Time under tension is one of the most commonly discussed concepts in the science of hypertrophy, and yet it remains poorly understood.
Technically, time under tension should be a good measure of the dosage of the hypertrophic stimulus provided by a workout. Unfortunately, researchers have as yet been unable to make connect time under tension with the amount of muscle growth that results, under all circumstances.
Indeed, there are many conflicting reports in the literature, some of which suggest that time under tension is closely linked to the amount of muscle growth that occurs after strength training, while others suggest it is not.
Research has reported a dose-response relationship between training volume and hypertrophy, but no such relationship between lifting (concentric phase) tempo and muscle growth, despite tempo being a very effective way for increasing the duration of time spent performing a set of a strength training exercise.
In my view, this confusion arises because we have traditionally not defined which muscle fibers are being subjected to tension, nor have we defined the level of tension that must be experienced. Indeed, when all muscle fibers are activated, and the tension is high, time under tension can be linked to the hypertrophy that results from training.
If we fix these problems with our definition of time under tension, I believe that these inconsistencies mostly disappear.
Let me explain.
What stimulates hypertrophy?
Hypertrophy is mainly the result of single muscle fibers inside a muscle increasing in volume. Single muscle fibers grow once they are subjected to a mechanical loading stimulus.
Some researchers have suggested that hypertrophy might also be triggered by metabolic stress or muscle damage, but these hypotheses are not necessary to explain the current research literature. Mechanical loading can account for all of the results that have been reported to date.
The mechanical loading stimulus that causes an individual muscle fiber to increase in volume is the force exerted by the fiber itself. This force needs to be above a certain threshold, because forces that are too low do not trigger hypertrophy.
To achieve this high force, a fiber needs to contract actively at a slow speed, because the shortening speed of a fiber is the main determinant of the force it produces. This is known as the force-velocity relationship. Slow shortening speeds are produced either when muscles contract against heavy loads, or contract under fatiguing conditions.
Slow fiber shortening speeds allow greater forces because they involve more simultaneously attached actin-myosin crossbridges, and it is the attached actin-myosin crossbridges that produce force.
Indeed, researchers have found that if they experimentally increase the force produced by a single muscle fiber, the number of its attached crossbridges increases. Conversely, when they experimentally increase the contraction velocity of the muscle fiber, the number of attached crossbridges decreases. When a fiber shortens more slowly, each of its crossbridges can remain attached for longer, and this increases the amount of force it can exert.
However, muscles contain many thousands of fibers, which are organized into groups of motor units. There are hundreds of motor units in each muscle, and they are recruited in order of size, from small, low-threshold motor units to large, high-threshold motor units.
Low-threshold motor units govern small numbers (dozens) of comparatively unresponsive muscle fibers, which do not grow very much after being subjected to a mechanical loading stimulus. High-threshold motor units govern large numbers (thousands) of highly responsive muscle fibers, which grow substantially after being subjected to a mechanical loading stimulus. Such motor units might control both slow twitch and fast twitch fibers, or solely fast twitch fibers, depending on the fiber proportions of the muscle.
Only those contractions that involve the recruitment of high-threshold motor units while muscle fibers are shortening slowly will stimulate meaningful amounts of hypertrophy. The recruitment of low-threshold motor units does not stimulate very much muscle growth, because such motor units govern only a small number of relatively unresponsive muscle fibers.
How can we define time under tension?
Traditionally, time under tension has been defined as the time spent carrying out muscular contractions as part a strength training exercise, usually by timing the duration of the sets and reps.
Unless heavy loads are used, this definition will include time when high-threshold motor units are not recruited, and may also record time when muscle fiber shortening speeds are too fast for mechanical loading to reach the required threshold to stimulate muscle growth. Clearly, this will not be a useful way of recording the dosage of the hypertrophic stimulus.
For time under tension to be a meaningful measurement of the hypertrophic stimulus, it needs to refer only to the biological conditions that lead to muscle growth.
