Time under tension (TUT) is the total amount of time the muscle spends contracting against resistance. The principle of TUT does not differentiate among concentric contractions, isometric contractions, or eccentric contractions. During a traditional set, some time will be spent in all three conditions. How much time depends on the specific repetition cadence that is used.
Researchers from Brazil and the U.S. performed a study with 22 male subjects with previous training experience. The subjects performed two training protocols (slow and fast) on the Smith machine bench press. Both protocols included three sets, three minutes of rest, and 60% of one-rep maximum (1RM). The slow protocol consisted of six reps with a six-second rep duration, whereas in the fast protocol the subjects performed 12 reps with a three-second rep duration. Muscular activation and blood lactate concentrations were measured during and until 12 minutes after the last set.
Both muscle activation and blood lactate concentrations were higher in the fast protocol when compared with the slow protocol.
Time under tension being equal between two protocols, faster rep speeds result in higher muscle activation and blood lactate levels.
When planning your rep cadence for your workout, there is no need to perform your reps slower than three seconds. A one-second up, onesecond down scheme works, as does a one-second up, two-seconds down scheme. The important thing is to adequately activate the target muscle while generating as much metabolic stress as you can to produce a good anabolic stimulus.