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Time Under Tension And Muscle Growth

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The Meal Prep Manual · Time Under Tension And Muscle Growth

How many times have you heard a gym bro say that time under tension is the key to muscle growth?  If you’ve spent a lot of time in commercial gyms over the years I’m sure it’s a lot.  I’ve even seen this idea begin to leak into some fitspo culture on social media as well which is why I felt the need to write about time under tension training for this month’s article.  Unfortunately for all of us, many of the trendy beliefs that cycle through the fitness world are evoked by these fitspos since they have the largest audiences.  Thank god the greatest hallmark of credibility in our industry is having a fat ass.  Kidding.  It makes me blood red mad.  Half of these people don’t even have training certifications and I’d be surprised if even 1% of them knew a goddamn thing about muscle biology.  But as many of you know, even a blind squirrel can find a nut once in a while.  Did the gym bros and fitspos get this one right?  Is time under tension training actually a beneficial and productive way to train? Let’s find out. 

NOTE: I think it is important for me to say that there are many ways to go about training and the type of training you choose to participate in should be specific to your goals.  If your goal for exercise is simply to improve your health then you’ll have more freedom to train in whatever way you want.  When I am writing and speaking about these topics I try to come at the idea from the standpoint of maximizing the effectiveness of your training dependent on the goal that is being discussed.  In this instance, we are discussing if increasing time under tension leads to greater muscle growth.  I am trying to convey my understanding of the scientific literature, what I’ve learned from experts in the field, and my own practical experience in the gym.  I am not saying that the methods and ideas discussed here are the only way to gain muscle, but they are probably the most effective from my understanding.  

What in the sam hell is time under tension?

Time under tension is exactly what it sounds like.  It is the time your muscles spend experiencing tension during an exercise.  Mechanical tension is detected by mechanoreceptors on your muscle fibers. It is a result of force generation or stretch in the individual muscle fibers.  Increasing time under tension can be increased by elongating the concentric portion of the movement, the eccentric portion of the movement or both.  It can also be achieved by increasing the number of repetitions performed.  Time under tension is additive.  It is simply the summation of time your muscles spend experiencing tension.  If the time spent under tension was the key to driving muscular growth, then that would mean measuring the dose of time under tension would be the optimal way to go about growing muscle.  It is fairly well established in the literature that muscle growth and training volume follow a dose-response relationship but when we look at lifting tempo (thus increasing time under tension) with muscle growth, the same result isn’t seen.¹  

What actually causes muscle growth?

The answer to this question is multifactorial and incredibly dense.  To answer it in a paragraph or two would not be doing it justice but to understand the question at hand (and really any question on growing muscle) you need to at least have an understanding of the basics.  Muscle growth occurs as a result of individual muscle fibers increasing in volume after being exposed to a loading stimulus and mechanical tension.  The stimuli are the forces that are exerted by the individual muscle fibers.  It is important to understand that the tension experienced by the individual fibers is what matters, not the tension felt by the muscle as a whole.  This makes sense because it is the individual muscle fibers that grow as a result of training, not the muscle as a whole. You have to make the things that make up the muscle bigger to make the actual muscle bigger.²  

Motor Units

A motor unit is defined as a group of muscle fibers and a motor neuron that innervates it.  When our body wants to move a muscle, our central nervous system sends an electrical signal down the motor neuron, to the motor unit, telling all of the muscle fibers within that motor unit to fire and shorten as quickly as possible to produce force.  We have both low threshold and high threshold motor units and they are recruited in order of the size of the motor neuron. Low threshold motor units are recruited when only low levels of force are required and they only govern a small number of muscle fibers (dozens).  High threshold motor units aren’t recruited until certain recruitment thresholds are met and the force requirement is high enough. This is to help conserve energy and to be more efficient as high threshold motor units govern many muscle fibers (hundreds or thousands) and make up the real meat of all of your muscles.  They are also more responsive to training than low threshold motor units.  It is hypothesized that the reason low threshold motor units don’t contribute much to muscle growth is 1) because they only govern a small number of muscle fibers and 2) because they are recruited first and at low force levels, they are activated many times throughout daily life so it is possible they have already achieved their maximum potential for size.  

