Fins are good for improving leg strength. They behave
as though you have an extra muscle; your legs therefore strengthen up
because they have to work this extra muscle in addition to the real ones.
We wouldn’t recommend the large fins though. As with the large paddles,
they are too much effort. We prefer the short stubby ones and, again,
supplementing them with a drag belt with cups if you want to.
Fins are also a good swimming training aid for assisting with difficult drills when you want to
concentrate on something else, however, don’t swim a lot of full stroke
with them on.