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Background information

Strength training: muscles don’t count reps

Claudio Viecelli
21.12.2021
Translation: Jessica Johnson-Ferguson

Ever been distracted during strength training? A thought that made your mind wander and forget which repetition you were on? Wouldn’t it be great if you could just ask your muscles which rep you’re on. Guess what? They’re not counting.

Muscles don’t have an internal counting mechanism. In other words, they don’t count repetitions. Instead, they evolved as an organ that stores and generates strength and allows us to interact with our environment.

Muscle physiology

Two mechanisms

Recruit and fire! These two mechanisms regulate force production in the body. It depends both on the number of motor units recruited and on the firing frequency with which the action potentials are triggered in the motor neurons. The greater the recruitment and the faster the action potentials follow each other, the higher the muscle strength.

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The motor units are recruited in order of size, the more muscle power is required [5,6]. This means that type S motor units are recruited first, followed by FR and then FF-type units in an additive manner. This generates a maximum arbitrary peak force (see graph).

Fatigue

Once all motor units have been recruited, you can still move the dumbbells as long as the internal, muscular torque is greater than the external torque. In other words, recruiting all motor units doesn’t necessarily require a high training load. You can also achieve full recruitment with low training loads, provided you train up to muscle failure.

From repetitions to fatigue

Two studies show the importance of the repetition factor for strength training. in once of them, the factor is related to the intensity and in the other one, to the stress duration:

Reps and intensity

A research group around Kumar et al. [7] investigated the relationship between muscular protein buildup and exercise intensity using a study design in which the work done was equal in two test groups. Strength training consisted of dominant leg extension exercises from 20% to 90% 1-RM in 25 young (24 ± 6 years) and older (70 ± 5 years) men with identical body mass indexes (24 ±). In order to average their workouts, participants were divided into groups:

  • Group 20% 1-RM doing 3 × 27 repetitions.
  • Group 40% 1-RM doing 3 × 14 repetitions.
  • Group 60% 1-RM doing 3 × 9 repetitions.
  • Group 75% 1-RM doing 3 × 8 repetitions.
  • Group 90% 1-RM doing 6 × 3 repetitions.

20% 1-RM x 3 x 27= 16.2
90% 1-RM x 6 x 3 = 16.2

Repetitions and tension duration

The researchers around Burd et al. [8] investigated the question what influence the stress duration per repetition has on the protein build-up. For this purpose, young men trained one leg at a time with a knee extension exercise with 30% of the 1-RM until muscle failure. To do this, they each completed 3 sets with 2 minutes of rest in between. In addition, the researchers specified the time for one repetition.

The participants trained at 6 seconds per movement phase (concentric and eccentric), so 12 seconds for each repetition. With the same number of repetitions, the participants now trained the other leg, but with a movement speed of 2 seconds per repetition. The number of repetitions was 12 ± 1, 7 ± 1, and 6 ± 1 for sets 1, 2, and 3, respectively, for both legs. Over the following 30 hours, the researchers then examined protein build-up as an indicator of muscle growth.

Verdict: Your muscles don’t count reps.

What can we take away from these two studies? As already described in detail, muscle growth is already stimulated by 60% of 1-RM. Increasing the intensity does not bring any additional effect. In this regard, we also see that in the study design used, the mechanical effort was the same, but the number of repetitions varied.

The concept of specifying a number of repetitions actually has the same intent, but fails in different scenarios.

Complete fatigue of high-threshold motor units (FF and FR) stimulates protein build-up and the muscle growth very sturdily. This means you should opt for a slow pace of execution so that the muscles are constantly exposed to a high muscular tension. Then try to maintain this tension for as long as possible. Choose the load so that you can keep up the exercise for about 120 seconds. Keep in mind that muscles don’t count reps.

References

Images: Shutterstock"

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Molecular and Muscular Biologist. Researcher at ETH Zurich. Strength athlete.


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