The Force Velocity Relationship

The amount of force a muscle fiber can produce when it is not fatigued is determined by the force velocity relationship.  The force velocity relationship shows that when a fiber contracts slowly during the concentric portion of the movement, it can produce higher levels of force.  If a fiber were to contract quickly, it wouldn’t be able to produce as much force.  The reason for this has to do with the contractile proteins that lead to muscle contraction.  Again, this is one of those topics that could be discussed for hours but we obviously aren’t going to do that here.  Actin and myosin interact with each other via crossbridge formation which is what makes muscles shorten to produce force.  At slow contraction speeds, more crossbridges are able to form leading to greater forces.  At fast contraction speeds, you can’t form as many of these crossbridges so force is diminished.  So slow contractions speeds means more force in the active fiber, great.  That must mean that slow reps and increasing time under tension is better for growing muscle right? Not exactly.  Stay tuned.  Maybe you’ve been able to piece together why this may not be the case already.

Applications of Time Under Tension Training

Tempo Repetitions

One way you will see people trying to increase the time under tension during training is to intentionally slow down the speed of the repetition on both the concentric (lifting) and eccentric (lowering) portions.  The fit chicks on instagram love doing this.  It is a classic example of the idea that what is hard must be better.  Just because you feel the burn and the exercise is difficult doesn’t mean that it is going to help you build muscle more effectively.  Consciously and intentionally slowing down the tempo of a repetition means that you are producing a submaximal effort.  You would not be able to control the speed of a repetition with a heavier weight that required a maximal effort and the speed of the rep would be dictated by your body’s ability to generate force.  

Force is equal to mass times acceleration.  In order to move a weight you must create a force with your muscles that is greater than the force that gravity is accelerating the given mass back down towards the earth.  If you were to take a lightweight and control the tempo of the repetition so that it was slower in order to increase the time under tension experienced by the muscle fibers, it would require less force than if you were to lift it with a maximal effort.  Because you are performing a submaximal effort and the force needed to complete the repetition is low, motor unit recruitment is also low and only a small number of muscle fibers are activated.  Deliberatley slowing down the speed of the rep of a submaximal load causes the weight to accelerate less and thus leading to less force production as the level of effort is low.  Our motor units are recruited at different force thresholds (low and high threshold) and the order of recruitment is based on the size of the neuron that innervates each motor unit and the amount of force required.  Low threshold motor units only govern a small number of fibers that are not very responsive to training.  In order to see meaningful growth, you need to recruit the high threshold motor units.  You can achieve this with slow repetitions but only when the speed of the rep is mediated and slowed by fatigue, not a conscious decision to slow it down.  As fatigue accumulates and the muscle fibers producing force are unable to keep up with the demand, higher threshold motor units will be recruited to continue with the movement.  

Remember our muscle fibers are what grow as a result of stimulative training and the majority of are muscle fibers are grouped together in the high threshold motor units which are only activated at certain recruitment thresholds.  If you don’t ever pass these recruitment thresholds, you won’t stimulate the muscle fibers and lead them to grow.  So if your slow tempo reps reach the place where the speed of the rep isn’t mediated by you intentionally slowing the rep and the slow contraction speeds are a result of fatigue, then it is likely just as good for muscle growth as normal speed reps that are also slowed by fatigue as the set progresses.  That means that utilizing slow tempo reps as a way to generate fatigue to recruit high threshold motor units can be a way to grow muscle, but it has not been shown to be better than any normal reps that achieve the same recruitment levels.  

Slow Eccentrics

Where the fitspo girls on instagram love the slow tempo reps, the gym bros that are entirely too big love the slow eccentrics.  Increasing the time under tension via slowing down the eccentric portion of the lift is probably the most popular way for exercisers to weave time under tension style training into their programs.  In this application, the concentric portion is performed as fast as possible and the eccentric is exaggerated to last a few to many seconds.  As the set progresses, the concentric portion of the lift becomes slower due to the effects of fatigue.  

If I were looking to incorporate time under tension style of training into my own plan this is the way I would do it.  Keeping the concentric portion of the lift at maximal velocity has been shown to be more beneficial in gaining strength.³⁴  As long as the set is taken close to failure and the velocity of the concentric portion of the lift is slowed due to fatigue, the outcomes in terms of hypertrophy are likely equivocal between slow tempo reps and maximal intended velocity.  In a previous section we discussed how slow contraction speeds are better for growing muscle because they allow each individual muscle fiber to experience greater tension and produce more force since more crossbridges between actin and myosin are allowed to form.  It is possible that the first few reps of a set are not ideal for growing muscle as the speed of the repetition is too fast to generate sufficient tension in the muscle fibers.  But the reason for continuing the set until you get close to failure is so that you can reach the point in which fatigue causes the subsequent rep speed to decrease and these muscle fibers of the high threshold motor units to experience more tension.  Slowing down the eccentric portion of the lift is one way to decrease the amount of repetitions it would normally take to reach this level of fatigue. 

