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For about a decade there, it was proposed that hypertrophy was potentially induced via several different mechanisms. - Namely, metabolic stress and muscle damage. - In fact, there's still people repeating that muscle is built through "micro trauma and tears and then built back up bigger". - Which is absolute nonsense and was disproven I don't know how long ago, but is still repeated. - But I digress. - But over the decade(s) or so, as more data came out it became obvious that once you removed mechanical tension from the equation, neither of these actually created a biological response that caused the addition of more contractile proteins. - But What IS mechanical tension? Think of mechanical tension as a type of pulling force. - The fibers experience a pulling force when they are trying to shorten but are resisted in doing so. This force increases as the shortening velocity decreases, and it is then when muscles experience high degrees of mechanical tension. - This is due to the force velocity relationship. - The force velocity relationship says that the force generating capacity of a muscle is predicated by contraction velocity. - So as you get closer to failure, there will be an involuntary slowing of contraction speed, no matter how hard you're trying to move the weight. - It is during these reps when the greatest amount of cross-bridging occurs between the functional proteins in the cell, which are known as actin and myosin, and this is how muscles produce force. - This is also when the active fibers will experience mechanical tension. - Now some misinformation that continues is that there's more mechanical tension due to heavier loads. Which isn't true. - Light loading to failure causes similar amounts of hypertrophy as heavier loading to failure. - It's also a myth that certain exercises cause more mechanical tension. - Mechanical tension exists on the force velocity curve. Not a heavy or light loading curve, or an exercise selection curve. - Mechanical tension also isn't something that is "accumulated" through volume. It is simply something the active fibers experience.