Momentum and weight lifting.
August 25, 2006 6:03 AM Subscribe
The physics of weight lifting: It's often anecdotally said that one should perform lifting reps slowly because the faster you lift the more one develops momentum and the less one actually lifts the weight. The implication is that this builds from rep to rep so that the last one is easier for this reason (developed momentum) than the first rep.
This doesn't accord with my understanding of momentum, however, as I would think that momentum would go to zero at the moment when one changes the direction of the lift.
1) Is my supposition true, that changing direction does away with produced momentum?
2) Do other people have this understanding of this piece of weight-lifting advice, that the effects of fast lifting are progressive within a set?
3) Are fast reps really easier only because of the momentum developed during that rep, in one direction?
I may well be asking this question in the wrong way, but helping me to refine the terms here would be something I'd appreciate.
This doesn't accord with my understanding of momentum, however, as I would think that momentum would go to zero at the moment when one changes the direction of the lift.
1) Is my supposition true, that changing direction does away with produced momentum?
2) Do other people have this understanding of this piece of weight-lifting advice, that the effects of fast lifting are progressive within a set?
3) Are fast reps really easier only because of the momentum developed during that rep, in one direction?
I may well be asking this question in the wrong way, but helping me to refine the terms here would be something I'd appreciate.
(1) Yes, though I can imagine an exception some "bouncing" type of movement with, e.g., a bench press. Or imagine the old and discredited standing hamstring stretch with bouncing at the bottom.
(2) No
(3) Yes but see (1)
Also, I think slower reps are generally safer.
posted by exogenous at 6:20 AM on August 25, 2006
(2) No
(3) Yes but see (1)
Also, I think slower reps are generally safer.
posted by exogenous at 6:20 AM on August 25, 2006
To build on what swordfishtrombones said, if you do them fast you tend to start with a jerk; the momentum you gather at that beginning point is what makes that specific rep just a little less difficult (as swordfish explained in the second paragraph). But no, there is no momentum built up between reps.
The other reason to do them slower is to make sure you're fully controlling the weight; typically the faster you try to do something, the tougher it is to retain the correct form (and hence the easier to get injured / use a different muscle group than intended).
posted by inigo2 at 6:22 AM on August 25, 2006
The other reason to do them slower is to make sure you're fully controlling the weight; typically the faster you try to do something, the tougher it is to retain the correct form (and hence the easier to get injured / use a different muscle group than intended).
posted by inigo2 at 6:22 AM on August 25, 2006
Any tendency to "cheat" is likely to become more pronounced as you progress through the set and get more tired, maybe that's something to do with what you've heard people talking about.
posted by teleskiving at 6:32 AM on August 25, 2006
posted by teleskiving at 6:32 AM on August 25, 2006
inigo2 has it - people advise against faster reps because it encourages poor form. However, fast reps do have their place if you can keep your form in check. Slow reps build muscle, but faster reps help keep your muscles quick.
posted by MrZero at 6:36 AM on August 25, 2006
posted by MrZero at 6:36 AM on August 25, 2006
Response by poster: Thanks for the answers. I understand the issues of "cheating" and form, and am careful about them, I was mostly just interested in what I took to be a mis-application of the term momentum in this little nugget of advice.
posted by OmieWise at 6:44 AM on August 25, 2006
posted by OmieWise at 6:44 AM on August 25, 2006
I could be way off, but my experience seems to suggest that faster reps are "easier" due to plyometrics.
posted by dead_ at 6:58 AM on August 25, 2006
posted by dead_ at 6:58 AM on August 25, 2006
According to the rules of Newtonian physics, work is applying a force over a distance. It takes a certain amount of force to move the weight from the bottom of the rep to the top. No matter how fast or slow you do it, the same amount of work (i.e. energy) is required.
However, the next thing you have to account for is the cost of simply resisting gravity. Physics doesn't consider this to be "work", but it certainly takes muscle just to hold a given weight in place against the force of gravity. I'm not sure how to compute this. But this is what causes slow reps to be more effective... you spend a lot more time suspending the weight in the air, causing your muscles do have to do a lot more "work" (not in the physics sense, however).
Finally, there's the efficiency of our muscles and our bodies at translating the muscle contraction into the movement of the weight. Some loss probably occurs, but I think it's minimal if you're doing the particular exercise correctly (because they are designed to use correct body movements).
posted by knave at 7:10 AM on August 25, 2006
However, the next thing you have to account for is the cost of simply resisting gravity. Physics doesn't consider this to be "work", but it certainly takes muscle just to hold a given weight in place against the force of gravity. I'm not sure how to compute this. But this is what causes slow reps to be more effective... you spend a lot more time suspending the weight in the air, causing your muscles do have to do a lot more "work" (not in the physics sense, however).
