Fast or Slow Reps for Muscle Growth Summary
- The study examined whether fast or slow reps for muscle growth.
- The study found both groups gained muscle, but a slower eccentric phase (4 seconds) resulted in greater hypertrophy/muscle growth in the vastus lateralis muscle than a fast eccentric phase (1 second).
- The slow eccentric group saw greater strength improvements, with an 11.1 kg (24.5 lb) increase in their one-repetition max compared to 5.6 kg (12.3 lb) in the fast eccentric group.
- Longer contraction time in the slow eccentric group suggests a shift toward slow-twitch fiber hypertrophy, which may have implications for endurance and recovery.
Do Fast or Slow Reps for Muscle Growth Work Best?
For years, lifters and bodybuilders have debated whether fast or slow reps for muscle growth are more effective. Some argue that slower reps increase time under tension (TUT), resulting in more hypertrophy, whereas others believe that fast reps maximize training volume and explosive power. But which method is scientifically superior? A recent study published in Frontiers in Physiology (2025) by Kojic, Mandic, and Duric titled “The Effects of Eccentric Phase Tempo in Squats on Hypertrophy, Strength, and Contractile Properties of the Quadriceps Femoris Muscle” examined how different eccentric tempos affect muscle hypertrophy and strength gains. Their research specifically analyzed whether prolonging the eccentric phase during resistance training leads to superior muscle growth compared to performing reps at a faster pace.1 The study outcomes suggest that there may be some advantages to lifting the weight slowly in terms of emphasizing muscle growth in certain regions.
Does Slowly Lowering the Weight Increase Muscle Growth
Modifying resistance training variables, including intensity, volume, rest periods, contraction duration, and range of motion, significantly impacts muscle adaptations.2 Tempo, often overlooked, has recently gained attention for its role in hypertrophy and strength improvements.3 Tempo denotes the duration of the eccentric (lowering), isometric (pause), and concentric (lifting) phases of a movement, directly influencing time under tension (TUT) and affecting.4
The enhanced hypertrophic response to eccentric training is largely due to the increased mechanical tension (i.e., heavier weight that can be lifted) and muscle damage associated with eccentric contractions. Eccentric contractions with a heavier weight cause greater muscle damage, serving as a strong stimulus for muscle growth.5,6 However, recent research questions whether muscle damage is necessary for hypertrophy, leading to further debate.7 The evidence for the superiority of slowly lowering the weight for muscle growth is mixed. Studies have found that slowly lowering the weight is superior for muscle growth8-12, while others have found that slowly lifting the weight is not beneficial 7,9,13
Overview of the Study on Fast or Slow Reps for Muscle Growth
The study by Kojic, Mandic, and Duric (2025) recruited 18 participants (10 men and 8 women) who had not engaged in resistance training for at least 8 months. Participants were randomly assigned to one of two groups:
- Fast Eccentric Group: 1-second eccentric, 1-second concentric
- Slow Eccentric Group: 4-second eccentric, 1-second concentric
Both groups performed squats twice per week for 7 weeks, using 3-4 sets per session at 60-70% of their one-rep max (1RM).
Results: What the Study Found
Strength Gains
Both groups experienced significant improvements in squat 1RM, but the slow eccentric group showed superior strength gains. This suggests that increasing eccentric duration may enhance maximal strength development. The slow eccentric group increased their 1RM by 11.1 kg (24.5 lbs.), while the fast eccentric group increased by 5.6 kg (12.3 lbs.)
Muscle Gains
Overall muscle growth increased in both groups, but the emphasizing a slow eccentric contraction group exhibited significantly greater hypertrophy in the vastus lateralis. No significant hypertrophy differences were observed in the rectus femoris, vastus medialis, or vastus intermedius between the slow and fast groups.
