Vixen, I can not fully understand what you are doing. What does 5# mean? 5 pounds? 5kg? In either case it does not sound as if significant improvement - neither of momentary power, nor of endurance - for the particular involved muscle groups may be expected - except as an initial result when you started up the routine.
When you say bluegrass "rhythm" do you use a plectrum? In such case it is expected to cause considerable static effort. When you say "strumming" is that left hand or right hand work?
Anyway guitar is quite different from concertina.Playing energetic rhythmical chords with a guitar may be heavy work indeed, classical guitar more a matter of tension from a strainful posture. With concertinas it should be possible to eliminate almost all static effort by using stable handles and adequate technique and consequently needs for muscular strength or endurance are minimal and no extra "work-out" ought to be needful.