If your goal is to maximize the potential benefit of an exercise while minimizing the risk of injury, you should move at least slowly enough to be able to do three things:
- Reverse direction smoothly between the positive (lifting) and negative (lowering) phases of the repetition, without yanking, jerking, or bouncing the weight
- Maintain correct body positioning over the full range of the exercise
- Focus on contracting the target muscles
Reverse direction smoothly
To hold a weight motionless or to lift or lower it at a constant velocity you must produce a level of force equal to the pull of gravity on the weight. To start lifting, or to reverse direction between lifting and lowering movements, however, requires a change in velocity, or acceleration, which requires a change in force. You must produce more force to slow to a stop and change direction between the negative and positive (the lower turnaround) and less force to slow to a stop and change direction between the positive and negative (the upper turnaround). The acceleration will be proportional to the force.
If the acceleration is low the force does not vary significantly from the amount required to hold or move the weight at a constant velocity, only a few percent even at typical repetition cadences, and the tension on the target muscles won’t vary significantly other than due to changes in leverage. If you attempt to lift the weight explosively and the acceleration is high, the forces can vary by a huge amount, increasing the force against the muscles to potentially harmful levels then decreasing it proportionally over a portion of the range of motion after positive acceleration stops, increasing the risk of injury while reducing the efficiency of muscular loading.
Reversing direction smoothly, with low acceleration, prevents the force encountered by the muscles from varying too much relative the resistance provided by the weight or machine, which minimizes your risks of injury and keeps the tension on the target muscles more consistent over the full range of the exercise. It is not necessary to move extremely slowly to accomplish this, however.
If you use a weight that allows you to perform at least a moderate number of repetitions, the average force your muscles are required to produce will be well below their concentric maximum, which is below their eccentric maximum, which is below the amount required to cause an injury (assuming healthy tissue and no pre-existing injuries). For example, if you perform an exercise with a load that is approximately seventy five percent of your one repetition maximum, you already have a margin of safety way over twenty five percent. With this sizeable safety margin a few percent variation in force is not going to significantly increase your risk of injury, and the difference in the variation of force between different repetition cadences is very small as long as the turnarounds are performed smoothly.
This has been demonstrated with force gauge experiments and can be proven mathematically. I’ve discussed this with several friends who are engineers, one of whom shared the following: Assuming a typical range of motion of about half a meter, if you calculate the force required to bring a one hundred kilogram or two hundred and twenty pound barbell to rest over a distance of about four inches at the cadences listed below, you get the following percentage of the load used (gravitational constant rounded up to 10 and results rounded off to the nearest whole number):
1/1 cadence (.5 m/sec): 113%
2/2 cadence (.25 m/sec): 103%
4/4 cadence (.125 m/sec): 101%
10/10 cadence (.05 m/sec): 100%
In other words, as long as the acceleration occurs over a distance of at least a few inches the difference in peak force between a 2/2 repetition and a 10/10 repetition is only a few pounds.
If you double the distance over which acceleration occurs during the 1/1 cadence to eight inches, which is more realistic if you are attempting to reverse direction smoothly at that cadence, the percentage of load required to bring it to rest comes down to only 106%, which would neither be a dangerous level of peak force or significant variation in tension. The faster you go, the more difficult it is to reverse direction this smoothly, however.
It becomes easier to turnaround smoothly as you go more slowly, but moving any more slowly than necessary to do so does not significantly reduce your risk of injury or improve the efficiency of muscular loading. With proper training and practice a three to four second cadence results in a slow enough speed on most exercises for most people to perform reasonably good turnarounds.
Maintain correct body position
Your body positioning affects the levers your muscles work against and the forces encountered. Correct positioning results in the muscles working against levers which provide resistance which is reasonably well balanced to the strength of the muscles over the range of motion and which do not expose the joints to potentially harmful compression or stretching. Incorrect positioning can result in poorly balanced resistance and underload the muscles over significant portions of the range of motion, as well as potentially harmful compression or stretching of joint tissues.
A lot of this depends on the exercise and the equipment being used. It is much easier to maintain correct positioning during certain types of movements and with certain types of equipment than others. However, as a general rule the faster you move the harder it is to maintain correct body position. Up to a point, moving more slowly will make it easier to maintain correct body position, as well as to detect and correct incorrect positioning or movements.
Focus on the target muscles
The goal of an exercise is not to use your muscles to lift the weight, but to use the weight to efficiently load the muscles. It is easier to focus on and feel the tension in the target muscles and to be able to adjust your form based on this feedback when you are moving more slowly. Like turnarounds and positioning, this is easier with slower movement, however it doesn’t require moving extremely slowly.
Downsides of moving too slowly
While I’ve written a lot over the years about the problems with moving too quickly during exercise, I have made little mention of the problems with moving too slowly, mainly because there are only a few and they are minor compared with the problems with moving too quickly.
Two important factors in stimulating muscular strength and size increases are metabolic stress and muscle damage, both of which appear to be reduced as speed decreases.
