This odd muscle may be causing your hand to go numb...

Subclavius posticus: an anomalous muscle in association with suprascapular nerve compression in an athlete - From The Abstract

"Subclavius posticus is a rare anomalous muscle that traverses from the costal cartilage of the first rib posterolaterally to the superior border of the scapula. We present an athlete who presented with incapacitating suprascapular nerve compression associated with a subclavius posticus diagnosed by MRI. Symptoms were relieved, and function was restored by decompression of the nerve and excision of the anomalous muscle. We present this case to bring awareness of the subclavius posticus as an anatomic variant and a potential additional source for compression in suprascapular neuropathy. We recommend high-resolution magnetic resonance imaging to evaluate cases of isolated suprascapular neuropathy."

Link To The Full Paper

Cogar, A.C., Johnsen, P.H., Potter, H.G., Wolfe, S.W. (2015). Subclavius posticus: an anomalous muscle in association with suprascapular nerve compression in an athlete. Hand (NY). 
https://www.ncbi.nlm.nih.gov/pubmed/25762884