I came across this excercise suggested for seniors, to build muscle strength in the arms and shoulders. It seems so easy, so I thought I'd pass it on. 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-kg potato sack in each hand, extend ur 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-kg potato sacks. Then 20-kgs potato sack, and eventually try to get to where you can lift a 50-kg potato sack in each hand and hold your arms straight for more than a full minute.
Once you feel confident at that level, put a potato in each sacks.