Time under tension has been a topic under continuous discussion when it comes to the science of hypertrophy. But, it is a model many individuals do not have a clear understanding of.

In a technical point of view, time under tension should be a wonderful tool of determining the hypertrophic stimulus resulting from a workout. Unfortunately, researchers have not succeeded in establishing a connection between time under tension and the muscle gained.

Many literary works have been written but they conflict with each other. Some agree that time under tension has a direct link to the amount of muscled gained after undergoing strength training exercise. Other literature reports say that there is no link between the two.

A report from a study shows there is an exposure-response relationship that exists between hypertrophy and the training volume, while there is none between the lifting tempo and the gain in muscles. However, the tempo is regarded as an effective method of improving the time used to undertake strength training exercises.

The confusion arises due to the absence of the ancient definition of which muscle fibers that get the impact from the tension. The magnitude of the tension that should be experienced has not been defined as well. It is true that after activation of all muscle fibers and having a high tension, time under tension can be connected to hypertrophy as the outcome of the training.   

If time under tension definition is aimed at solving problems human beings face, many of inconsistencies may vanish.

Let’s get to the explanation.

Factors that stimulate hypertrophy.

Hypertrophy happens when the volume of single muscle fibers within a muscle increases. The fibers increase due to mechanical loading stimulus.

A number of researchers suggest that the damage of muscles or the metabolic stress are some of the factors contributing to hypertrophy. They are hypotheses with no truth to back them up. Most of the results reported are attributed to mechanical loading.

Mechanical loading stimulus is explained as the force exerted by the fiber causing an increase in the individual muscle fiber’s volume. The force has to be beyond a given threshold to be able to initiate hypertrophy.

The high force is achieved by actively contracting the fiber at low speed. The fiber’s shortening speed determines the force exerted. The process is commonly referred to as the force-velocity relationship. To achieve slow shortening speeds, contraction of muscles should be against heavy loads or under wearing circumstances.

With slow shortening speeds of the fiber, there is the exertion of high magnitude forces for they include more concurrently attached actin-myosin crossbridges where the force is produced.

Researchers have realized that, through experiments, they are able to increase the number of attached crossbridges by improving force exerted by any single muscle fiber. If they increase the muscle fiber’s velocity of contraction, it leads to a decrease in the number of attached crossbridges. If the shortening of the fiber happens slowly, the attached crossbridges can stay attached for long, which results in an increase in the force exerted.

You need to know that muscles have countless fibers structured in groups of motor units. Each muscle has hundreds of motor units arranged in terms of size, from small motor units with a low threshold to the large units with a high threshold.

The low-threshold motor units manage small numbers of muscle fibers that are relatively unresponsive, which are less affected by mechanical loading stimulus. After being subjected to the stimulus, they do not show much growth. On the other hand, the motor units with a high-threshold control many thousands of muscle fibers that are very responsive, which grow rapidly after application of mechanical loading stimuli. Such motor units can go overboard into controlling the slow and fast twitch fibers.

During the slow shortening of the fibers, contractions that employ motor units of a high threshold are known to contribute to hypertrophy. The use of the units with low threshold contributes to less growth in muscle since they control small numbers of muscle fibers that are less responsive.

How time under tension can be redefined

From ancient times, time under tension was explained as the time used in undertaking muscular contractions during a strength training exercise, by ensuring that the time difference between sets and reps are recorded.

As long as there is no use of heavy loads, the definition will hold to include the time in which motor units of high threshold are not employed, and may also include the duration when the shortening speeds of muscle fiber are too swift for mechanical loading to attain the minimum threshold to contribute to growth of muscles. Evidently, such is not an effective way to record hypertrophic stimulus quantity.

To bring usefulness to time under tension, it should solely refer to biological conditions that cause growth in muscles. 

With an updated comprehension of the working of hypertrophy, the definition has to refer to the time when the only involved motor units are those of high threshold, while there is a slow shortening of muscles. Therefore, the definition will refer to the type of muscle fibers to which tension is exerted and the magnitude of tension applied by referring to the shortening speed of the muscle fibers.

