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Pain is unaltered by neck or arm movements. Pain is spontaneous, often with no apparent cause. Gradual decrease in pain followed by marked weakness of upper arm. Neurologic deficits indicate that more than one nerve is involved (i.e., lesion in the plexus).
Muscle strain. There are several muscles of the chest and arms that could cause armpit pain from overuse or injury. The pectoralis major is a large chest muscle that runs up into the shoulder. It can be injured playing sports or lifting weights.