Mechanical Tension is related to exercise intensity (the amount of weight you are lifting); in other words, to get big you have to train heavy. Eight-time Mr. Olympia Lee Haney once said, "The key to building massive, powerful muscle is to doggedly increase the training weights you use." Science anecdotal evidence backs Haney and common sense says when a bodybuilding icon like Haney speaks on building muscle, we should all listen.
Muscle damage is associated with muscle soreness, this inflammatory response aids in the muscle building process. Of course, assuming the lifter recovers properly.
Metabolic Stress is a result of the byproducts of anaerobic metabolism in the 30-60 second range of set duration (i.e., hydrogen ions, lactate, inorganic phosphates). In other words, lifting all-out for this duration of time, scientists believe, causes a hormonal response that will illicit the greatest muscle growth.