The principle of time under tension (TUT) in training is a concept that refers to the amount of time during which a muscle is under tension or actively working during a certain exercise.

It is a critical variable in resistance training and is believed to play an important role in the development of muscular strength, endurance and hypertrophy (muscle growth).

The principle is based on the idea that by increasing the time in which the muscle is under tension, it will be subjected to greater stress, which leads to greater adaptations.

The basics of time under tension involve manipulating the speed of each repetition to create an extended period of tension in the muscles.

This can be done by slowing down the eccentric (lowering) and/or concentric (lifting) phase of the exercise, using isometric holds, or performing partial repetitions. For example, a traditional biceps exercise involves lifting the weight up and lowering it down in a relatively quick motion. However, by slowing the descent phase of the twist, the muscle will be under tension for a longer period, thus increasing the time under tension.

Research has shown that different TUT can cause different outcomes in terms of muscle adaptations.

For example, a study by Burd et al. (2010) found that longer time under tension (40-60 seconds per set) resulted in greater increases in muscle protein synthesis (MPS) compared to shorter time under tension (10-20 seconds per set). MPS is the process by which muscle cells build new proteins, which are necessary for muscle growth and repair. Therefore, a longer TUT may be more beneficial for muscle growth and hypertrophy.

However, it is important to note that the relationship between time under tension and muscle growth is not straightforward, and that other factors such as intensity and volume of training may also play a role.

Furthermore, excessive TUT can lead to fatigue, which can negatively affect training quality and increase the risk of injury.

Therefore, it is recommended that you vary the time under tension in your training program and not rely solely on long TUT to achieve muscle growth.

In terms of practical application, the time under tension principle can be applied to any resistance training exercise. For example, in the squat, a longer TUT can be achieved by slowing down the descent phase of the movement or by performing partial squats where the muscle is under tension for a longer period. Similarly, in the push-up, a longer TUT can be achieved by performing a slow descent phase or holding the bottom position for a few seconds before pulling back up.

Taking all of this into account, the time under tension principle is a key variable in resistance training that can be manipulated to achieve specific muscle adaptations.

While longer TUTs may be more beneficial for muscle growth, it is important to vary the TUT in a training program and not rely solely on this principle to achieve muscle growth.

By applying time under tension principles to resistance training exercises, individuals can challenge their muscles and achieve greater adaptations. Time under tension (TUT) refers to the length of time a muscle is under stress during exercise. In other words, it is the length of time the muscle contracts during repetitions of the exercise.

Ultimately, the effect of time under tension on muscle growth may depend on a number of individual factors, including genetics, training experience, and training goals. However, incorporating a variety of TUT into a training program, such as high and low TUT, can help provide a well-rounded approach to muscle growth and overall fitness.

Alternatively, as it pertains specifically to the concentric and eccentric phases, tempo depends on the speed at which the external load is moved.

Increasing the time under tension can be considered a way to increase volume, and it is commonly believed that increasing the time under tension could be a strategy to enhance the hypertrophic stimulus.

However, fast movements require more neural control and depend on the capacity to maximally engage the largest number of motor units. Furthermore, fast vs. slow eccentric phases resulted in greater muscle damage when equalizing total time under tension, so more recovery was required between sessions.

In addition, since muscle damage is considered one of the factors that initiate the hypertrophic process, fast movements may be more effective, contrary to what is believed in practice.

Another study compared the chronic effects of 1–0–1 s versus 1–0–3 s pacing for the concentric, isometric, and eccentric phases. After the 8-week intervention, proximal muscle strength and size increased similarly in both groups, although the hypertrophic response at the distal site was more pronounced after the rapid protocol.

Regarding dynamic strength gains, a recent meta-analysis observed similar results between fast and slow movements (effect size: 0.07, 95% CI – 0.13 to 0.27).

However, the authors noted that fast movements may have been more beneficial (effect size: 0.31, 95% CI − 0.01 to 0.63) when moderate loads (60–79%1-RM) were used, while there was no difference between fast and slow repetition training with low (< 60%1-RM, effect size: − 0.06, 95% CI − 0.45 to 0.32) or high load (> 80%1-RM, effect size: − 0.08, 95% CI − 0.41 to 0.25).<60%1-RM, effect size: − 0.06, 95% CI − 0.45 to 0.32) or high load (>> 80%1-RM, effect size: − 0.08, 95% CI − 0.41 to 0.25).

