Your underarms are not actually a muscle, but are made up of the conjunction of many muscles. You have to work each of these muscles to tone the entire area under your arms. The primary muscles that tie into the underarm are the lats, the biceps, the triceps, the pectorals and the anterior deltoid. Training all of these muscles correctly with five to 10 sets of maximum repetitions and spot-on nutrition will lead to toned underarms. Always consult a doctor before starting this or any exercise regimen.
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Training the Lats, Pecs, Front Delts, Biceps, and Triceps
To train the upper lats where they tie into the underarm, engage in exercises that involve pulling down from an overhead position. Examples of such movements are pull-ups, chin-ups, swimming, ab rollouts, dumbbell or barbell pullovers, and ski erg training. Each of these movements involves an overhead stretch and an intense contraction that closes the underarm with great force. This will attack the entire underarm, especially emphasizing the back and bottom portion.
The good news about the biceps is that the lat movements that require you to flex your elbow joint to complete the movement – pull-ups and chin-ups – also work the biceps. Still, you'll want to do extra work in this area. To do so, you'll need to do elbow flexion exercises, which means bringing the arm from a straight position to a bent position. Exercises for this can be the standard biceps curl with a barbell or dumbbells, preacher curls, hammer curls, reverse grip curls, and using rings to curl your body weight.
The triceps form the back of your arm, and tie into the upper back side of your underarm, next to the biceps and above the lat. To work these muscles you need to perform elbow extension movements, which simply means taking the elbow from a bent position to a straight position. Great exercises are any variation of push-up, bench press, overhead press, or dips. These compound movements also work your pectorals, shoulders, and lats substantially, giving them priority over isolated triceps exercises, because they are far more specific to the underarm area.
The pectoral and anterior deltoid muscles tie into the front of your underarm, the area everyone sees when talking to you face-to-face. If you have performed all the above exercises, especially the triceps exercises, you have already worked on the pecs and delts substantially. But, you can also perform pec flies using dumbbells, a pec deck machine, or gymnastics rings. Like a bird flapping its wings mid-flight, flies involve taking your straight arms from an open position, adjacent to your shoulders and chest, to a closed position directly in front of you.