Slow Reps vs Fast Reps for Muscle Growth: Which Is Better?

If you want to build muscle, should your reps be slow or fast? What’s going to give you the best results? Here’s a quick summary of the latest research on rep speed and muscle growth.

A study from a team of Brazilian scientists compared two different rep speeds [1]:

• 2 seconds per rep
• 6 seconds per rep

Subjects taking part in the study trained each leg twice a week, doing 3-4 sets on the leg extension machine, with each set separated by 3 minutes of rest.

With leg extensions, you normally work both legs together. But in this study, the men trained one leg at a time, which meant that each leg could be trained using different rep speeds.

All sets were taken to failure, where the subjects were unable to complete another rep through a full range of motion.

The amount of weight lifted, number of sets, as well as the amount of rest taken between each set, was identical.

So, what happened? Who gained the most muscle?

After 14 weeks of training, ultrasound scans showed no significant difference in muscle growth between the two legs.

Irrespective of rep speed, rectus femoris and vastus lateralis – two of the muscles that make up the quads – grew at much the same rate.

One of the main limitations of this study is the fact it was done on untrained beginners, who tend to grow no matter what they do.

Would trained participants respond in the same way?

A follow-up study, published in the Journal of Strength and Conditioning Research, done with trained versus untrained subjects, also shows no advantage to slower rep speeds [2].

This time, a group of men who’d been lifting weights for at least three years, and were able to squat an average of double their bodyweight, took part in the study.

The study was set up in much the same way. 

Participants trained one leg with a fast rep speed, taking 1 second to lift and lower the weight. The other leg was trained with a slower tempo, 1 second to lift the weight and 3 seconds to lower it.

The men did sets of 8-10 reps in weeks 1–4, with a fourth set added in weeks 5–8. 

A 2-minute and 3-minute rest interval was allowed between sets and each leg, respectively.

To ensure that differences in training volume didn’t skew the results, volume load (sets x reps x weight) was equated between legs. 

After eight weeks of training, there was no significant difference in muscle growth whether the reps were done using a fast or slow speed.

Much like the findings in untrained subjects, slower reps had no benefits for hypertrophy.

Most other research on the subject shows that intentionally slow training speeds fail to deliver superior gains in muscle mass compared to regular training speeds. 

While slowing down your reps may make your workout feel harder, it’s unlikely to help you build muscle any faster.