Another systematic review without meta-analysis summarized the effects of fast and slow movements on muscle hypertrophy, considering exercises involving upper or lower body muscles with particular reference to biceps brachii and quadriceps, respectively.

Interestingly, fast movements seemed to increase the size of the biceps brachii, while slow movements were more beneficial for the quadriceps. As the authors discussed, this may depend on the difference in fiber type prevalence between the two muscle groups, with more type II fibers in the biceps brachii than in the quadriceps, leading to the idea that specific movement speed strategies may be used depending on muscle morphology.

Notably, the studies included in this previous meta-analysis manipulated both concentric and eccentric phases, so the results were not related to the intrinsic characteristics of shortening versus lengthening contractions.

This further emphasizes that time under tension is a necessary variable to report when reporting resistance training protocols and should be considered when determining exercise volume. Interestingly, an analysis of the relevant literature challenges the common belief that slow movements favor hypertrophy.

Although no studies have directly investigated the effects of the isometric phase on different muscle lengths, a recent review examined this factor when only isometric training is performed, providing useful indications. For example, a greater hypertrophic stimulus derived from duration-matched isometric training performed at long versus short muscle length. Furthermore, more pronounced changes in muscle architecture were induced in long versus short muscle lengths, as well as greater improvements in tendon function. Therefore, the position in which the isometric phase occurs may influence muscle structure and should be reported.

Basically, a longer time duration leads to a greater accumulation of metabolites and a possible increase in the hypertrophic stimulus.

Short-term studies have reported markers of muscle damage mainly induced by a single eccentric-based session, although such damage is necessary to provide the muscle with a protective effect against subsequent eccentric exercise, ie. repetition effect.

Chronically, although not all included studies matched training protocols for total volume, various meta-analyses reported possible advantages in strength and hypertrophic gains for eccentric versus concentric training.

Interestingly, eccentric or concentric training is associated with different typical adaptations in muscle architecture, with the former causing a more pronounced elongation of the fascicles, while the latter leads to an increase in the pennation angle.

However, while training based on the eccentric-concentric principle has attracted much attention, the inclusion of traditional concentric-eccentric protocols in the comparison is rare.

In a study in which training volume was matched with a combination of total repetitions, load, displacement, and total time under tension, the authors reported that concentric, eccentric, and traditional concentric-eccentric resistance training performed by trained men produced similar gains in bench press 1-RM, although the eccentricity-based protocol was the only protocol that induced hypertrophic effects and improved muscular endurance and maintained all adaptations after the detraining period.

Using a more comprehensive design, another study reported that iso-loading concentric-based, eccentric-based, and a traditional concentric-eccentric protocol led to increases in concentric, isometric, and eccentric strength, although eccentric strength gains were lower after concentric training.

Moreover, the inclusion of the eccentric phase alone stimulated fascicle elongation and all protocols induced a widening of the pennation angle, while the hypertrophic stimulus was greater when the eccentric phase was included.

Finally, retention of results after the detraining period was more pronounced when the eccentric phase was performed. For all the above reasons, it should be reported whether the exercise includes the performance of both concentric and eccentric phases or only one of them.

By the way, performing one or two phases has repercussions on the overall displacement of the external load and should therefore be taken into account when calculating exercise volume.

Time under tension, muscle damage, activation of motor units, and mind & muscle connection can have an impact on hypertrophy, but only if the variable of mechanical tension is satisfied.

In isometric contractions, a greater hypertrophic stimulus causes training of a controlled duration with greater muscle length, as well as better tendon function. Basically, a longer duration leads to a higher accumulation metabolites and possible increase in hypertrophy. Training with low weights to failure provides the same hypertrophy stimulus as training with heavy weights. The difference is that with less weight loss, greater fatigue occurs centrally and peripherally, while with larger weight loss, a greater increase in strength is observed. Fast eccentric contractions, although they engage a larger number of motor units, do not necessarily cause hypertrophy, for that mechanical tension is required – for example, supramaximal eccentric training.

Hypertrophy occurs when a single muscle fiber increases its volume. It can increase it longitudinally (increasing the length of the fascicles) or diametrically (by increasing the CSA). If mechanical tension comes from active elements, it results in diametrical hypertrophy – example concentric training. If the mechanical tension comes from passive elements, then it most often results in longitudinal hypertrophy. The best example for that is eccentric training.

A proper and individual approach to exercise requires a good knowledge of the human body, traditional and modern training methods, application of technology and experience. Also, the desired results and the best and fastest way to achieve them, while reducing the risk of injury, must be kept in mind all the time.