Time under tension is a huge buzz term in the bodybuilding community. Everything gains-related has to deal with time under tension (TUT) and just about every guru will tell you that you have to perform slow, controlled reps at submaximal weights to maximize TUT. Is TUT the main driver for muscle growth? Let’s discuss.
The first thing worth understanding is that focusing on TUT can help maximize two specific components of training that are related to muscle growth with the first being volume. Typically, one focusing on increasing their total TUT will do so by increasing their total training volume. Training volume has been shown to have a dose-response relationship with muscle growth (11), so increasing training volume should increase gains. The second component is tension. When a muscle contracts to lift a weight, the muscle fibers experience mechanical tension while they shorten to move the joint. Mechanical tension is probably the main determinant of muscle hypertrophy (16), so the more tension you can create, the more gains you’ll get.
Now, let’s examine the typical strategy when trying to maximize TUT: moving submaximal weights at slower tempos so that the muscle is constantly under tension – and tension is a good thing right? According to the force-velocity curve, a muscle produces its greatest force and experiences greater tension at slow contraction velocities (5). However, the tension a muscle experiences will only be an equal and opposite force to the load that it is lifting (2). Therefore, if you are using a lighter weight and moving at slow velocities, you still might not be undergoing much tension.
This directly leads us into our next physiological speed bump: the Size Principle. The Size Principle states that motor units will be recruited based off of their size, i.e. low threshold motor units (LTMUs) are recruited for easy movements at lighter weights while high threshold motor units (HTMUs) are recruited to move heavier weights and/or higher velocites (5). Therefore, if you are moving submaximal weights slowly, you’re not going to tap into the high threshold motor units very much (2). This represents a significant problem when considering gains, because low threshold motor units don’t really grow in response to training (8). To develop a muscle, you need to recruit the HTMUs to a high degree as they are the muscle fibers that are going to respond by growing (8).
We see this phenomenon occur with long distance runners. Long distance runners undergo insane amounts of time under tension. However, long distance running has actually been shown to decrease muscle fiber size (15). Why is this the case? Running does not require much force production and therefore does not recruit HTMUs to a nearly high enough degree to cause growth.
Okay, so we know we need to maximize tension and volume to grow, but moving lighter weights slowly doesn’t do the trick. What can we do? First of all, we need to examine the ways that you can fully recruit the HTMUs of a specific muscle group. The first way is through performing maximum velocity movements, like jumping (2). However, even though jumping produces high amounts of muscle activation, it still does not result in muscle growth (3). Why? Jumping doesn’t involve any external load, so the muscle fibers still don’t undergo much tension.
The second way to fully recruit HTMUs is by training close to, or to, failure. Studies have shown that muscle recruitment increases throughout a fatiguing set (1) and that a muscle is maximally recruited in the final 3-5 reps of a set leading to failure (14). The best part about this method is that the velocity of movement typically slows during fatiguing conditions (7,9) which means that the muscle is producing a ton of force (due to the force-velocity curve) and therefore undergoing maximum amounts of tension. The problem with training to failure, besides safety issues of course, is that training to failure multiple times in a workout can decrease the total volume for that workout (17) which would directly affect the volume-gains relationship we mentioned earlier. Training to failure too often can also result in high amounts of muscle soreness which may impair training throughout the rest of the week (10).
Training to failure is a great way to recruit HTMUs, but there is one more way you can attack these muscles to force growth: train heavy! Studies show that lifting at intensities above 80% of your 1RM maximizes muscle activation (6). Moving weights above 80% of your 1RM will also result in slower contraction velocities which will induce insane amounts of tension on the muscle.
So what do all of these points mean? There’s a few different ways to maximize the tension that muscle fiber can experience – training to/close to failure and using heavy loads are the primary ways. But we still haven’t discussed the concept of time. Why? Because it seems pretty irrelevant. Several studies have shown that the repetition speed does not affect muscle gains when equated for load (12). Another study found that moving heavy loads at faster velocities resulted in significantly higher muscle growth than moving light loads at slow velocities (13). Everything boils down to tension being WAY more important than time.
At the end of the day, don’t worry so much about your total time under tension. Instead, think about the amount of stimulating reps you delivered to the muscle you were training (2). Stimulating reps are the reps that are performed at high levels of muscle activation and at slower contraction velocities. AKA, the last few reps in a set to failure, or all of the reps in sets at or above 80% of your 1RM for that particular exercise. Use training to failure as a tool in your program as training to failure too often can negatively affect gains (10). Your best bet for increasing the amount of stimulating reps you perform is by lifting heavy and doing so often!
Special thanks to Dr. Chris Beardsley for his phenomenal write-up on Time Under Tension – if you’re interested in an even more in-depth discussion of the topic check out his article here for more info.
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