decrease the amount of weight you're lifting or opt for bodyweight exercises.

This arm challenge incorporates two rest days per week -- because even though it's a challenge, recovery days are always important. "My professional opinion is to alternate upper-body and lower-body training days to improve your overall fitness level and to avoid creating muscle imbalances," Triolo says, which is why you'll see "lower body" listed on the plan. On these days, you can do lower-body exercises (like squats or glute activation exercises) or start with walking, going for a run, biking, or hiking.