Justin Jackson packs on 20 pounds of muscle in offseason Posted Aug 15, 2019 Dallas Mavericks forward Justin Jackson has been working hard this summer to work his way up in the rotation. The 24-year-old already has added 20 pounds of muscle this offseason, making him more ready to go toe-to-toe with bigger wing players in the NBA. Jackson said last month he is looking forward to having a successful season with the Mavericks. (Clutch Points)