#1. The muscle fibers experiencing the tension

During muscular contractions, whether aerobic exercise or strength training, force production is controlled by the motor units. Generally, endurance activities like cycling, running and swimming lead to slow repetitive limb movements. Thus, shortening of muscle fibers happens at a slow speed giving room for each working fiber to produce high force. When closely observed, the effort related to each movement is relatively low compared to the highest limit that can be exerted. It can be said that the force is produced by motor units of the low-threshold fibers.

Exerting tension for an extended time to the motor units of low-threshold fibers through aerobic exercises is not a direct contributor to gaining muscles. All kinds of muscle fibers reduce in size when one is involved in long distance running, despite the involvement of long period of time under tension for the fibers that are controlled by the motor units of low-threshold.

If the type of muscle fiber being subjected to tension is not considered in the definition, then there may be the wrong assumption that slow movement endurance exercises can result in great hypertrophy in the muscle fibers that are under the control of motor units of low threshold. Therefore, the definition of time under tension ought to refer to the time when tension is exerted to motor units of the high-threshold fibers only.

#2. The magnitude of the applied tension

When low tension is applied to the motor units of high-threshold muscle fibers and allowing quick shortening does not result in the growth of the muscle. High-speed movements incorporate motor unit of high levels but vertical umping programs do not result in essential hypertrophy. The velocity of movement is essential in determining the level of muscle growth which emanates from strength training, with no regard to the involvement of the motor unit.

Motor units of a high threshold can be employed without necessarily motivating growth of their fibers since hypertrophic stimulus magnitude determines the mechanical loading, and not the magnitude of the motor unit being involved. From studies, it can be deduced that inhibition of myosin functioning in muscle contractions causes the prevention of hypertrophy. Therefore, muscle growth is triggered by tension emanating from actin-myosin crossbidges. It does not rely on the activation of the muscle fibers.

Thus, if the definition of time under tension will not include the level or magnitude of the tension on muscle fibers, we could assume that performing a high volume of quick movements in succession without getting tired can result to hypertrophy. Thus, the definition for the time under tension ought to refer to the timespan for which tension of a given magnitude above a stated threshold is exerted on muscle fibers. The process will need the muscle fiber to experience slow shortening velocity.

Importance of the New Definition

With the traditional definition, the time recorded will vary with the lifting tempo applied. Slow lifting tempos will take a longer time under tension as compared to the faster ones. Such recording will pose a problem when it comes to hypertrophy science. Slow lifting tempos won’t trigger higher levels of muscle growth yet time under tension is to be established as a great measure of hypertrophic stimulus dosage.

Good news! The new definition helps in explaining why this occurs. The new definition involves the amount of time of exposing the fibers of high-threshold motor units to mechanical loading as an outcome of slow shortening. The process can be called ‘stimulating time under tension.’

When one makes a comparison of stimulating time under tension for several sets of exercises on strength training with the slow and fast lifting tempos, there is no difference.

Why will the stimulating time under tension stay same amid the changing lifting tempos? You will realize that the involvement of the motor unit is increased with fatigue. Therefore, we will note reps performed when there is fatigue and in its absence.

Absence of Fatigue

In this case, the force exerted by the whole muscle at any given speed depends on:

  1. How many motor units are used determined by effort level and hence a number of muscle fibers that are activated.
  2. The activated muscle fibers’ shortening speed  

With Fatigue

In this case, the force exerted by the whole muscle at any given speed depends on:

  1. Number of motor units used
  2. Activated muscle fibers’ speed of shortening
  3. The level of fatigue for the muscle fibers that are in use.

Summary

Hypertrophic stimulus resulting from a workout is well determined with time under tension on condition that there is a recording of time when high tension is exerted to motor units of the high threshold. The rate of tempo is not a determinant. The final reps contribute to the stimulation of muscle growth.

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