Tempo

Every rep has four distinct phases, and tempo controls all of them. The standard notation uses 4 digits: the first indicates the seconds of the eccentric phase (when the muscle lengthens under load), the second the pause at maximum stretch, the third the concentric phase (the actual contraction) and the fourth the pause at peak contraction.

Concrete example on the bench press: a 3-1-1-0 tempo means 3 seconds to lower the bar to the chest, 1 second pause on the chest, 1 second to push it up and no pause at the top. With the same weight and the same reps, a controlled tempo makes the set far more demanding than uncontrolled execution.

Manipulating tempo is useful in several contexts. A slow eccentric phase (3-4 seconds) increases muscle damage and time under tension, both relevant factors for hypertrophy. A pause at maximum stretch (like the floor press or paused squat) eliminates the stretch reflex and forces the muscle to generate force from a dead stop. An explosive concentric tempo, on the other hand, is ideal for developing strength and power.

You don't need to obsess over the stopwatch on every single rep. The practical guideline is: control the eccentric (at least 2 seconds), don't bounce at the turnaround points and move the concentric with intent. Do that and your tempo is already in good shape for most goals.