Muscle Growth: Slow Reps vs. Fast Reps - Which is More Effective?

When we train to increase our strength and muscle mass, we often focus on factors such as the weight lifted, the number of repetitions, and the exercises performed. However, there are less controlled variables, like the speed at which we execute movements. Should we perform the movements slowly or quickly? Should the lifting phase be slow and the lowering phase fast, or vice versa?

Muscle Growth - Slow Reps vs. Fast Reps

Components of Movement: Concentric, Isometric, and Eccentric

A single repetition consists of three main phases: the concentric phase, the isometric phase, and the eccentric phase. Each of these phases has different requirements, so it may be interesting to vary the speed for each one.

By adding up the time taken for all phases, we get the total seconds that the repetition lasted. When we sum up all the repetitions in a set, we get the total time under tension (TUT) that we've experienced during that set. This time under tension is crucial in promoting muscle growth.

Let's consider an example: if each repetition lasts five seconds, and we perform ten repetitions in a set, the accumulated total time under tension for that set would be 50 seconds. We continue adding up time under tension for all sets targeting the same muscle group until the end of the session.

That's why it is often recommended to perform exercises slowly to increase the time under tension. However, as we'll see, this is only partially true, as different phases require different speeds.

Concentric Phase

The concentric phase occurs when we overcome the resistance, not the other way around. Examples include pushing during a bench press or standing up during a squat. When performing pull-ups, we conquer the resistance of our body weight to raise ourselves up.

For increasing muscle mass, the speed of the concentric phase doesn't matter much. Performing it slowly can increase the time under tension, but it reduces the weight lifted as each repetition becomes more fatiguing.

To improve strength, it is vital to perform this phase as fast as possible, or rather, as explosively as possible. In exercises like bench press or squats, we should have the intention to push the barbell to the ceiling, even if the actual speed seems slow from the outside. The actual speed and intentional speed are different, and our focus should be on moving the weight as fast as possible, regardless of the actual speed.

Isometric / Pause Phase

The isometric phase occurs between the concentric and eccentric phases. In the bench press, it's when the barbell is fully extended or touching the chest. In this phase, it's essential to take the time needed, regardless of speed.

To increase muscle mass, it may be beneficial to hold for a second at the point of maximum contraction in exercises where applicable. While the bench press has an isometric phase, it's not as effective for hypertrophy as the isometric phase in pull-ups.

When the arms are fully extended during the bench press, we don't feel as much activation in the chest compared to the feeling in the lats when we hold ourselves up with the chin above the bar during pull-ups.

For exercises like pull-ups, hip thrusts, bicep curls, or lateral raises, it's beneficial to hold for a second between the concentric and eccentric phases to provide an extra stimulus.

For increasing muscular strength, the isometric phase should allow us to take a breath and prepare ourselves physically and mentally for the next repetition. When training basic strength exercises like deadlifts, bench press, or squats, this phase helps stabilize after performing the explosive concentric phase.

Eccentric Phase

The eccentric phase occurs when the resistance overcomes us, and we lower the weight. Lowering the barbell during bench press, squatting down, or lowering ourselves during pull-ups are examples of the eccentric phase.

To maximize muscle growth, the eccentric phase should be controlled but not excessively slow. It should last longer than the concentric phase to control the weight and gradually let the resistance take over.

When lifting heavy weights in basic strength exercises, this phase may naturally be slow to ensure safety. However, it shouldn't be too slow, as it can lead to fatigue that hampers the entire set. The speed of this phase for strength training should be such that it allows us to control the weight, no more and no less.

The Perfect Repetition, According to Science

After reading all of the above, you may wonder how slow or fast each phase should be. Numerous studies have explored this training variable to shed light on the matter.

For a long time, it was believed that extending the eccentric phase as much as possible would generate more hypertrophy. While longer sets indeed result in more time under tension, having high durations per repetition would require reducing the weight or the number of repetitions to manage the load.

Both speed and load need to be considered. A 2015 systematic review found that there is no difference in hypertrophy for repetitions lasting 0.5 to 8 seconds. However, super-slow repetitions lasting over eight seconds may negatively impact hypertrophy.

One explanation is that performing very slow repetitions requires using very light weights, which leads to different adaptations in muscle fibers (study).

The isometric component should be short to maintain constant tension, muscular ischemia, and hypoxia (stimulators of hypertrophy). However, this aspect depends on the muscle area where the peak force occurs.

See also: How to Increase Muscle Mass: Strength or Hypertrophy?

For example, in a standing bicep curl, the peak tension occurs halfway through the movement. Therefore, it doesn't make much sense to rest with fully extended or fully flexed arms. However, in a hip thrust or lateral shoulder raises, where the peak occurs at the end of the movement, it is beneficial to pause briefly at that point.

Practical Application

There is no perfect repetition. Move the weight in a controlled manner and focus on the muscle group you're working rather than the time you take for each repetition.

Slower repetitions increase time under tension but may limit the weight lifted compared to slightly faster repetitions. On the other hand, faster repetitions reduce time under tension but allow for higher weight lifted.

Therefore, aim for a balance between load and speed, avoiding excessively slow speeds, especially during the concentric phase. Based on research comparing different execution speeds, the following tempos are recommended:
  • For muscle growth: A tempo of 3:1:1 or 3:0:1 (three seconds for eccentric phase, one or none for isometric phase depending on the exercise, and one second for concentric phase) may serve as a guide. Each repetition lasts about five or six seconds.
  • For muscular strength: A tempo of 3:0:X or 2:0:X (three or two seconds for eccentric phase, minimal time for isometric phase to control the weight, and the concentric phase as explosive as possible) may serve as a guide. Each repetition lasts about three seconds.

By applying these principles, you can optimize your training to achieve both muscle growth and increased strength effectively. Remember, individual variations and goals may require adjustments to find the best approach for each person.
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