Just came across this exercise suggested for seniors, to build muscle strength in the arms and shoulders. The article suggested doing it three days a week.
Begin by standing on a comfortable surface, where you have plenty of room at each side. With a 5-lb. potato sack in each hand, extend your arms straight out from your sides, and hold them there as long as you can.Try to reach a full minute, then relax.
Each day, you'll find that you can hold this position for just a bit longer. After a couple of weeks, move up to 10-lb. potato sack. Then 50-lb. potato sacks, and eventually try to get to where you can lift 100-lb. potato sack in each hand and hold your arms straight for more than a full minute.
After you feel confident at that level, put a potato in each of the sacks.