Time under tension refers to how long a muscle is working against an opposing force. While it is a factor in muscle growth, attempts to leverage it by doing reps slowly is based on a misunderstanding. Until you are close to failure, going intentionally slow does not recruit high-threshold motor units containing fast-twitch muscle fibers because it uses a lighter weight. Most of the set involves slow-twitch fibers, the type that don’t grow with training. At normal speeds with heavier weights, reps will slow down involuntarily as you approach failure, all while recruiting higher-threshold motor units for more of the set. Even if you only have access to light weights, whether you decide to do your reps slowly or do as many reps as possible at a normal pace makes little difference, since the time under tension experienced by high-threshold motor units is about the same in both cases. In other words, don’t worry about time under tension. Just take your sets close to failure using more weight or more reps.
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