The aircraft collided with the runway at the mid-point in a 30 degree nose low attitude during takeoff. According to the pilot, the takeoff was started at the north end of the 5000 foot runway and after 500 feet the aircraft lifted off normally. The pilot then relaxed the back pressure on the elevator to accelerate to best climb speed. The pilot further stated that at the mid-point of the runway the aircraft began to pitch down gently so he applied some nose up trim but the aircraft continued to pitch down and the pilot noticed that the control stick would not move any farther aft than the neutral position. The aircraft continued to pitch down until it impacted the runway. An inspection of the aircraft revealed an unusual amount of tools and plywood in the tail section. No items were found lodged so as to interfere with the flight controls. All the flight controls had continuity but the elevator cables had an unusual amount of slack but not enough to make the aircraft uncontrollable.