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Slow vs. fast reps

13 Oct 2020   ·   

A training programme planning includes the handling of variables such as exercise selection, training volume and intensity, movement speed, rest interval or the duration of a repetition. Sometimes, in spite of not receiving the necessary attention, the duration of a repetition –defined as the time taken to perform the concentric and eccentric muscle action during a repetition– is a very important concept within strength training. It is usually expressed in a sequence of three digits; the first number represents time (in seconds) taken to complete the concentric action, the second number represents the isometric transition phase between the concentric and the eccentric action, and the third number represents the time invested in completing the eccentric action. For example, a 2–0–2 tempo points out that the concentric action is performed in 2 seconds, with no isometric pause and then 2 seconds again for the eccentric phase. Therefore, in this case, the repetition would last 4 seconds. In other cases, we can find it represented in a sequence of 4 digits; the fourth number would represent the isometric or transition phase between the end of the eccentric phase and the beginning of the concentric phase. Thus, following the previous example, in a 2–0–2–1 tempo, number 1 would represent the final phase of the complete movement, and in this case, the repetition lasts 5 seconds.

Up to a certain point, the repetition duration will depend on the load intensity. Great effort is required to move heavy loads in a concentric way, and therefore, the lifting speed will be relatively slow. Speed during the concentric phase will gradually reduce as we get close to muscle failure. As an example, in an interesting research performed in the 1990s, Mookerjee y Ratamess1 observed that trained men completed the concentric movement of the first 5 MR repetition on a bench press in 1.2 seconds, while the fourth and fifth repetitions took them 2.5 and 3.3 seconds, trying to perform the movement as fast as they could in all the repetitions.

Suggestions about the speed at which a repetition for muscle hypertrophy should be performed are very different. While some authors advocate explosive movements, others prefer slow movements, mainly affecting the eccentric phase. With the aim of providing more information for this matter, Brad Schoenfeld, an expert regarding strength training and hypertrophy, performed a meta-analysis with the objective to determine if the variation in the repetition duration could affect muscle growth.2 According to the results of the study published in Sports Medicine, the optimum interval for a repetition to increase muscle mass, should be between 0.5 and 8 seconds. This interval suggests that there is a wide range in which muscle mass can be increased. However, the authors did not mention the duration of the different movement phases (for example, eccentric versus concentric), which makes it difficult to draw a definite conclusion.2

Similarly to the results obtained by Schoenfeld et al., in a study published in The Journal of Physiology, the experts Robert Morton and Stuart Phillips did not observe any differences in the activation of muscle fibres type I and II and subsequent anabolic response in a group of well-trained young individuals who performed a protocol until muscle failure with different intensities (80% of 1 MR versus 30% of 1 MR) and repetition durations (1–1–1 versus 3–1–3).3 Therefore, regardless of the intensity, for total durations between 3 and 7 seconds per repetition (remember that the interval proposed by Schoenfeld went from 0.5 to 8 seconds) the strength training seems to produce the same effect on muscle mass.3

There seems to be a threshold in 10 seconds (i.e. very slow repetitions) above which the stimulus of muscle mass increase would diminish.2 In another paper, it was observed that for the same total workload, a group that took 4 seconds per repetition (2 seconds for the concentric phase and 2 for the eccentric phase) the cross-sectional area of the muscle increased by 39% compared to only 11% in the group that performed very slow repetitions (14 seconds in total, 10 for the concentric phase and 4 for the eccentric phase).4 In other words, despite the time under tension, was three times longer in the group that performed super slow repetitions, the benefits of muscle mass increase, were almost four times less.4

Although in this regard evidence is still inconclusive, at least it does suggest that the repetition duration can cover a wide range when the main objective is to increase muscle mass. Fitness coaches should always respect and consider training personalization given the variability of responses by different subjects.

Javier S. Morales


  1. Mookerjee S, Ratamess N. Comparison of strength differences and joint action durations between full and partial range-of-motion bench press exercise. J Strength Cond Res. 1999;13:76-81.
  2. Schoenfeld BJ, Ogborn DI, Krieger JW. Effect of repetition duration during resistance training on muscle hypertrophy: a systematic review and meta-analysis. Sports Med. 2015;45(4):577-85.
  3. Morton RW, Sonne MW, Farias Zuniga A, Mohammad IYZ, Jones A, McGlory C, Keir PJ, Potvin JR, Phillips SM. Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure. J Physiol. 2019;597(17):4601-4613.
  4. Schuenke MD, Herman JR, Gliders RM, Hagerman FC, Hikida RS, Rana SR, Ragg KE, Staron RS. Early-phase muscular adaptations in response to slow-speed versus traditional resistance-training regimens. Eur J Appl Physiol. 2012;112(10):3585-95.


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