A hiker is climbing up a steep path Halfway he finds the muscles in his legs are experiencing burning and aching sensation What most likely cause of these symptoms?

Lactic Acid

The burning and aching sensation he feels is most likely caused by a build-up of lactic acid in his leg muscles. Lactic acid is produced when muscles use glucose for energy in the absence of sufficient oxygen. This often occurs during intense exercise, such as climbing a steep path. As lactic acid accumulates, it can cause the muscles to feel sore and fatigued.