This refers to the speed at which you perform a lift. It is generally designated in a 3 or 4-number sequence in seconds, which looks like this: 212(0). Sometimes that last number is left off.

Let’s use the squat as an example for a visual.

The first number (2) is the Eccentric phase or muscle lengthening (lowering down).

The second number is the isometric (no change in muscle length/no movement) pause at the bottom.

The third number (2) is the concentric phase or muscle shortening (standing up).

The fourth number (0) is the pause at the top or resting phase of the lift. 2120 is a pretty general lifting tempo, maybe 2121 for a slight reset.

Reasons to use tempo variation:

1. Time under tension especially in the eccentric phase creates muscle damage which when recovered properly = muscle growth (hypertrophy), if that is your goal then slowing down your lifts can be beneficial. [3030]
2. Slowing down your lifts can increase the difficulty of a lift without adding weight (at-home workouts) [5252]
3. If you find yourself failing a particular range of a big lift, spending more time in it (IE. isometric section at the bottom of a squat) can help build strength in that range. [2511]
4. Beginners learning to lift can slow lifts down to better focus on form with lighter weight while still stressing the muscles [3131]
5. Using faster tempos can work more fast twitch muscles and create different adaptations for sport or power athletes [3011]