Mostly no. Across 14 studies that changed only rep speed, slower reps (about 3.5 seconds) and faster reps (about 1 second) grew muscle about equally, and the small gap there was leaned slightly toward the faster reps5.
Rep speed, within a wide normal range, looks like a minor lever for growth. How hard the set is, and how many hard sets you do, look like bigger ones.
What the terms mean
Tempo is how fast you lift and lower each rep. Coaches often write it as three or four numbers, for example 3-0-1: three seconds lowering, no pause, one second lifting.
Each rep has two main phases. The concentric phase is the lift, when the muscle shortens. The eccentric phase is the lowering, when the muscle lengthens under load.
Repetition duration is the total time of one rep, all phases added together2. Time under tension is the total time a muscle works across a set. Slowing each rep stretches it out.
Hypertrophy is muscle growth, measured in these studies as changes in muscle or lean mass, thickness or cross-sectional area.
In plain terms: "slow reps" means spending more seconds on each rep, usually on the lowering.
What was studied
The two reviews at the centre of this question pooled many small trials.
| Review | Studies | What it compared | Who |
|---|---|---|---|
| Schoenfeld 2015 | 8 | Rep durations, sets to failure, at least 6 weeks | Adults without chronic disease |
| Enes 2025 | 14 | Slower (1.7–4.5 s, about 3.5 s) vs faster (0.3–2 s, about 1 s) reps | Healthy adults |
| Amdi & King 2025 | 9 (166 people) | Longer vs shorter lowering phase | 46% trained, 77% men |
The 2025 Enes review included only studies that kept every other training variable the same, so tempo was the one thing that differed5. Nine of its studies changed the lowering speed, four the lifting speed, and one both5. It used Bayesian statistics, which give a credible interval: the range the true difference most likely falls in, given the data.
The 2015 review required every set to be taken to muscle failure, the point where another rep with proper form is not possible, and growth to be measured by biopsy, imaging or body scans2.
What the studies found
Both speeds grew muscle. In the 2025 review, the average within-group growth was 0.43 standard deviations for faster reps and 0.34 for slower reps, which the authors called similar5. A standard deviation is a measure of how much people vary, so these are sizes of change in units of that spread.
Muscle growth with faster vs slower reps, pooled within groups · SD
The difference between them was trivial. Comparing slow and fast head to head, the pooled difference was 0.09 in favour of faster reps, with a credible interval from −0.04 to 0.225. That range crosses zero. The authors reported a low probability that the true difference was even small (0.45), and almost none that it was medium (0.001) or large (below 0.001)5.
Subgroups did not change the picture much. Splitting by lowering or lifting, upper or lower body, and whether sets went to failure gave trivial to small differences, with varying certainty5.
A wide range of rep durations worked. The 2015 review found that reps lasting from 0.5 to 8 seconds produced similar growth2. It also found that very slow reps, over 10 seconds each, appeared to produce less growth, though the authors noted a lack of controlled studies2.
Slowing just the lowering phase. A separate 2025 meta-analysis of 9 studies compared longer and shorter lowering phases. For muscle growth the estimate was near zero (effect size 0.05, 90% confidence interval −0.22 to 0.33) and uncertain6.
Slower vs faster reps: pooled difference in muscle growth · SMD
In plain terms: within about half a second to eight seconds a rep, speed barely changed how much muscle grew.
The nuance
Single trials point both ways. In 13 trained men, each leg did leg extensions at a different lowering speed for 8 weeks. Near the knee, thigh thickness grew more with the fast tempo (5.5% vs 2.2%); near the hip, growth was similar3. In 18 adults with no recent training, doing squats for 7 weeks, a 4-second lowering phase grew the vastus lateralis (the outer quad) more than a 1-second one4. Results like these, one muscle or site at a time in small groups, are why the pooled estimates matter more.
Slow reps feel harder. In the leg-extension trial, the slow-tempo leg rated its sessions as harder (8.59 vs 7.98 on a rating-of-effort scale) while growing no more overall3. Effort felt is not the same as growth.
Slow tempo can make a light load work. In a 2006 trial of 24 untrained young men, knee extensions at about 50% of max with a slow 3-second lift and 3-second lowering grew the thigh on MRI, as did a heavy load (about 80%) at normal speed. The same light load at normal speed did not1. Both growing groups were working at about an 8-rep max, meaning sets were hard. The slow tempo appears to have mattered because it made the light load hard, not because slow is special.
"Time under tension" is not the target by itself. If time under tension drove growth, the 8-second reps would beat the 1-second reps. They did not2. What the trials held in common is that sets were demanding. See RIR vs RPE for how effort is rated.
Speed on purpose vs speed by fatigue. Reps slow down on their own near failure. That is different from choosing a slow tempo from the first rep, which is what these trials tested.
In plain terms: pick a controlled tempo you can train hard with; the stopwatch matters less than the effort.
What this means in practice
The trials that grew muscle used a wide spread of tempos, and most rep durations between half a second and eight seconds did about equally well2. Slowing reps to add "time under tension" did not add growth in the pooled data5.
For progress over time, adding load or reps at the same controlled tempo is the variable most programmes track; see what progressive overload is. How many hard sets a muscle gets each week is covered in how many sets per muscle group per week, and rest between them in how long to rest between sets.
The studies at a glance
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Tanimoto & Ishii, *J Appl Physiol*1 | 24 untrained young men, 12 weeks | Light load with slow tempo grew thigh muscle like a heavy load; light load at normal speed did not | 2006 |
| Schoenfeld et al., *Sports Med*2 | 8 studies, sets to failure | Similar growth from 0.5 to 8 s per rep; over 10 s may be worse | 2015 |
| Pearson et al., *J Strength Cond Res*3 | 13 trained men, one leg each tempo, 8 weeks | Similar growth near the hip; fast tempo ahead near the knee | 2022 |
| Kojic et al., *Front Physiol*4 | 18 adults without recent training, squats, 7 weeks | 4-s lowering grew the outer quad more than 1-s lowering | 2024 |
| Enes et al., *J Strength Cond Res*5 | 14 studies, healthy adults | Pooled difference 0.09 (−0.04 to 0.22), trivially favouring faster | 2025 |
| Amdi & King, *J Sports Sci*6 | 9 studies, 166 people | Longer vs shorter lowering: near-zero, uncertain difference in growth | 2025 |
Limits worth knowing
- Abstracts only. We could not open the full text of any of these six papers; every number here is from an abstract.
- Small trials. The single trials had 13 to 24 people each; the meta-analyses pool many small studies.
- Short. The single trials ran 7 to 12 weeks; the 2015 review required at least 62.
- Mixed populations. Trained and untrained, mostly men, mostly young.
- Missing details. The 2025 Enes abstract does not give the thresholds for "small" or the total number of participants5.
- Different measures. Muscle thickness, cross-sectional area and lean mass do not always agree.
Latest research
The newest synthesis is the 2025 Enes meta-analysis, published in late 20255. It is the most tightly controlled pooling so far, because it included only studies where tempo was the single difference. Its conclusion is that tempo "appears to have minimal overall effect on muscle hypertrophy, with potential differences emerging under specific conditions." We have seen only the abstract, so we cannot say which conditions those are.
The short version
- Slow and fast reps grew muscle about equally in the pooled studies.
- The small gap that exists leaned slightly toward faster reps, and it was trivial.
- Rep durations from about half a second to eight seconds all worked; much slower may work less well.
- Slow tempo made a light load effective in one trial by making the sets hard.
- Effort and weekly sets look like bigger levers than rep speed.
Educational overview only — not medical advice.
