The graph represents the relationship between running time and energy system contribution - HSC - SSCE Personal Development and Physical Education - Question 20 - 2007 - Paper 1

At 40 seconds of running time, the dominant energy system primarily switches from anaerobic alactic to anaerobic lactic processes. During this transition, the accumulation of lactic acid plays a significant role in fatigue. Lactic acid builds up when the body relies on anaerobic metabolism for energy, leading to metabolic acidosis, which can impair muscle function and contribute to feelings of fatigue. Therefore, the correct answer is (B) Increased lactic acid.