Finally, there's the efficiency of our muscles and our bodies at translating the muscle contraction into the movement of the weight. Some loss probably occurs, but I think it's minimal if you're doing the particular exercise correctly (because they are designed to use correct body movements).
posted by knave at 7:10 AM on August 25, 2006
knave wrote: "Physics doesn't consider this to be 'work'..."
Physics does consider this to be work, just not mechanical work done on the weight. There is, however, chemical (and, on a small scale, mechanical) work done inside the muscles themselves to maintain muscle tension and keep the weight off the ground.
That's the reason it's harder to do slow reps than fast reps; you don't just do the work of lifting the weight, but also the work maintaining muscle tension over a period of time.
posted by dseaton at 7:19 AM on August 25, 2006
Physics does consider this to be work, just not mechanical work done on the weight. There is, however, chemical (and, on a small scale, mechanical) work done inside the muscles themselves to maintain muscle tension and keep the weight off the ground.
That's the reason it's harder to do slow reps than fast reps; you don't just do the work of lifting the weight, but also the work maintaining muscle tension over a period of time.
posted by dseaton at 7:19 AM on August 25, 2006
In some circumstances, it actually is possible to store energy between reps by going faster. For instance, consider a bicep-curl where you start with your arm hanging vertically, and pull it up until it's perpendicular to the ground. If you start going quickly, then when your arm descends, you will swing a little past hanging vertically ---and when you start pulling up again, you get a little gravity assist by swinging back down to vertical. Also, your muscles and tendons have some elasticity to them, so under some circumstances you can use the bounce in them to save energy. Your body does this to a certain extent when walking and running. Both of these effects will be eliminated if you stop and pause before changing directions.
posted by Humanzee at 7:57 AM on August 25, 2006
posted by Humanzee at 7:57 AM on August 25, 2006
Humanzee: Even if you're not swinging the weights past vertical, and are stopping the weights at the bottom of your curls, you're still "cheating" if you accelerate the weights quickly and cheaply while their motion is largely horizontal and resisted only by inertia, and then use that momentum to help lift them.
posted by nicwolff at 8:20 AM on August 25, 2006
posted by nicwolff at 8:20 AM on August 25, 2006
I agree. I didn't mean to imply that what I was describing was the only (or even dominant) effect. I just wanted to point out that it is possible to store up energy between reps.
posted by Humanzee at 8:53 AM on August 25, 2006
posted by Humanzee at 8:53 AM on August 25, 2006
My understanding is that slow reps come closest to duplicating isometric excercise. Imagine slowing down the reps until there is no motion at all--just the holding of the weights in various positions.
Isometric Exercise provides the fastest stimulation for muscle tissue and increases nervous conductivity and enervation (the ability to contract more muscle fibre). As such it increases the strength of the participant faster than any other natural method.
posted by weapons-grade pandemonium at 9:07 AM on August 25, 2006
Isometric Exercise provides the fastest stimulation for muscle tissue and increases nervous conductivity and enervation (the ability to contract more muscle fibre). As such it increases the strength of the participant faster than any other natural method.
posted by weapons-grade pandemonium at 9:07 AM on August 25, 2006
weapons: you are going to need more then a link to wikipedia to back up that claim. Isometrics play a role in strength training, but as with anything the speed of the movement depends on your goals, training level, etc. Most strength experts would tell you you should use a variety of training methods to maximize strength, and direct iso work would probably be a pretty small piece of that.
posted by rsanheim at 9:34 AM on August 25, 2006
posted by rsanheim at 9:34 AM on August 25, 2006
It's often anecdotally said that one should perform lifting reps slowly because...
... you might drop the weight on your foot or your neck.
Don't forget that there's a safety factor here. "Slow and in control" is more than just an exercise thing. It's just good practice in general.
posted by frogan at 10:50 AM on August 25, 2006
... you might drop the weight on your foot or your neck.
Don't forget that there's a safety factor here. "Slow and in control" is more than just an exercise thing. It's just good practice in general.
posted by frogan at 10:50 AM on August 25, 2006
I agree with rsanheim. But here's my take on repetition speed:
When you take your reps fast, you tend to start cheating in ways that are not regulated by simple newtonian physics. I'll explain:
When benching quickly, people often bounce the bar off their chest, giving them extra momentum on the way up. Furthermore, by allowing the weight to drop faster, you are relenquishing the support generated from your muscles in holding the weight up, which can be called "microfraction rest" or something of that ilk (I just made it up, don't take it as a standardized term).
