Employee was moving timbers to lift truck with flat tire off the ground. Setting down timber, heard left knee pop, felt ""bolt of lightening"" shoot through left leg and fell to ground. Worked thru shift. Next morning left knee still sore. Went to Dr.; xray taken. Dr. felt tendon or muscle pull; will heal. Knee still unstable, saw Dr. 2/21/05 put in brace; light duty; reportable.