Kine 3030 Lecture 20

hi welcome back to can thirty thirty online today we're going to talk about balance and stability but uh first we're just gonna do a quick review of static at the equilibrium at the joint level and i left you with this question um that you are you have a that client who is doing biceps curls using the uh the bands so basically this and yet they were worried about their shoulder it was uh aggravating and you need to figure out uh which muscle group is active at the shoulder if any and by how much um and you should always start with an appropriate free body diagram i've given you the information about the mass of the client um and what percentage of the client would be um of their total body weight would be the uh segment weight which is five percent point zero 0.05 um the actual force coming from the resistance band is 15 newtons um and it acts 70 degrees to the horizontal and we have a uh a distance for the from the moment arm band to the shoulder of 15 centimeters and a location of the center of gravity of the arm which is uh from the arm to the shoulder would be 15 centimeters or sorry five centimeters apologize five centimeters so um this is the free body diagram of the segment again if we want to figure out what's going on in the shoulder we cut through the shoulder right cut through the shoulder if you need to figure out what's going on at the shoulder and then we look at either everything distal to the shoulder which would be the um the whole arm forearm hand and upper arm or it would be the rest of the body um except for the arm and it's easier to go uh distal so it's easier to go with the arm because then we don't have to worry about what the ground reaction forces are etc and that's just how we typically do it and you can see here we have the weight of the arm we have the force of the band showing there we have the the two reaction forces um at at the shoulder the x and y components of those reaction components and then we have the torque we're just trying to figure out the muscle torque here because we don't know exactly which muscle group it is is it the flexors or is it the extensors do you know that ahead of time and if you don't then you have to start with just a torque and as i've drawn the torque right now you could see it would try to bring arm up this way which means as i've drawn it right now it's a if i get a positive value for that torque we have a flexor torque okay i've put some dimensions in there so first of all we need to figure out the weight of the arm weight of the arm is the mass of the person times the the fraction of the whole body weight total body weight that is the arm that's the 5.05 and times of course 9.81 to make it into a force so 34.3 newtons now we are going to try and figure out the torque around that shoulder so what it is and and if you have some sort of um band of this nature um at home try it and figure out where you feel it where do you feel which muscle group do you think it's going to be um and do you really feel it in the shoulder like i mean would this be a shoulder activity and i'm going to tell you yes yes you are going to feel it in the shoulder um so the sum of the torques around uh the shoulder equals zero we're assuming that it's you know kind of held in place making it a static activity so we have the if again counterclockwise i'm considering positive so i drew my um muscle torque at the shoulder as counterclockwise just because i wanted it to be flexor is positive and as i've drawn that free body diagram it just turns out that flexor positive would be a counterclockwise direction and then i have the weight of the arm times its moment arm which is .05 and that would be negative because it would try and rotate my arm the other way the weight of my arm is going to try and pull it this way same with the force at the at the hand um which is 15 newtons times the 0.15 and again negative because it would also try and pull my arms down and so it would try to extend them in that posture at the shoulder so from that you can solve for the torque at the shoulder which would be 3.97 newton meters so it's positive 3.97 newton meters it's a positive value as i've drawn that my positive value would be my flexors and again try it and see where you feel it but you should feel it in your shoulder flexors we can also figure out the joint reaction forces again these will not be as large as the actual um bone on bone forces in the glenohumeral joint because we are using the torque approach and the net torque approach does not include the force of the muscle so it underestimates that uh um the actual force is in the shoulder but it gives you a bulb a a first pass um and so sum of forces in the x direction equals zero and that's going to be the um joint force in the x direction minus the force of the band in the x direction and that's where you're going to have to use that cos 70 because the 70 degrees so in the end you're going to have 5.13 newtons in the x direction and in the y direction you have the joint force in the y direction positive as i've drawn it weight of the forearm and hand and upper arm so the whole arm weight of the upper arm is a force which of course is down so it'll be negative and the force of the band um which is um also down in the y direction and that'll be the 15 newtons times the um sine 70 so it's the component that is down when i rearrange that i am going to come up with 48.4 newtons in the joint force in the y direction so that's how you solve that and that's an example of the equilibrium at the joint level where we can find out that's a torque example if we knew strength of torque strength of the shoulder flexors for either the client or for a population that might include the client um you could then figure out what percent nvc they were working at so now into balance and stability where we're going to again we're going to define stability and understand the factors that affect it and understand the strategies we will use to improve it so what can a wrestler do to prevent being pinned basically they want to stay upright they don't want to get down if they can help it so we'll come back to that one stability uh the capacity of an object to return to equilibrium or its original position after being displaced or perturbed is another word you here for that static equilibrium of course is that situation where you're uh there is no acceleration and the sum of the forces equals the uh equals zero and the sum of the torques equals zero and balance is the ability for us to include to control our equilibrium so there's a lot of forces that are sports that require high stability the main ones these are examples of ones wrestlers football linemen an elderly person walking why well i think you can see there's these are sports where you don't want to be um tipped over you don't want to be uh you you want to stay upright you want to stay involved and certainly the implications if you go over are high an elderly person walking if they