What is considered a living organisms?

anything that is living, like plants, animals, humans, etc

A living object that has an organized structure, can react to stimuli, reproduce, grow, adapt, and maintain homeostasis is referred to as an organism. Any animal, plant, fungus, protist, bacterium, or archaeon on Earth qualifies as an organism. Subcellular structures can also be used to categorize organisms.

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.