Unique exercise for building muscles in "older" people

Just came across this exercise suggested for older people, to build muscle strength in the arms and shoulders. It seems so easy, so I thought I'd pass it on to some of my friends. 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 sacks.

Then try 50-lb potato sacks and then eventually try to get to where you can lift a 100-lb potato sack in each hand and hold your arm straight for more than a full minute. (I'm at this level)

After you feel confident at that level, put a potato in each of the sacks......