Biomechanically Based Exercise Selection
Biomechanically Based Exercise Selection
People tend to put a lot of thought into the exercises they use in their program. Many people will lean toward the exercises they are good at, others will focus on weak points in strength or physique. However, there are others that just want to make the best choices for the long-term, or to work around past and current injuries. There is a way to make good exercise choices with one's joints in mind based on simple biomechanics.
First, you must understand what the center of mass is. For the purposes of this discussion, think of it as the point through which the force of gravity acts. Gravity always acts directly downward, toward the center of the Earth. It is the 3-D balancing point for an object. So, if you were to place an object on a fulcrum, it would balance perfectly about this point. For something symmetrical like a barbell, the COM position is pretty easy to determine (Dead center along the bar). For something asymmetrical, like a human, that position is going to vary based on body position, external load that may now be part of the system (Like a bar on your back), limb proportions, etc. Keep in mind, during a movement, the COM actually changes position. Sometimes it will sit OUTSIDE the body.
Second, I think you should understand the concept of a lever. Torque is a twisting force. So, if you rotate a crank of some kind, you are applying torque to that system. The amount of torque placed on that crank is the force applied to the crank multiplied by the lever arm distance. Just imagine trying tighten a bolt by grabbing really close to the bolt versus grabbing at the end of the wrench handle. Obviously, the latter will be way easier. Even if the same amount of force is being applied by you, there is less torque if your force vector moves closer to the axis of rotation.
Okay Cow, what the fuck are you talking about? You are blathering about nerdiness with no end in sight. Well there is a reason I said this. Virtually all forces your muscles produce about a joint result in torque, or rotation. Imagine your knee as the center of a clock. Your lower leg is the minute hand. It moves just like that. Other joints like the hip and shoulder do that same thing, but in 3 dimensions instead of two (Really the knee can rotate in 3 dimensions too, but not very much, little enough to be ignored). More importantly, the forces acting upon your body impose torques as well.
Hopefully, if you understand this concept, you will be able to apply these principles to make your own exercise modifications. Nonetheless, I will give you some examples.
Exercises for those who have lower back issues
Trap Bar Deadlift
Okay, so you want to stimulate your glutes and hammies without killing your lower back. Deadlifts are out; they aggravate your lower back like Kanye West at an award show. If you have access to a trap bar, then you can reduce the load in your spine in two ways.
First of all, the center of mass of the various forces acting on your spine are shifted closer to the spine. You have reduced the lever arm distance from your spine to the center of mass. You now have less torque acting on your spine, even with an equivalent amount weight on the bar. The side effect here is that you do shift some of the load from your hips to your knees, but there is still plenty of hip drive required I assure you.
The second way you reduce stress on the spine is by allowing for a more upright posture. Unfortunately we can't change the direction in which gravity acts. All we can do is change our posture to alter the relationship between gravity and our body. This reduces shearing forces, and shifts some of that load to compressive force placed on the spine. The structure of the spine allows it to be far more resistant to compressive force versus a shearing force.
Remember when I said you can't make gravity act any way but down? Well, cables setups are an ingenious way of converting that downward pull on the stack of weights into a force vector that follows the line created by the cable. So, this exercise results in some of the same benefits of a trap bar deadlift. In the very bottom position, the cable is almost parallel with your spine. Once again, less shearing forces and more compression.
As well, because the cable is always running very close to the lumbar spine, the lever arm distance is short even when you are standing up and your hips are fully extended. Plus, once in a standing position, because of the unusual force vector created by the cable, torque is further minimized.
Another movement that is great for leg development but give the low back some troubles in people is the back squat. If you don't have any contraindications, then back squat away. If you do, then work on improving mobility or tissue quality and strength where necessary and front squat in the meantime (Not that the front squat isn't always a great exercise).
So if you are getting this lever concept down, you might be thinking: well how does a front squat position make for reduced torque on the lumbar spine when the bar is now sitting in front of you, further away from the low back? The answer lies in trunk angle. Your trunk angle is far more upright in a front squat. Your knees are further forward, probably past your toes some. So, the resultant force vector of your center of mass plus the external resistance is sitting closer to the low back compared to a back squat, especially if you tend to back squat powerlifter style. Once again, because of the more upright posture, you are also allowing the spine to be subjected to more compression and less shearing.
A little bit of a bitch to setup the exercise, but a great way to take the load off your spine. Beacuse of where you place the weights, you almost totally avoid loading up the spine because all the force is applied below the lumbar spine. Give them a whirl if your back really can't tolerate a load.
