There is no singular best exercise for toning the upper arms; rather, it's a combination of strength training to build your muscles, and decreasing body fat through caloric expenditure. The best upper-body exercise that uses most of the muscles in your entire arm is the overhead shoulder press, specifically the Arnold press. Incorporate Arnold presses twice a week into your upper-body routine to strengthen and tone your arms.
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Your arms consist of your shoulder muscles, which are the frontal deltoid, medial deltoid and rear deltoid, and your biceps and triceps. Exercise all three areas of your shoulders to build the right size and shape. Your shoulder muscles are responsible for overhead lifting, and extending and rotating your arms. Your biceps are responsible for bending your arms at the elbows, and your triceps straighten your arms back out and assist during overhead lifting. All three of these muscle groups are activated during the Arnold press.
According to ExRx.net, the Arnold press specifically activates your front and medial delts. In addition, your biceps remain active the whole time stabilizing the weights, and your triceps assist your shoulders in pressing your dumbbells overhead. Grab dumbbells and bring them up in front of your head, palms facing toward you, elbows below your wrists. Open your arms out wide, finishing with the dumbbells on either side of your head at about ear level, palms facing forward. Press the dumbbells up overhead and pause at the top. To include your rear delts, lean forward during the press and continue rotating your arms slightly so your palms face laterally away from your body once extended. Bring them back down to ear level, then together in front of your head again.
The first step in toning your arms is building your muscles. The National Strength and Conditioning Association recommends building muscle by lifting a weight that fatigues your muscles by eight to 10 reps for about four sets. Once you have developed a little more size and shape, increase reps to three sets of 12 to 15 to further tone and sculpt the muscles. You may need to lower your lifting weight slightly to accommodate the increased repetitions.
Lose excess body fat to tighten your arms by being in a caloric deficit, which means burning more calories than you consume. Add in cardiovascular exercise and monitor your diet. For general health, the American College of Sports Medicine suggests at least 150 minutes per week of moderate-intensity exercise, or five days at 30 minutes per day. To burn more calories in a shorter amount of time, try high-intensity intervals such as sprints on a bike or treadmill. Start with a 20-second sprint followed by a 40-second active recovery and continue for 20 minutes, three days per week. Gradually increase time and days per week.