Hi there,

I don't think it is the subscapularis as it is a very deep muscle.

It could the supraspinatus, anterior deltoid, or a move involved issue.

If you were using 7kg of weight, I would imagine that during the isolation movement it would have caused the recruitment of the deltoids, and it may be the anterior deltoid that is strained, but I can't figure that out from the movement described. Were you completing both movements at the same time?

Could you describe exactly your position and movements?

Regards