Does It Matter How Slow (Or Fast) You Lift Weights? Here’s the Science

Speed might seem like a more important metric for your runs or rides, but it comes into play when you’re strength training, too. The tempo of your reps—how fast or slow you complete them—can change how a move stimulates your muscles and prompts your body to adapt, including how sore you might get and what results you’ll ultimately see from your strength work. 

If you’re already factoring in how many reps and exercises to do in a workout and which workout split to tackle (among other things), worrying about your rep tempo might sound like a lot—but rest assured, it doesn’t need to be complicated. Here’s everything you need to know about doing slow reps vs. fast reps in strength training, how to find the perfect lifting tempo for you, and when it really matters.

Slow Reps vs. Fast Reps: What’s Better? 

Well, neither. Both slow and fast reps have value and should be included in your training. “It really depends on 1) the goal of the workout and 2) the goal of that specific block of work,” says Peloton instructor Andy Speer. Your overall fitness goals (think: build muscle, get stronger, develop speed and power) also factor into which rep tempo is best—though, in general, most people can benefit from incorporating a range into their workout routine.

For instance, a research review published in the International Journal of Sports Medicine found that a wide array of rep speeds offer benefits when you’re strength training and that the best one for you will depend on your fitness goals.

Rep tempo is directly related to time under tension (or TUT), which refers to the amount of time a specific muscle or muscle group is active and under tension during an exercise. Like doing more sets or reps or lifting heavier weights, playing with time under tension is one way to change the demand an exercise puts on your body. “The more time you spend under tension in a movement, generally, the muscle breakdown and metabolic buildup, all of that will increase,” Andy says. “So it's a great way to safely load your muscles over a little bit of a longer period of time.”

To dig into the science of rep tempo and time under tension, it helps to understand the three key muscle actions (or phases) involved in exercise: concentric, eccentric, and isometric. The concentric phase of a movement is when the targeted muscle is shortening, eccentric is when it’s lengthening, and isometric is when it’s staying the same length, according to the National Academy of Sports Medicine (NASM). It helps to imagine an overhead tricep extension: The concentric phase is when you’re straightening your elbow, and the muscle on the back of your upper arm is contracting while getting shorter; the eccentric phase is when you’re lowering the weight over your head, and your tricep muscle is contracting while lengthening; and isometric would be if you paused halfway up the movement, with your tricep muscle under tension, where it’s contracting but not changing in length.

To describe rep tempo, exercise science research often uses a format like 2/1/2/0 or 2:1:2 to show the number of seconds spent in each phase of the movement. There’s no standardized method of expressing rep time, but a recent review published in the Strength and Conditioning Journal recommends using a four-digit combination—eccentric (lowering), isometric (holding at the bottom), concentric (lifting), and isometric (holding at the top)—with slashes in between. To make things easy, that’s how we’ll write out rep tempos for the rest of our discussion here. 

How to Choose Lift Speed Based on Goals

As we established above, the ideal rep tempo depends on your overall fitness goal, plus your target for the workout and this specific part of the session, Andy says. For example, your main MO may be to get stronger, and you could have the same goal going into a resistance training workout, but during the two-minute AMRAP finisher, you may be more focused on getting your heart pumping and reaching muscular fatigue. Below, we’re breaking down which rep speed is best for different needs.

Are Slow Reps or Fast Reps Better for Beginners?

If you’re new to strength training or coming back from a hiatus, it’s smart to take it slower. “If I'm trying to teach, be more educational and instructional, and give someone an opportunity to learn a movement, we're going to slow things down,” Andy says. “We might do a three-, four-, or five-second eccentric, pause at the bottom, and then do a two- or three-second concentric on the way back up, just so we're really controlling the movement.” (So, that could look like 3/1/2/1 or 4/2/3/1.) 

As a blanket recommendation, the American College of Sports Medicine (ACSM) also advocates for a moderate or slower tempo for novice- and intermediate-trained individuals. In general, this helps build muscular endurance, which most beginners should focus on before progressing to hypertrophy or maximal strength work, per NASM.

Taking it slow also helps you get a feel for where your body is in space and gives you plenty of time to monitor and master your form while in motion. “The faster you move, the less time you have to observe what's going on in your body,” Andy says. “And the faster you move, the more compensatory movements might come into the exercise,” which could potentially result in injury or take the emphasis off the target muscles. 

Are Slow Reps or Fast Reps Better for Getting Stronger?

The good news is that you can’t go wrong with slow or fast reps if your goal is to get stronger. The researchers behind the International Journal of Sports Medicine review concluded that both fast reps and moderate-slow reps help improve muscular strength (think: how much weight you can lift). In this case, fast is considered reps with an eccentric phase of 1–3 seconds and concentric phase of less than 1 second, and moderate-slow is reps with an eccentric phase of 1.7–3 seconds and a concentric phase in the same range. 