Okay, not fancy shmancy science needed here. Basically, any one legged variation of an exercise requires less total load to stimulate the target musculature. So, you don't have to load up your spine as much!
Well, just because I had to be a nerd, there is some biomechanical advantage too. With unilateral exercises, you have a lesser base of support on the frontal plane (Left to right), but an increased base of support on the saggital plane (Front to back). So, you can keep a completely vertical trunk position without losing your balance like you would with most bilateral movements. Once again, compression is better than shearing.
Bent Rows Low
Okay, so mostly lower body exercises are potentially troublesome for the lower back, but so are bent over rows. A lot of people pull the bar kind of high on their trunk (To the chest or so). If you take a closer grip, tuck your arms closer to your sides, and pull the bar toward your belly button/waist area, then you dramatically reduce the torque on your lower back each time you pull the bar toward you.
Exercises for those with knee issues
Ah the box squat, a favorite of many powerlifters. The powerlifter back squat is great for shifting some of the load to the glutes and hammies. Once again, think about your trunk angle on a box squat where you keep your shin angle almost vertical. You have to lean forward quite a bit more. This shift your center of mass along with the bar forward quite a bit. Now you are moving the center of mass of the system closer to the knees, and further from the hips. Granted, your quads will become less important, but it's better to stimulate them some without pain than not stimulate them at all.
Once again, another exercise that requires a bit of forward lean, but also the bar sits out in front of you quite a bit because of the nature of the way you support it. Again, this has the same effect as the box squat. Some of the torque necessary to move the thing is borne by the hips and reduced at the knee because the center of mass is shifted toward the knee and away from the hips.
Leg Press High
Another good exercise for stimulating the quads, but the knees can take a little beating here. If you place your feet higher on the foot pad you accomplish two things though. First, you are moving your knees closer in line with the center of mass of the sled moving toward you. Second, you are reducing knee range of motion to some extent, which is desireable if more knee flexion is an issue for you.
Deadlifts are probably fine for you. However, if you are really trying to limit the stress placed on your knees this alternative might be better. Because you hardly bend your knees at all, you don't put them in a position to have much shearing force imposed upon them. As well, beacuse the bar is almost rubbing your shins on the way up, like in a regular deadlift, the lever acting on the knees is quite small. This really limits the torque on the knees.
I hope that gives you a taste of what I'm talking about. I'll try to add more to this later. Happy lifting.
Last edited by CowPimp; 09-09-2011 at 11:40 AM.
Good stuff! Very informative.
Wow, that's a really good post.
I've been doing front box squats with the box ATG low. I find it's a good exercise for me and I seem to get the best development that a single exercise can give. Furthermore, I only need about half the the weight on the bar as I would need for a just past 90 back squat.
Do you mind running through the angles and forces on that?
Oooh, very nice.
Suggestions for subsequent entries: why wide-grip chins and upright rows suck for your rotator cuff; and perhaps edit in links to youtubes of the exercises in the first post over time.
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Well upright rows it's not really about the biomechanics so much as the structure of your body. The position in which you put your shoulder decreases subacromial space to a minimum; thus, you maximize your chance of shoulder impingement. In fact, you are basically performing the shoulder impingement test under a load!
Originally Posted by Built
Some people will be able to do this fine. About 1/3 of the population will trash their shoulders with it because of the structure of their clavicle and limited subacromial space. Another substantial chunk of the population has a good chance of causing problems too. As such, I always advise against it as an exercise.
Going excessively wide with you grip on pullups would seem to unnecessarily increase torque placed on the shoulders. As well, because of your elbow angle and line of pull, you are forcing your shoulder muscles to generate a ton of tension in what's considered a fairly at risk shoulder position: abduction and external rotation. Granted this is only at the beginning of the movement, but if you plan on using the full range then it's not the greatest idea. I would rather increase tension in a more biomechanically safe position. Similar idea in some ways as to why going really wide on the bench press is retarded.
With that said, going SOMEWHAT wider such that you can get some should adduction in as part of the primary shoulder articulation is not the worst idea, just don't go overboard with it.
It should be essentially the same as a regular front squat. I mean, the main reason for the difference in the box squat I mention is the assumption that it is done squatting like a powerlifter (Low bar, more forward lean). You can only change your trunk angle so much on a front squat before the bar rolls off your shoulders, heh.
Originally Posted by Hoglander
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