Based on our current understanding of how hypertrophy works, such a definition needs to refer to the time for which only the high-threshold motor units are recruited, while the muscle shortens slowly. This means that the definition needs to refer to: (1) which muscle fibers are being subjected to tension, and (2) the level of tension that is applied, by reference to the speed that the muscle fibers are shortening.
#1. Which muscle fibers are subjected to tension
Motor units control the production of force in much the same way during all types of muscular contractions, regardless of whether these contractions are classified as strength training or aerobic exercise.
In most cases, the repetitive limb movements of endurance activities like running, cycling, and swimming are not quick. Therefore, muscle fiber shortening speeds are slow, and this allows a fairly high force to be produced by each working fiber. Given that the level of effort involved in each movement is low compared with the maximal amount that could be exerted, this force is likely produced by the fibers of low-threshold motor units.
Exposing the fibers of low-threshold motor units to tension for long periods of time in the form of aerobic exercise does not stimulate any meaningful muscle growth. Long distance running reduces the size of muscle fibers of all types, despite involving a very long duration of time under tension for the fibers governed by low-threshold motor units.
Therefore, if we do not word our definition of time under tension to refer to which muscle fibers are subjected to tension, then we might wrongly assume that endurance exercise involving slow movement speeds would produce a lot of hypertrophy in the muscle fibers controlled by low-threshold motor units. Consequently, our definition for time under tension should refer to the time for which only fibers of high-threshold motor units are subjected to tension.
#2. The level of tension that is applied
Subjecting the muscle fibers of high-threshold motor units to low levels of tension, by allowing them to shorten quickly, does not cause muscle growth.
Vertical jumping programs do not cause meaningful hypertrophy, although high-velocity movements involve very high levels of motor unit recruitment. And animal model studies have confirmed that actual movement velocity is critical for the amount of muscle growth that results from strength training, irrespective of the level of motor unit recruitment.
High-threshold motor units can be recruited without their fibers being stimulated to grow, because it is mechanical loading that determines the magnitude of the hypertrophic stimulus, and not the degree of motor unit recruitment.
Studies that have inhibited the actions of myosin during muscle contractions (without affecting the calcium ion activity resulting from motor unit function), have shown that hypertrophy is prevented. This reveals that it is the tension produced by actin-myosin crossbridges forming that triggers muscle growth, and not whether a muscle fiber is activated.
Therefore, if we do not word our definition of time under tension to refer to the level of tension that is experienced by muscle fibers, we might assume that hypertrophy could result from doing a high volume of fast movements without fatigue, otherwise known as “jumping up and down all day long.” Consequently, our definition for time under tension should refer to the time for which muscle fibers are subjected to a level of tension that is above a certain threshold, which requires a slow muscle fiber shortening velocity.
How does this new definition help us?
If we apply a traditional definition of time under tension, the time we record differs quite a bit depending on the lifting (concentric phase) tempo that is used. Slower lifting tempos typically involve a much longer time under tension than faster lifting tempos.
This is a big problem for hypertrophy science, because slow lifting tempos do not stimulate greater muscle growth, but time under tension is supposed to be a good measurement of the dosage of the hypertrophic stimulus.
Fortunately, our new definition of time under tension can help us explain why this happens.
Our new definition only includes the amount of time for which fibers of high-threshold motor units are subjected to the level of mechanical loading that results from them shortening slowly. We could refer to this as the “stimulating time under tension.”
When we compare the stimulating time under tension between sets of strength training exercise with fast and slow lifting tempos, we find that it is not that different.
Here is why.
Why is stimulating time under tension similar regardless of lifting tempo?
To see how stimulating time under tension differs between sets of strength training exercise performed with a different lifting tempo, it helps to consider reps performed with and without fatigue, since fatigue increases motor unit recruitment.
