It depends Ivan..there are plenty of slim players out there still..look at Simon, Isner, or even Federer who aren't anymore "muscular" than many players of the past, especially really fit players from the past such as Laver, Borg, or Lendl. See Borg in these pics here for example. The guy tested off the charts in leg strength in Sweden. Borg liked to keep very slim in general though because he wanted to be extremely mobile, but he actually became very strong in the upper body as well by the time he was 24-25. In general though, there is now more emphasis on players using weights to strengthen the torso, arms, legs, etc..but there are diminishing returns there. You certainly don't want to be too bulky and muscular as a tennis player. It's certainly more about the legs now, but guys like Vilas, Lendl, and Borg were very strong in the legs even back then. It's just that now most all players play the way they did. They led the way towards the current brand of tennis.