By taking the reps slow, you are controling the weight with your muscles, thus taking advantage of each direction of the set (not just the lift, but the return as well). Much better for your muscles.
posted by stratastar at 10:52 AM on August 25, 2006
When you take your reps fast, you tend to start cheating in ways that are not regulated by simple newtonian physics. I'll explain:
When benching quickly, people often bounce the bar off their chest, giving them extra momentum on the way up. Furthermore, by allowing the weight to drop faster, you are relenquishing the support generated from your muscles in holding the weight up, which can be called "microfraction rest" or something of that ilk (I just made it up, don't take it as a standardized term).
By taking the reps slow, you are controling the weight with your muscles, thus taking advantage of each direction of the set (not just the lift, but the return as well). Much better for your muscles.
posted by stratastar at 10:52 AM on August 25, 2006
A Review of Research on SuperSlow® High Intensity Strength Training
posted by ecrivain at 11:59 AM on August 25, 2006
posted by ecrivain at 11:59 AM on August 25, 2006
From a friend who was a physics major:
Typically, within any given rep, some parts are harder than others. So if you move fast, when you hit the “hard part”, you can allow the momentum to get you through. Typically when you do reps to failure, you can still do part of the rep on which you fail. So if in early reps, you save the muscles that support the weakest part of your rep, when you get to the last rep, they have more lifting capacity left.posted by mmascolino at 12:06 PM on August 25, 2006
Look at the clean and jerk. There are portions of that rep that are almost 100% momentum.
As for if its cheating, or best practice, it really depends on what you are looking for. Fast reps with heavy weight can build your capacity for delivering power. In most athletic events, that is what you are really trying to accomplish – acceleration. However, slow reps will build more consistent strength through the rep, and build more stabilizing muscles.
Response by poster: All really good answers. Thanks.
posted by OmieWise at 1:12 PM on August 25, 2006
posted by OmieWise at 1:12 PM on August 25, 2006
You should lift weights slowly to minimize momentum on a per rep basis.
Rapid repetitions are often "thrown" at the beginning of the repetition, and your muscular work goes down to near zero as your body parts coast through the motion. (giving your muscles less time under load and giving them the ability to partially recover during the work)
This causes a high impact at the beginning and end of the motion (as the force + momentum catch up and your muscles and joints have to rapidly decelerate the force)
So, when weights are moved rapidly:
They subject the muscular/skeletal system to high forces - and high forces are where the body gets injured.
The work under load isn't constant, and defeats the whole objects of anaerobic fatigue.
Slow repetitions reduce the ability to cheat (it's always possible).
Don't confuse Skill exercises: like the Clean and Jerk, with exercises that put the body under constant load. The clean and jerk is a competition exercise (and skill).
The "hard part", or sticking point of any exercise is normal (and necessary!) It usually has to do with either the point that your motion arm (your limbs) are at the most severe disadvantage. Well designed machines (or leverage arm equipment like Hammer) try to reduce this.
Fast reps do not develop power. Strong muscles develop power. Stronger muscles develop more power. Realistically specific skill practices helps power more than anything else.
posted by filmgeek at 9:11 PM on August 25, 2006
Rapid repetitions are often "thrown" at the beginning of the repetition, and your muscular work goes down to near zero as your body parts coast through the motion. (giving your muscles less time under load and giving them the ability to partially recover during the work)
This causes a high impact at the beginning and end of the motion (as the force + momentum catch up and your muscles and joints have to rapidly decelerate the force)
So, when weights are moved rapidly:
They subject the muscular/skeletal system to high forces - and high forces are where the body gets injured.
The work under load isn't constant, and defeats the whole objects of anaerobic fatigue.
Slow repetitions reduce the ability to cheat (it's always possible).
Don't confuse Skill exercises: like the Clean and Jerk, with exercises that put the body under constant load. The clean and jerk is a competition exercise (and skill).
The "hard part", or sticking point of any exercise is normal (and necessary!) It usually has to do with either the point that your motion arm (your limbs) are at the most severe disadvantage. Well designed machines (or leverage arm equipment like Hammer) try to reduce this.
Fast reps do not develop power. Strong muscles develop power. Stronger muscles develop more power. Realistically specific skill practices helps power more than anything else.
posted by filmgeek at 9:11 PM on August 25, 2006
This thread is closed to new comments.
As to the reason why faster reps seems easier: consider the limit case for a very, very slow rep: you need to hold the weights at a certain level, which costs energy. It's not just a matter of potential energy (which would cancel out over one rep, in fact). your muscles are still requiring energy.
posted by swordfishtrombones at 6:18 AM on August 25, 2006