What is a pain in left scapulae that runs under left armpit and to left pectoral muscle caused from?

Seratus anterior : 40's male has pain in "armpit to pectoralis", and he queries as to cause. The serratus anterior muscle is implicated ; pushing his arm forward against resistance would be expected to arouse pain. Pushing and weight lifting stressors might be expected to cause this type of muscle injury ; reflexive muscle spasm. Rest and heat might benefit. If persists see an osteopath or a physiatrist.