Tempo training refers to controlling the speed of each phase of a lift. A tempo is typically written as four numbers: eccentric (lowering), pause at bottom, concentric (lifting), pause at top. For example, a 3-1-1-0 tempo means 3 seconds down, 1 second pause, 1 second up, no pause at top. Manipulating tempo is a powerful but underused tool for increasing muscle growth and breaking plateaus.
Slowing the eccentric (lowering) phase is the most effective tempo modification for hypertrophy. Research consistently shows that the eccentric phase generates the most muscle damage and growth stimulus. A 3-4 second lowering phase dramatically increases time under tension compared to the typical uncontrolled drop that most gym-goers use. Try this on your next set of squats: lower yourself for 3 seconds, pause for 1 second at the bottom, then drive up normally. You'll be surprised how much harder a familiar weight feels.
Tempo training is particularly effective for intermediate and advanced lifters who have stalled on regular progressive overload. It's also valuable for beginners learning movement patterns - slowing down gives you time to feel the target muscle working and reinforces correct technique. Use slower tempos (3-4 second eccentric) for 4-6 weeks as a hypertrophy block, then return to regular tempo with heavier weights. You'll often find your strength has carried over significantly. Don't use slow tempos for maximal strength work - heavy compound lifts should be performed with intent to move the bar as fast as possible.