Effects of 32-year leisure time physical activity discordance in twin pairs on health (TWINACTIVE study): Aims, design and results for physical fitness - Leskinen, T; Waller, K; Mutikainen, S; Aaltonen, S; Ronkainen, P H; Alen, M; Sipila, S; Kovanen, V; Perhonen, M; Pietilainen, K H; Cheng, S; Suominen, H; Kainulainen, H; Kaprio, J; Kujala, U M (Twin Res Hum Genet, 2009)