I’ve spent much of the last few months (and posts) thinking about the basic training variables that influence hypertrophy, namely how altering training intensity alters the hypertrophic results from training. The main idea of these posts was that, when taken to failure, light training loads (30%-1RM) can produce comparable hypertrophy to high intensity loads (80-90%-1RM). While the location of growth may vary, being higher in type I fibres with low-load, type II fibres with high intensity (1), ultimately, whole-muscle hypertrophy is very similar across intensities when trained to failure (2).
From a training perspective, this suggests that load (intensity) compensates for time-under-tension, so that as load decreases, total work and time under tension (TUT) increase (3).This was clearly demonstrated by Burd et al (3), who found a somewhat similar protein synthetic response to a single training session at either 90%-1RM or 30%-1RM to failure, an effect that didn’t occur with light load, work-matched training (non-failure). Their subsequent training study validated that comparable hypertrophy occurs in untrained subjects trained in a similar fashion (80%-1RM vs 30%-1RM) over a number of weeks (2).
One assumption in these previous posts was that both work and time-under-tension increase as training intensity decreases. This assumption is absolutely correct assuming that all your repetitions are performed at a similar tempo but tempo, work and time-under-tension, at any given training intensity, are competing variables. If you slow down your repetitions, you ultimately perform less work, however the TUT is usually higher than when reps are performed faster. So slower tempos come with greater TUT at the expense of the total work performed during the set.
Tempo disrupts the relationship between time-under-tension and work
You probably don’t need a peer-reviewed paper to tell you this, but if you slow both the eccentric and concentric rep speed, you’ll perform fewer reps than at a faster rep speed for a given training intensity (4-6). As the number of reps impacts work, less work is performed in a slow set at an equivalent intensity, but the time-under-tension is usually much greater (5,7). Tempo then, disrupts the relationship between time-under-tension and work. This leaves us with the question, when training at a similar intensity (%1RM), is training with a slow tempo to maximize TUT, or a fast tempo to maximize work optimal for muscle hypertrophy?
Does time-under-tension trump work?
Spend any time searching for tempo and time-under-tension, and more often than not you’ll see a recent study from Burd et al (8) offered up in favour of extending time-under-tension through slow-tempo lifting. The authors compared the protein synthetic response following unilateral leg extensions completed at 30%-1RM performed either at a slow tempo (6/6) to failure, or a work-matched faster tempo (1/1). The myofibrillar, mitochondrial and sarcoplasmic protein synthetic responses were greater following the slow-tempo to failure than the work matched condition, suggesting that greater TUT is favourable when tempos are work-matched at an equivalent training intensity.
While this study may seem like increased TUT can promote a favourable protein synthetic response, we can’t be certain that it’s not related to the fact that one group trained to failure while the work-matched group did not. To really get at this question, it would’ve been great to have a third condition in the study where training was completed at the faster tempo, but to concentric failure (non-work matched). As is always the case, this study also only represents an investigation of the acute response to a single training session, which may not be indicative of the ultimate adaptations to training. Fortunately we have more to go on than studies of the acute protein synthetic response to a single training session that further our understanding of the role of tempo in hypertrophy.
Tanimoto and Ishhi (9) compared the effects of slow tempo training at 50%-1RM (3/0/3/1) to failure with training at 50%-1RM at a ‘normal’ tempo (1/0/1/1; work-matched to the slow training) and heavy, ‘normal’ training (80%-1RM, 1/0/1/1) to failure to clarify the role of intensity and tempo and muscle hypertrophy. Following 12 weeks of training, both the low-intensity slow training and heavy training produced comparable quadriceps hypertrophy (5.4%, 4.3%), whereas the work-matched low-intensity group had no growth. All groups increased their strength, however as expected, those that trained with heavy weights improved the most.
This study suggests that, when performed at an equivalent training intensity, time-under-tension compensates for reduced work with slow-tempo lifting. The fact that both the high and low intensity groups had comparable growth agrees with previous literature on training intensity (2), and the fact that they used different tempos suggests the speed of your reps may not matter for muscle growth when training to failure. Just as we saw with Burd et al (8) this study would have benefitted from another group that trained with low-intensity to failure to really clarify the relationship between tempo, work and TUT when training to failure.
In agreement with Tanimoto and Ishhi (9), Claflin et al (10) compared high and low velocity training with heavy and light training loads respectively, in both young and old males and females who completed a 14 week training program. Participants were randomized to perform training for three sets at 10RM (third set to concentric failure) at two different training velocities that influenced training load (fast = 250-250 degrees per second at the hip, 100-160 degrees per second knee, slow =30-90 degrees per second at the hip, 20-40 degrees per second at the knee).
Type I fibres were unaffected by the training program in either condition, whereas type II fibres increased in size (8.3%), and this change was independent of the velocity (and load) of training. In fact, of all the single fibre contractile properties investigated, only the increase in the peak power of the type II fibres could be explained by lifting velocity. Seemingly counterintuitive, the increase in peak power occurred due to low velocity training. Either way, these data suggest that the majority of training adaptations occurred independent of age and sex, but more importantly, of velocity (tempo) as well.
While these studies cumulatively suggest no main hypertrophic detriment to slow tempo training, the case for the indifference of work and TUT with respect to tempo is not air-tight. Recently Schuenke et al (11) investigated an array of training variables, namely the role of various training intensities and tempo on the adaptations to training. Participants were divided into four groups performing traditional strength training (TS; 80-85%-1RM, 6-10 RM, 1-2/1-2), endurance-oriented training (TE; 40-60%-1RM, 20-30 RM, 1-2/1-2), super-slow training (SS; 40-60%-1RM, 6-10RM, 10/4), or acted as sedentary controls. The TS and SS groups were matched for the number of repetitions, although performed at different intensities, and TE and SS were equated for intensity, but NOT the number of reps and ultimately work performed.
After six weeks of training, only the traditional strength and super-slow group had appreciable muscle fibre growth (TS=39%, SS=11%) although the large difference between the two groups was not statistically significant. The traditional strength group had growth across all fibre-types (I, IIa, IIx), whereas the SS group affected Type IIa and Iix fibres only. The fact that the SS group outgrew the TE group who performed more repetitions and total work indicates that, at least in this study, time-under-tension trumps work at an equivalent training intensity.
While there are some disagreements in the literature, this data supports that, when training to failure, the total amount of work and time-under-tension, both influenced by tempo, do not impact the amount of growth following training. Future studies are definitely required in order to conclusively determine the statement above, however, I wouldn’t spend too much time worrying about how your lifting tempo may be impacting your hypertrophy results.
At the end of the day, pick the speed you feel best lifting at
From a performance perspective, it’s hard to make a compelling case for slow rep speeds. Decreased work, power outputs, reps at an equivalent training intensity (6,12,13,14), and ultimately reduced strength and power gains (17-21) won’t win over anyone concerned with how well their muscles actually work (4,5,16,20).
But as far as pure hypertrophy is concerned, the culmination of these studies indicates that tempo is largely irrelevant when training to fatigue/failure (21), although TUT may still be more important than total work (11,22). The tempo of your reps determines whether you perform a high work (fast reps) or greater TUT (slow reps) set, however the muscle growth in either case may largely be the same.
I see this literature as largely freeing from a programming perspective, as it means one less training variable to lose sleep over, and indicates that you can pick the speed at which you feel most comfortable lifting without having to worry about compromising your gains. You can vary tempo across exercises without worry, or stay firmly entrenched in either the “grip it and rip it” or “move slow to grow” ideologies without missing out on extra pounds of muscle mass.
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