For those getting along in years, here is a little secret for building your arm and shoulder muscles. Three days a week works great! Begin by standing straight, with a 5-LB. potato sack in each hand. Extend your arms straight out from your sides and hold them as long as you can - try to reach a full minute.
After a few weeks, move up to 10-LB. potato sacks, and then 50-LB. potato sacks, and eventually try to get to where you can lift a 100-LB. potato sack in each hand and hold your arms straight out for more than a full minute.
After you feel confident at that level, start putting a couple of potatoes in each of the sacks, but be careful not to overdo it.