Q: Does the (tempo) speed of your exercise repetitions matter? Is there a “best”? I’m guessing it depends on what you’re doing which is why we do pulses and holds sometimes.
A: The short answer is tempo matters. For the most part there are 2 tempos that research show are effective – “normal” and explosive. Research on exercise tempo also shows that super slow* is the least effective. The pulses and pauses at the end of some sets are “bonuses” we add to try and squeeze out a little more stimulus out of your muscles.
*Super slow tempo would be taking 10 seconds to lower a weight, 10 to raise it and then a 10 sec hold… which is 30 seconds per rep, or 6 minutes per set for 12 reps. 3 sets would be 21 minutes for just one exercise with only one minute of rest between sets. 6 exercises would take more than 2 hours, while being less effective and nobody has time for that.
Fast or Normal
Some exercises are only effective done explosively – like slamming a medicine ball. (Explosive is relative, but using the medicine ball slam example it’s most effective if you slam it as hard/fast as you can.) Some exercises are meant to be done at more of a “normal” pace or tempo – like a bicep curl. While many exercises can be done effectively at either a “normal” or explosive tempo – like squats can be done “normal” or explosive (aka jumping).
What if you slow down?
If your body doesn’t slow down then you aren’t working hard enough. For an exercise like a medicine ball slam, as people get tired the speed of the slam and the frequency of the slams both go down. You do want to fight the slowing down, but the slowing is both inevitable and desirable.
If you can’t slow down, then what you are doing is too easy for you and/or you are not pushing hard enough. Effort is essential. Effort creates exhaustion, and the path to exhaustion and the metabolic byproducts of exhaustion are a BIG part of what stimulates positive change in your body.
Can you go too fast?
On an explosive exercise like a slam you can’t slam it too fast, however on a “normal” tempo exercise you can lower the weight too fast. For example, on a bicep curl, if you let gravity take over and let the dumbbell yank your hand down that is too fast. It’s too fast because your muscles aren’t doing any work to slow the lowering of the weight. It’s not so much that there’s an optimal lowering time, but rather that what you’re doing needs to be effortful until you take a rest break.
On the topic of “optimal lowering time,” there is research comparing 1 second lowering speed to 2 seconds lowering speed to 4 seconds lowering speed, and, on the whole they are all similarly effective. In reality the speed at which you lower a weight will decrease as you get tired if you are working hard enough to stimulate positive change in your body.
What about isometrics?
We use isometrics or “pauses” at the end of a set of “normal” tempo exercises to try and squeeze a few more myokines* out of your muscles after the muscles are exhausted doing the full range of motion. To use a Thanksgiving metaphor, isometrics (or holds) are the gravy. Gravy is great on top of many things, but I would not want to sit down to a meal that was just a bowl of gravy.
*Myokines are signaling proteins that are created by muscular contraction that have positive impacts in every part of your body – from brain health (preventing dementia and Alzheimer’s), to cardiovascular health, to mood stabilization, to immune function, and more.
A more technical answer on the limitations of isometric-only exercise is that this style of exercise has very limited effectiveness – it will only make you stronger in the exact position you hold +/- 10 degrees. To use bicep curls as an example, if you only trained your biceps by holding a weight with your elbow at 90 degrees then your bicep would only get stronger from 80 degrees to 100 degrees. There would be no change in strength outside this limited window. Since the elbow moves about 160-170 degrees you would need to do isometric holds in about 8 different positions to get similar (but still inferior) benefit as doing a regular, full range of motion bicep curl.
The problem with planks
Furthermore, the narrow range for increased strength is one of the big problems with using isometric exercises such as planks (or other exercises where your midsection does not move) as the focus of your core training. With isometrics only (or mostly) your core muscles only get stronger in one position (so called “neutral”), but stay weak or get weaker outside of this narrow range. In other words, planking (or similar no-core-movement-exercise) only improves the ability of your core to get stiff. You do need your core to be able to get stiff, but you also need to be able to pick up a piece of trash without throwing your back out. (More on core training here.)
Bottom line regarding the speed of your exercise
Use you full range of motion most of the time, and work hard enough that you slow down.