An apparatus for strength training of an athlete in preparation for playing a sport that requires swinging of a bat or other implement comprises an attachment mechanism that is removably attachable to the proximal “grip” end of the implement, and a weight attachable to the attachment mechanism. A principle axis of inertia of the weight is aligned with a longitudinal axis of the implement, such that added weight does not significantly impact the athlete's use of the bat during practice drills and games. Concentration of the weight near the athlete's hand(s) also reduces risk of injury. In embodiments, additional weights can be added to the apparatus as needed. In embodiments, the apparatus includes a flexible inner member that is pressed against the shaft of the implement by threading thereon of an outer member. A longitudinal spring can be included to create tension between elements of the apparatus.