Tempo

Tempo is the speed of each phase of an exercise repetition, usually written as three or four digits representing seconds.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 312.} An optional fourth digit sets the gap before the next rep.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 312.} Adding the phases gives the rep’s duration: 2-0-3 produces a 5-second rep.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 124.}

Heavy loads move slowly even when the client tries to lift fast, and reps slow as fatigue builds.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 124.} Loads at or below about 80% of one rep max (1RM) leave room to vary lifting speed.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 124.}

Where sources differ

The sources use different digit orders: Schoenfeld puts the concentric (lifting) phase first, so 2-0-3 means 2 seconds up, no pause and 3 seconds down.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 124.} The International Personal Trainer Academy and Helms and colleagues put the eccentric (lowering) phase first.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 312.},^{3Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 195.}

In practice

• A meta-analysis found no difference in muscle growth for rep durations up to about 6 seconds; very slow “superslow” training is likely worse.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 124.}
• Deliberately slow reps forced loads down by about 30% in one study, while trying to move the bar fast suits heavy multi-joint lifts.^{4Hoffman, NSCA’s Guide to Program Design, p. 110.}
• Control the lowering phase, then avoid deliberately slowing reps unless a beginner needs more time to maintain good form.^{5Helms et al., The Muscle & Strength Pyramid: Training, 2nd ed., p. 199.}