I came across this exercise, suggested for older blokes, to build strength in the arms and shoulders.
It seemed so easy 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 2 kg 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 and then relax.
Each day you'll find you can hold the position for just a bit longer.
After a couple of weeks move up to 5kg potato sacks and then eventually try to get to where you can lift a 20kg potato sack in each hand and hold your arms straight for a full minute.
Once you feel confident at that level, put a potato in each of the sacks.