Most of us who run have felt that burning sensation in our legs during the end of a heavy tempo session, or a few repeats from finishing a session of 400’s. You’re urging your legs to keep going despite the very obvious slow down in pace. The session finishes and you walk off the track. The talk about the session as you gather for your cool down is how tough but good the session was. Your mind is screaming, “well it wasn’t good for me”. You chip in your two cents worth about how difficult the last few reps were. “I was fine, then all of a sudden my legs began to burn and I couldn’t keep the pace up”. Club mates nod in knowing agreement. “That’s the lactic acid in your legs” It slows you down when you go above your lactic threshold. You’ll probably be a bit sore tomorrow”.