The trunk and its associated muscles make up the core. The extremities are the arms, forearms, hands, thighs, legs, ankles/feet. Strong core muscles provide a foundation for the extremities to attach to and work more efficiently. It is hypothesized that a weak core can cause excessive stress on the extremity muscles, tendons, ligaments and joints. Core strengthening is a multi-joint exercise, involving larger muscle groups such as the chest, abdominals, back, hip/thigh, and shoulder blade muscles. Core strengthening is often incorporated as part of a low back or neck rehabilitation program. Because recovery or enhancement of core strength provides a stable base for the extremities, it is also commonly part of an arm, forearm, thigh, leg or ankle program.