The story
The first settlers in the area came from Sääksmäki, as evidenced by village names such as Voipala in Sysmä, which is named after Voipaala in Sääksmäki. Rapala may also be named after Rapola.
Sysmä was first mentioned in 1442, when it was already a separate parish. Jämsä was separated from it on the same year. Sysmä was a large parish, though most of its northern lands lacked a permanent population. The northern parts were eventually settled by Savonians and in 1561, the area became the Rautalampi parish which included most of modern Central Finland north of the Päijänne; from Toivakka to Pihtipudas.
The original dialect spoken in Sysmä was a Tavastian dialect. It was replaced by a Savonian dialect soon after Ivar Tawast killed 400 peasants from Sysmä, Jämsä and Kuhmoinen in Padasjoki during the Cudgel War. Many farms in Sysmä were left without owners, so they were taken by Savonian settlers.
Sysmä was also occasionally known in documents as Sysmäki due to association with the word mäki (hill), compare Kärsämäki which used to be Kärsämä.
Hartola was a part of Sysmä until 1784. At the time, the Hartola parish also included Joutsa, Luhanka, Leivonmäki and the western part of Pertunmaa.
From Wikipedia’s article on Sysmä, shared under CC BY-SA 4.0.5