The speed of the eccentric can really be whatever you want it to be as long as you are not just dropping the weight and allowing gravity to take over.  Control the weight down in a way that is comfortable and allows you to maintain proper form.  Whether that is 1 second or 5 seconds probably doesn’t matter.  However, it’s likely that there is a point of diminishing returns when it comes to slowing down the eccentric.  For example, doing a 30 second eccentric on your bicep curl is surely overkill.  The slower the eccentric portion is the less you will be able to utilize the stretch reflex to assist with the concentric. The stretch reflex is why the start of the second rep (and each following) feels easier than the first rep.  The stretch reflex makes it easier than starting from a dead stop, like bouncing out of a squat for example.  It’s a protective mechanism for your muscles to prevent over-stretching.

Adding More Volume

A less obvious way to increase time under tension in your training is to just add more volume by either increasing sets or reps.  Many people don’t consider this as time under tension training because time under tension training is so commonly defined as being slower reps.  Theoretically any way that you increase the amount of time your muscles feel tenison is a way to increase time under tension.  Now, there is a dose-response relationship between muscle growth and training volume.  The literature has pretty consistently shown that more generally equals better but overtraining is a real phenomenon.  The relation between muscle growth and total volume likely follows an inverted U style curve where there exists a point that more volume would eventually lead to less gains.  The point at which this occurs isn’t exactly well defined.  It will be different for everyone.  Your ability to recover, sleep quality, nutrition, daily life, and many other factors will all contribute to how much volume you can handle.  The point being, increasing volume as a way to increase time under tension probably will increase muscle growth as long as you are training intelligently.  

By train intelligently I am referring to maintaining all other best practices when it comes to growing muscle.  A good example of when adding more volume won’t lead to more muscle growth is in high velocity movements like jumping.  Jumping leads to fast contraction velocities which means that each individual muscle fiber produces less force and the tension they are subjected to is less than if it were to contract slowly.  This isn’t going to lead to any meaningful muscle growth but you could theoretically jump all day long and have a very high level of time under tension with unimpressive growth.  Another example would be long distance runners.  It’s not uncommon for these people to run for 2 hours at a time.  That’s a lot of time under tension so why are they string beans and not jacked out of their minds? Because aerobic exercise utilizes the low threshold motor units, the high threshold motor units and muscle fibers inside of them do not experience any tension.

So Time Under Tension Style Training Isn’t Better?

You’ve got it.  Training is actually pretty straightforward.  Lift weights.  Make sure it’s hard.  Be specific.  And be consistent.  If you get those things correct then you are almost certainly going to grow.  Any fancy training methodologies or fads are really just round about ways to get to the topics we discussed in this article.  I am not aware of any of these trends that work to grow muscle in a different mechanism than the one that we already know works.  Train in a way that leads to the recruitment of high threshold motor units and slow contraction velocities in order to generate as much tension as possible in the individual muscle fibers of these high threshold motor units.  The mechanism behind why just about any of these advanced and trendy training methods work is because they achieve just that.  If you prefer to keep your reps slow and controlled or you like to maintain 3 second eccentrics, be my guest.  The amount of muscle growth that you see from this will probably be equivalent if you trained normally as long as all of your sets were brought close to failure.  This is fantastic news.  It means that training isn’t this complex puzzle that people make it out to be.  As with all things in the worlds of fitness and nutrition, do what you like doing.  Adherence is the underlying principle that rarely gets discussed when it comes to trendy ideas.  If you can’t be consistent in your diet or training then it is not going to work.  Brick by brick baby.  Put in the work and you will reap what you sow.


  1. Schoenfeld, B. J., Ogborn, D., & Krieger, J. W. (2016). Dose-response relationship between weekly resistance training volume and increases in muscle mass: A systematic review and meta-analysis. Journal of Sports Sciences, 35(11), 1073-1082. doi:10.1080/02640414.2016.1210197
  2. Schoenfeld, B. J. (2010). The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training. Journal of Strength and Conditioning Research24(10), 2857–2872. doi: 10.1519/jsc.0b013e3181e840f3
  3. Pareja-Blanco, F., Rodríguez-Rosell, D., Sánchez-Medina, L., Gorostiaga, E., & González-Badillo, J. (2014). Effect of Movement Velocity during Resistance Training on Neuromuscular Performance. International Journal of Sports Medicine, 35(11), 916-924. doi:10.1055/s-0033-1363985
  4. Paddon-Jones, D., Lonergan, A., Abernethy, P., & Leveritt, M. (2001). Adaptation to chronic eccentric exercise in humans: The influence of contraction velocity. European Journal of Applied Physiology, 85(5), 466-471. doi:10.1007/s004210100467

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