Without fatigue
The amount of force that a whole muscle exerts at any speed when fatigue is absent is largely determined by two factors:
- The number of motor units that are recruited, and therefore the number of activated muscle fibers.
- The shortening speed of the activated muscle fibers, which is determined by the force-velocity relationship.
Broadly speaking, motor unit recruitment levels are determined by the level of effort, while the force-velocity relationship determines the actual amount of force that corresponds to that level of effort.
What happens in practice?
In fact, the effects vary depending on the load.
When lifting light or moderate loads, using a submaximal effort (a slow tempo) does not recruit high-threshold motor units. Therefore, time spent doing these reps cannot be counted as stimulating time under tension.
When lifting light or moderate loads, using a maximal effort does recruit high-threshold motor units, but the shortening speed of each fiber is too fast for mechanical loading to reach the required threshold. Therefore, time spent doing these reps cannot be counted as stimulating time under tension.
When lifting heavy loads (equal to or heavier than 5RM), lifting a weight with either maximal or submaximal effort does recruit high-threshold motor units and involves a slow fiber shortening speed. Time spent doing these reps can be counted as stimulating time under tension. Even so, stimulating time under tension will not differ substantially between maximal or submaximal effort tempos, because maximal bar speed is already slow!
In those rare cases where an extremely slow tempo is used with a heavy load, and the resulting bar speed differs substantially from the bar speed achieved when applying maximal effort, this extremely slow tempo will necessarily involve fewer reps being done before failure, and this will broadly equate the stimulating time under tension.
Under fatiguing conditions
The amount of force that a whole muscle exerts at any speed when fatigue is present is largely determined by three factors:
- The number of motor units that are recruited, and therefore the number of activated muscle fibers.
- The shortening speed of the activated muscle fibers, which is determined by the force-velocity relationship.
- The state of fatigue of the working muscle fibers.
Again, motor unit recruitment levels are determined by the level of effort, while the force-velocity relationship and the state of fatigue of the working muscle fibers together determine the resulting amount of force corresponding to that level of effort.
What happens in practice?
When lifting heavy loads, the effects are the same as when lifting without fatigue.
When lifting light or moderate loads with a submaximal bar speed, fatigue increases the level of motor unit recruitment, activating new muscle fibers, which compensate for the reduced force produced by previously activated, but fatigued fibers. As failure approaches, the level of motor unit recruitment reaches the high-threshold motor units. This stimulates hypertrophy.
When lifting light or moderate loads while using a maximal bar speed, fatigue decreases bar speed. This reduction in bar speed increases the force that each of the working muscle fibers can produce. As failure approaches, fiber shortening speed becomes slow enough to produce a high level of mechanical loading in the working muscle fibers, which are those associated with the high-threshold motor units. This stimulates hypertrophy.
When lifting with a maximal bar speed, the actual bar speed reduces towards the end of the set such that the speed of fast and slow tempos becomes similar, just like when using heavy loads when fatigue is not present. Therefore, the duration of stimulating time under tension is very similar.
Again, in those rare cases where an extremely slow tempo is used, and the resulting bar speed in the final reps of a set to failure differs substantially from the bar speed achieved during these reps when applying maximal effort, this extremely slow tempo will necessarily involve fewer reps being done before failure, and this will broadly equate the stimulating time under tension.
What is the takeaway?
Time under tension is a good measure of the dosage of the hypertrophic stimulus provided by a workout, but only when we only record the time for which the fibers of high-threshold motor units are subjected to high levels of tension, as indicated by a slow fiber shortening speed.
Whether we use a fast or a slow tempo, the stimulating time under tension is largely the same. Only in the final reps, when bar speed has slowed down in the fast tempo set, and when motor unit recruitment has increased in the slow tempo set, is muscle growth stimulated. In these final reps, actual bar speed is largely the same in fast and most slow tempos. When bar speed still differs, the slow tempo involves fewer reps because fatigue terminates the set earlier.