The idea is that muscles won't keep growing once you achieve a certain amount of strength, and you need to always make the exercise harder.
But is it the only way? Is there any evidence that muscle pump alone is enough to cause muscles to grow?
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It should be understood that muscle pump is not a form of stimulus, but rather a physiological reaction to that stimulus. Thus, our question should really be, what causes muscle pump, and is that conducive to muscle growth?
Muscle pump is cellular swelling caused by an increase in intra-cellular hydration, which is stimulated by muscle contraction. It occurs in response to repetitive work at moderate loads, and appears to occur maximally after numerous medium-to-high-repetition sets (8-20 repetitions) with short (30-60 second) rest periods, or by loading variations such as drop sets. Muscle pump has been shown to stimulate protein accretion, especially in Type II (fast twitch) fibres, by increasing synthesis and decreasing break-down.
Given the observation of the hypertrophic mechanisms involved with muscle pump, and given that the repetition ranges and training protocol associated with muscle pump are known to maximise hypertrophy, it is fair to conclude that muscle pump alone can facilitate muscular growth.