Complex sentence: There is also some doubt about whether Aran sweaters were ever widely used by fishermen and many argue that the orignal sweaters which with their untreated yarn were quite thick and stiff, which would probably restrict the movements of a fisherman.
Split into simpler sentences: There is also some doubt about whether Aran sweaters were ever widely used by fishermen and many argue that the orignal sweaters which with their untreated yarn would not have been suitable for this use. They were quite thick and stiff, which would probably restrict the movements of a fisherman.