Demmink played collegiately at Clemson under Jack Leggett, lettering all four years and serving as the team captain in 2005 and 2006. He was named the team's Most Dedicated Player and helped lead the Tigers to the NCAA Tournament in each of his four seasons, including an appearance in the College World Series in his senior campaign. He was also selected to the Clemson Baseball All-Decade Team as its utility player.
During his time at Clemson, Demmink played seven of the nine positions on the field, claimed Atlantic Coast Conference Academic Honor Roll accolades on three occasions and earned Strength and Conditioning Athlete of the Year honors from the National Strength Coaches Association in 2004. His best year on the field came as a junior when he started 63 of 65 games and hit .345 with four home runs, 41 RBIs and nine stolen bases.
Following graduation, Demmink was drafted in the 28th round of the 2006 Major League Baseball First-Year Player Draft by the Philadelphia Phillies and spent four seasons playing professionally.
Since retiring from baseball, Demmink has become the president and owner of 3D Performance Traning, LLC, and is responsible for training a vast array of professional athletes, including players in the MLB, NFL, MLS, as well as members of the PGA Tour, Olympic Track and Field participants and tri-athletes around the world.
Among the Major League Baseball players that condition with Demmink in the offseason are former Tennessee standouts Blake Forsythe, Charley Thurber, Zach Osborne, Stephen McCray, Steven Gruver and Nick Hernandez as well as 2012 All-Star closer and current Texas Ranger Joe Nathan and 2008 All-Star and current Baltimore Oriole Nate McLouth. He is a member of the National Strength and Conditioning Association and his credentials include Certified Strength and Conditioning Specialist, Certified Fitness Trainer (ISSA), and Athletic Republic Level 2.
After graduating from Clemson with a bachelor's degree in business management in 2006, Demmink spent five months as a graduate strength and conditioning coach with the Tigers in the fall of 2006. He also worked as a personal sports performance trainer in Charlotte for six years before moving to Knoxville.