How Slow Should You Go? New Research on Tempo Training and Rep Speed (2025)

Fast reps vs. slow reps: Which are better for muscle hypertrophy?

Time-under-tension and tempo training. Two methods of bodybuilding that manipulate the speed of a repetition in an attempt to increase muscle growth. Just like adjusting the load of a lift or increasing training frequency, this training approach treats the speed of a repetition as a variable that can be altered to achieve different results.

But does it work? Does merely performing a slow rep result in great gains, or is it better if you go faster? A large meta was just released that has an answer to: how fast should a rep be?

Key Points You Need To Know! Fast reps and slow reps both increase muscle mass There is little, if any, difference between the speed of a rep Each portion can be anywhere from 0.25s to 4.5s Use faster speeds for concentric and slower reps for eccentric to try to optimize

The Study:  How slow should you go? A systematic review with meta-analysis of the effect of resistance training repetition tempo on muscle hypertrophy.

Resistance training tempo, or the speed at which you lift and lower the weight, has long been a topic of debate in the world of bodybuilding and strength training. The conversation goes further than simply suggesting to perform a rep, "slow and controlled".

Rather, proponents suggest that purposefully performing a rep slower or faster can elicit different results. General thoughts included;

• Slower reps increase "time under tension" and mechanical stress, resulting in greater muscle growth.
• Faster reps cause greater muscle activation and higher force output, resulting in greater strength gains.

In the past, older reviews tended to lump total rep duration together rather than isolate the phases. However, a new systematic review and Bayesian meta-analysis (2025) just published is the first to separate eccentric (lowering) and concentric (lifting) tempos to see if manipulating tempo truly impacts hypertrophy.

Instead of just looking at "slow vs. fast reps", they looked at eccentric vs. concentric tempo effects on muscle growth.

Results & Key Findings

The researchers performed a large meta-meta, pooling from a large supply of studies. A few key points about the study selection include

• Study pool: 14 randomized trials, 278 participants (mostly young adults, only 73 women, and a few trained lifters)
• Study duration: Most studies lasted 6–12 weeks, with participants training 2–4 times per week.
• Muscles studied: Mainly quadriceps and biceps; mix of single-joint and multi-joint exercises.
• Measurement tools: Ultrasound, MRI, DXA, biopsies, or limb circumference.
• Analysis method: Bayesian hierarchical meta-analysis, which estimates the probability of one tempo being superior rather than relying on simple "significance."

Researchers ran the numbers and came up with 5 important findings.

1. Fast vs. slow, both increased muscle growth. When examining muscle growth within each group, both fast and slow tempos yielded solid gains.

This means that researchers examined whether fast repetitions and slow repetitions can build muscle independently.

When comparing the effect size (i.e., the magnitude of the improvement), faster tempos showed a medium effect size of 0.43, while slower tempos had an effect size of 0.34.

This means both groups built noticeable muscle with neither showing a significant advantage.

2. Direct comparison: no clear winner. When researchers directly compared faster reps and slower reps, faster reps had a slight statistical edge (mean difference of 0.09). However, the probability that this edge held any significance in the real world was only 45%.

This means the chance of it having an actual effect is low.

3. Optimal speeds for concentric and eccentric phases. When broken down by phase, the concentric phase trended slightly in favor of faster tempos. The eccentric (lowering) portion showed almost no difference. However, any differences were essentially negligible.

This means they made little to no difference.

4. Upper vs. lower body. This was an interesting component as the researchers examined whether different body parts might be affected by rep speed. However, tempo didn't matter for the body region.

This means that slowing down squats won't increase the mass of your legs, while speeding up curls doesn't result in bigger biceps. Both upper and lower bodies responded the same way regardless of tempo.

5. Training to failure. This was another interesting component of the study, as it looked at whether these speeds make a difference in relation to training to failure. Maybe it has a larger effect when you don't train close to failure? Well, they did find some interesting results.

• When participants trained short of failure, faster eccentrics seemed slightly better.
• When participants trained to failure, slower eccentrics had a modest edge.

Still, both effects were small and inconsistent.

This means the differences weren't strong enough to justify changing how most people should train.

How It Fits with Previous Research

The findings above primarily built upon information from past research with no major contradictions, but there were some.

1. Schoenfeld et al. (2015) reported similar hypertrophy across tempos ranging from 0.5 to 8 seconds. This new meta-analysis confirms the finding but builds upon it by ensuring this is true for eccentric versus concentric movements independently.

2. Hackett et al. (2018) suggested slower tempos might help quads, while faster tempos benefited biceps. The new analysis finds no consistent regional advantage once variables are controlled.

3. Wilk et al. (2021 narrative review) hypothesized that slower eccentric contractions and faster concentric contractions may be optimal and are likely ideal for strength and mass development. This is somewhat contradictory to the new 2025 meta, which shows no clear hypertrophy benefit for either phase.

In summary, this study primarily strengthened the evidence that the speed of a repetition has little to no effect on muscle growth, assuming the duration is at a reasonable speed (.25s-4.5s per contraction)  The only difference is that previous research suggests a slight benefit may be achieved by performing the concentric portion faster.

Practical Takeaways

The clearest message from this paper is that tempo doesn't significantly affect hypertrophy, as long as you're training hard and with sufficient volume. Both fast and slow reps build muscle effectively.

When we combine this information with the data we discussed above concerning strength, here are 3 guidelines on rep speed for strength and hypertrophy.

1. Concentric Contraction: Perform quickly and under control
2. Eccentric Contraction: Slow and controlled eccentrics 
3. Always Control The Load: We'll say it again, always control the load

We examined this topic in its entirety before this study was released and came to a similar conclusion - you can review it here.

That said, tempo can still be a useful tool depending on your situation:

• Rehab or joint pain: Slower tempos reduce joint stress and help maintain control.
• Power and explosiveness: Faster tempos are more sport-specific and build rate of force.
• Variety and fatigue management: Changing tempo can provide novelty and help manage training stress without affecting growth.

With all that said, the big levers for hypertrophy still remain: training volume, progressive overload, and proximity to failure. Tempo can just be a fun little variable to play with if you're interested.

References

1. ENES, A., PIÑERO, A., HERMANN, T., ZAMANZADEH, A., HENNESSY, T., MONTENEGRO, D., PARNELL, C., JIA, A., WEITZMAN, T., WOLF, M., KORAKAKIS, P.A., SWINTON, P.A. and SCHOENFELD, B.J. [2025]. How slow should you go? A systematic review with meta-analysis of the effect of resistance training repetition tempo on muscle hypertrophy. Journal of strength and conditioning research (pre-print)
2. Hackett, D. A., Davies, T. B., Orr, R., Kuang, K., & Halaki, M. (2018). Effect of movement velocity during resistance training on muscle-specific hypertrophy: A systematic review. European Journal of Sport Science, 18(4), 473–482. https://doi.org/10.1080/17461391.2018.1434563
3. Schoenfeld, B. J., Ogborn, D., & Krieger, J. W. (2015). Effect of repetition duration during resistance training on muscle hypertrophy: A systematic review and meta-analysis. Sports Medicine, 45(4), 577–585. https://doi.org/10.1007/s40279-015-0304-0 
4. Wilk, M., Zajac, A., & Tufano, J. J. (2021). The influence of movement tempo during resistance training on muscular strength and hypertrophy responses: A review. Sports Medicine, 51, 1629-1650. https://doi.org/10.1007/s40279-021-01465-2