Another review published in the journal Sports Medicine came to a similar conclusion: right now, we just don’t have data to suggest that rep speed makes a real difference in strength gains. The researchers note that if you’re trying to improve your maximal strength, it’s more important to lift heavy, which often necessitates moving slower. However, they caveat that faster resistance training is thought to provide a better stimulus for neural adaptations, which could lead to greater strength gains. Overall, they say that the consensus for building strength is to include a low number of reps (one to five), with two to five seconds of TUT each, for a total TUT of two to 20 seconds per set. 

Are Slow Reps or Fast Reps Better for Power and Explosiveness?

To train power and explosiveness—i.e., producing a maximal amount of force in a minimal amount of time—you’re going to want to move a little bit faster, Andy says. A review published in the Strength and Conditioning Journal, for example, found that reps taking a total of four to eight seconds are suitable for developing power as long as the concentric motion is performed quickly or explosively (in less than three seconds).

“If I'm trying to teach somebody to be explosive and train athletic movements, then there are times when we want to move very quickly,” Andy says. It might go without saying, but these skills are reserved for more advanced athletes; fast, explosive movements generally come only after you’ve built a solid base. Really, we want to be able to do both—move slowly and quickly—in life and in sport. “Things generally don’t happen that slowly, but those eccentrics and isometrics give us a chance to develop the control positions, and then when we put them into a faster pace,” Andy says.

Are Slow Reps or Fast Reps Better for Building Muscle? 

Like with building strength, we don’t yet have evidence confirming that one rep tempo is best for hypertrophy training (building muscle). The review published in the International Journal of Sports Medicine found that a wide range of repetition tempos (0.5–8 seconds) conferred similar hypertrophy benefits as long as sets were performed to failure (when you physically can’t do any more reps). The Strength and Conditioning Journal review came to a similar conclusion, recommending a total rep time of eight seconds or less.

It could make a difference to manipulate the amount of time spent in each movement phase, though the ideal breakdown still isn’t crystal clear, according to research. In the Sports Medicine review, the researchers suggest that the most favorable speed could include a slower eccentric phase and faster concentric phase, given some research suggests that longer eccentric periods are linked to more muscle gain. 

So, if your goal is to build muscle, doing slow or fast reps may not hugely affect your results. In general, to encourage muscle gain, it’s more important that muscle cells are exposed to a high training volume (typically 3-4 sets of 8-12 reps) and enough time under tension (between 40–70 seconds per set, according to NASM). NASM recommends a cadence of 2/0/2/0—but to up the ante, you can try playing with even longer eccentric phases.

Are Slow Reps or Fast Reps Better for Metabolic Conditioning / Cardio?

“To do any metabolic conditioning work—for example, doing 50 squat jumps to get your legs smoked and your heart rate up—you need to move quickly,” Andy says. In these types of workouts (think: HIIT, circuit training, metcon) it’s less about the time under tension and more about pushing the pace so you get the benefits of cardio, too. You could be doing one second up, one second down, and just working to maintain that, Andy says. No matter what, you want to find the sweet spot where you’re moving fast but still controlling the movement.

How to Find Your Lift Speed

Most people should worry first and foremost about performing the exercise correctly and with control—and dial in their rep cadence later. In general, the ACSM recommends sticking to a slow or moderate pace if you’re at a beginner (i.e., untrained with no resistance training experience or without consistent training for several years) or intermediate (approximately six months of consistent resistance training). More advanced exercises (with years of resistance training experience) can use a combination of slow, moderate, and fast tempos, depending on how much they’re lifting, how many reps they’re doing, and their goals. 

Andy also advocates for keeping it simple: “In my classes, if we're not working on a specific tempo, it's always control, hold, and go,” he says. “I'm not asking you to count out three seconds on the way down every single time, but you’re not letting gravity take over. You’re controlling the descent, owning that bottom position, and then executing that push or pull with confidence.” If you were to actually time it, this might look a little like 2/1/2/1, but it’s OK to feel it out and go with the pace that’s best for your body and the move you’re doing. From there, if you want to play with different tempo techniques, you can—but if you stick to this approach, “you're going to be in really good shape,” Andy says. 

The Takeaway 

When it comes to rep tempo during strength workouts, there aren’t a lot of right or wrong answers. “You really want to work on the whole spectrum—and you can do it all in the same class or workout,” Andy says. You might do an isometric hold or super slow reps during a warm-up, then some moderately-paced sets in the bulk of your workout, then finish with reps that are speedy and explosive. “It's a really fun way to experience and be thoughtful about how the body can move at all these different speeds and control itself in different directions,” Andy says. All that said, “programming things with an intent and a purpose really goes a long way,” he adds. If you’re gunning for a specific fitness goal, experimenting with fast and slow reps may help you dial in your explosiveness, build endurance and stability, and get strong—it just shouldn’t be the first thing on your list.