A medical instrument having a tool assembly including a pair of opposing tissue engaging surfaces for clamping tissue therebetween. The medical instrument includes a housing having a fixed handle and a movable handle mounted to the housing and selectively movable relative to the fixed handle from a first position in spaced relation relative to the fixed handle to a second position closer to the fixed handle to actuate the clamping of tissue. The medical instrument includes a selectively activatable drive assembly including a power source and a motor which is operatively coupled to the movable handle, wherein upon actuation the motor actuates the pair of opposing tissue engaging surfaces. The drive assembly includes a controller configured to variably control the rate at which the motor actuates the pair of opposing tissue engaging surfaces in response to the force exerted on the movable handle.