Reduced metabolic stress
Although metabolic stress and fatigue are more strongly associated with tension than mechanical work, as isometric exercise proves, mechanical work does play a role, and it has been demonstrated that a higher rate of mechanical work produces a faster rate of fatigue, most likely due to affects on several related factors. All else being roughly equal, more repetitions will result in greater muscle damage or microtrauma, which is a contributing factor. Additionally, since the muscles are significantly stronger and use less energy during eccentric contractions, it appears a longer negative may reduce the rate of fatigue, similarly although not to the same degree as rest-pause.
When I began to suspect this in the mid 2000’s I performed an experiment with several clients and invited other trainers to do the same and report the results: Perform as many repetitions of an exercise as possible at a 2/2 cadence, recording the time to concentric failure. Wait thirty minutes and repeat the exercise with the only change being to use a 10/10 cadence, recording the time to concentric failure. I had clients perform this with both upper and lower body exercises (usually leg extension and arm curl, but some times leg press and pulldown) and almost all were capable of continuing the exercise for up to fifty percent longer with the slower speed only thirty minutes after the first set. I repeated this later comparing a 2/2 and 2/10 cadence with similar results. The longer negative appeared to reduce the rate of fatigue with the selected resistance, allowing for a longer time to concentric failure or time under load (TUL).
I repeated these tests only a few years ago with equipment specifically designed for very slow repetitions with the same results; the rate of fatigue is lower with slower repetition speeds. While it could be claimed the additional time is beneficial, what it really means is it takes longer to recruit and fatigue all the motor units in the targeted muscles. It could also be claimed this allows for the use of greater loads than with faster reps for a given TUL (and the force/velocity curve supports this), but then it would make more sense to only perform the negatives for a longer duration rather than both phases (and in a study by Westcott discussed in his book Building Strength & Stamina a 4/10 protocol appeared to be more effective than 10/4). Also, while tension would increase, it might be at the expense of microtrauma. A more moderate repetition speed would probably be a good compromise between high tension and high muscle damage while allowing for more efficient inroad than very slow protocols.
Reduced muscle damage
Most of the muscle damage or microtrauma occurs during eccentric contractions and appears to be related to the volume of mechanical work. The slower the repetition cadence, the fewer repetitions performed within a set amount of time, the less muscle damage is likely to occur, potentially reducing the stimulus for muscular strength and size increases.
Not too fast, not too slow
While any repetition speed can be effective as long as you train hard and progressively, if you want to maximize benefit while minimizing risk, as I wrote above you should move at least slowly enough during exercise to be able to reverse direction smoothly, maintain correct body positioning, and to be able to focus on contracting the target muscles. If you’re not sure about the proper speed you’re better off moving too slowly than too quickly, but moving extremely slowly is not necessary and does not provide any benefits over more moderate speeds in terms of effectiveness or safety.
For the majority of people, a three to four second cadence (three to four second positive phase and three to four second negative phase) results in a good average speed of movement for the majority of exercises, not being too fast on exercises with a longer range of motion or too slow on exercises with a shorter range of motion.
Can you make gains with SuperSlow?
While discussing rep speed on my high intensity training facebook page someone asked me whether it was possible to make long-term gains using SuperSlow protocol, which uses a very slow 10/10 cadence.
Long term gains are possible using any repetition cadence or method if you are doing it hard, progressively, and consistently (assuming you still have further potential for growth and are doing everything else reasonably correctly). Slow reps, fast reps, negative-only, rest-pause, static holds, timed static contractions, etc. Anything done with a high level effort will eventually get you there, some will just get you there faster, more efficiently, and with less risk of injury and less wear and tear on your body.
I know some people who have gotten very strong and muscular using SuperSlow and there are a lot of personal training studios out there using it exclusively with good results. I am not disputing the fact slow reps are effective, but rather the claim they are safer and more effective than more moderate repetition cadences.
Without getting too far off topic, I think the primary reason some people have reported poorer results with SuperSlow than when using conventional repetition methods is not the slower speed but the very long set durations and significant reduction in load they require.
While research shows relative effort to be more important than load, consider most research is not done with cadences and rep ranges resulting in sets lasting three minutes (the current recommendations for SuperSlow are a TUL of 100 to 180 seconds). Load may not be the most important factor, but it does matter, and when load is too low strength and size gains will suffer regardless of the relative effort.
As an extreme example, imagine if you were to cut your loads to only ten percent of what you normally use. You’d end up doing very long sets, and even if you performed every exercise to momentary muscular failure you’d probably end up with much slower strength and size gains. When using typical rep ranges and loads – from around five to twenty at typical cadences and TULs – effort appears to be key, but if you keep dropping the weight and increasing the time you’re going to hit a point where it starts becoming less effective for strength and size gains.
Anecdotally, I had noticeably better results with SuperSlow when using a much shorter rep range of three to four (under 90 seconds) than when using a rep range of four to eight, and most clients responded better to the shorter rep range as well.
The reason I recommend a more moderate speed and TUL (around six to ten reps at 3/3 to 4/4 as a starting point, adjusting based on individual response) is it seems to be the best compromise of what I consider the most important factors: tension, microtrauma, metabolic stress, safety, ability to maintain focus, learnability, ability to observe, identify and correct discrepancies, etc.