Fast or Slow Reps for Muscle Growth Verdict
The outcomes suggest this was not a clear-cut win for slower lifting for muscle growth. Both groups increased muscle growth, but the slow-lifting eccentric group tended to have greater gains in muscle mass in one area (i.e., vastus lateralis). The author stated, “These results suggest that slower eccentric tempos may be more effective in targeting hypertrophy in specific QF muscles, such as the VL, and could support fiber-type-specific training adaptations.”
Practical Applications
- Both fast and slow tempos can be effective for muscle growth as long as training volume and intensity are sufficient. Incorporating a mix of tempos may provide the best results by targeting different muscle fibers and promoting balanced development.
- A slower eccentric phase (4s) in squats is optimal for hypertrophy, particularly in the vastus lateralis, and for maximizing strength gains.
- Faster tempos are generally more effective for developing strength and power, allowing for heavier loads and greater neural adaptations.
References
1 Kojic, F., Mandic, D. & Duric, S. The effects of eccentric phase tempo in squats on hypertrophy, strength, and contractile properties of the quadriceps femoris muscle. Frontiers in Physiology 15 (2025). https://doi.org/10.3389/fphys.2024.1531926
2 Androulakis Korakakis, P. et al. Optimizing Resistance Training Technique to Maximize Muscle Hypertrophy: A Narrative Review. Journal of Functional Morphology and Kinesiology 9 (2023).
3 Farthing, J. P. & Chilibeck, P. D. The Effects of Eccentric and Concentric Training at Different Velocities on Muscle Hypertrophy. European Journal of Applied Physiology 89, 578-586 (2003). https://doi.org/10.1007/s00421-003-0842-2
4 Burd, N. A. et al. Muscle time under tension during resistance exercise stimulates differential muscle protein sub-fractional synthetic responses in men. J Physiol 590, 351-362 (2012). https://doi.org/10.1113/jphysiol.2011.221200
5 Hortobágyi, T. et al. Changes in Muscle Strength, Muscle Fibre Size and Myofibrillar Gene Expression After Immobilization and Retraining in Humans. The Journal of Physiology 524, 293-304 (2000). https://doi.org/10.1111/j.1469-7793.2000.00293.x
6 Norrbrand, L., Fluckey, J. D., Pozzo, M. & Tesch, P. A. Resistance Training Using Eccentric Overload Induces Early Adaptations in Skeletal Muscle Size. European Journal of Applied Physiology 102, 271-281 (2007). https://doi.org/10.1007/s00421-007-0583-8
7 Ünlü, G., Çevikol, C. & Melekoğlu, T. Comparison of the Effects of Eccentric, Concentric, and Eccentric-Concentric Isotonic Resistance Training at Two Velocities on Strength and Muscle Hypertrophy. Journal of Strength and Conditioning Research (2020).
References
8 Azevedo, P., Oliveira, M. G. D. & Schoenfeld, B. J. Effect of different eccentric tempos on hypertrophy and strength of the lower limbs. Biology of Sport 39, 443 – 449 (2021).
9 Burd, N. A. et al. Muscle time under tension during resistance exercise stimulates differential muscle protein sub‐fractional synthetic responses in men. The Journal of Physiology 590 (2012).
10 Horwath, O., Paulsen, G., Esping, T., Seynnes, O. R. & Olsson, M. C. Isokinetic resistance training combined with eccentric overload improves athletic performance and induces muscle hypertrophy in young ice hockey players. Journal of science and medicine in sport 22 7, 821-826 (2019).
11 Marzilger, R., Bohm, S., Mersmann, F. & Arampatzis, A. Effects of Lengthening Velocity During Eccentric Training on Vastus Lateralis Muscle Hypertrophy. Frontiers in Physiology 10 (2019).
12 Alves, R. C. et al. Training Programs Designed for Muscle Hypertrophy in Bodybuilders: A Narrative Review. Sports 8, 149 (2020). https://doi.org/10.3390/sports8110149
13 Mallinson, J. E. et al. Longitudinal hypertrophic and transcriptional responses to high‐load eccentric‐concentric vs concentric training in males. Scandinavian Journal of Medicine & Science in Sports 30, 2101 – 2115 (2020).
