US Air Force SENIOR AIRMAN Frank Zientek from the Air Mobility Command (AMC) team tests his strength and endurance on the "Belly Robber" obstacle during the Chiefs Challenge event at Lackland Air Force Base, Texas, for DEFENDER CHALLENGE 2000. Defender Challenge is the annual Air Force wide competition sponsored by Air Force Security Forces. This competition showcases the talents and capabilities of 13 international Security Forces teams in seven physical fitness, base defense, and policing skills over 6 days