fall down they can you know obviously the implications are high that's when you get your hip fractures so it's the importance of not losing it and the difficulty in maintaining on the other hand we have the sports where less stability can that can be good these are examples sprinters swimmers at the start downhill skiers soccer goalie well the reason is because we're often at the edge of stability in order to get a quick start in order to change directions quickly um so we're trying to do things by staying right on the edge we can make movements quicker so if we're looking at somebody who is going to lose their balance and uh what does that mean it means to be balanced your center of gravity is within your base of support okay and unstable means that your center of gravity is either outside your base of support in which case you're going to fall or near the edge of your base of support which means it would be easier to put it outside the base of support in which case you will fall now fall doesn't necessarily mean that you're going to land on the floor it might mean that you have to take a step or change your body posture in some way to react to it so you don't fall so we remember what a center of mass is of the whole body that's the you know for we're looking at the whole body so it includes all of the different body segments together and the um base of support is the area around the outer most uh regions in contact between a body and this and support surfaces when we're talking walking on flat surfaces or doing things on a flat surface that literally means the area that our feet or our hands depending how we're doing are in contact with the ground in the event that there's something else that we can grab onto that actually would include our base support because now we have something up here that we're holding on to that to is increasing our base of support there are a number of factors that affect stability we have newton's first two laws and we're going to see how that's important we have the actual basis support we have the location of the center of gravity uh with respect to that base of support um we need to talk about what the height of the basically the height of the center of gravity is and what the weight of the object is person or something else the potential energy you want to minimize the potential energy and i'll get to that and we have to remember in the case of humans we have the uh we do have that neuromuscular system we have control over things so we can modify where our center of gravity is within our base of support so think about newton's laws with respect to an angular situation a rotating body will maintain a state of constant rotational motion unless acted upon by an external torque and also we have of course the sum of torque equals i alpha um and basically an object is not is going to maintain its stability unless it is acted by a torque that's really what it comes on to if i i'm standing here doing nothing um then i'm going to stay pretty stable unless some sort of external torque acts on me so if we push an object over it really is an active rotation see that it's it's an active rotation to tip something over you are actually rotating it around the outer side on the side that it's going to tip over and if we're going to prevent the rotation then the torques must be balanced in other words the sum of the torques equals zero okay so let's think about the book there we go and if i'm going to push it over you can see it's going to rotate on that edge it's a essentially a rotation around that edge and the distance from the table up to where i'm pushing and i'm pushing horizontally so that's going to be the moment arm of the force of my pushing force and at the same time we have the weight of the uh the force for the weight is going down the center here and its moment arm is literally that that width of where that center of gravity is with respect to that side so in the end if you have the sum of the torques right now it's in equilibrium and if we want it to remain in equilibrium that means the torque created by that push which is this distance times however much i'm pushing by is going to be the same as the force down which is the weight of the book times its moment arm which is quite small in this particular case and if you look at those four factors how much i'm pushing how high up i'm pushing the weight of the object and the distance from where the center of gravity is to that outer side of the base of support um you are essentially going to get uh the four factors that really talk to the fat the things that affect stability and balance so you need to keep this the push times its moment arm to be less than the weight times its moment arm for this knot to tip over as soon as the push times its moment arm the torque created by me pushing on this book exceeds the weight by its moment arm the book is going to fall down in other words you want to minimize this guy and maximize the other the torque created by the other one so let's start with the base of support which is really maximizing b and the area the base of support is the area around the uh outermore regions of contact as i said um in a horizontal projection of the center of gravity moves outside that base of support you are going to lose your balance and the wider the base of support the increasing the stability because it's just farther to push my center of gravity to an edge than it is to um than it is if my feet are close together so if a force comes this way um and my feet are close together it doesn't take that much to tip me over whereas if i'm like this and the force comes i have a greater area that i will cover before i fall over and that is the b portion of that uh um of the equation that we've been talking about what does that mean so there's some pictures of basis support and you can see the line around all of the different pit places that are in contact if i was just on one foot it would literally be a line around my shoe um if i've got one hand down it's going to be that that triangle if i have a cane in my hand it's going to be a triangle that's going to include the cane so you can now understand why walkers are so beneficial for elderly people who do have balance issues because it widens their base of support we want to know where that location of the center of gravity is within that base of support and the closer it gets to the edge the easier it is as i've said before to tip over in that direction now you can actually play with that in order to increase your stability if you put position it um towards an oncoming force or it are towards the base of support on the side that you aren't likely to be tipped to so if i'm going to be tipped this way if somebody's pushing me here it's going to tip me there it's going to be then if i move my basis support over here to begin with um i have an advantage again it is the b the distance between that center of gravity and the edge of the base of support is the b on the side that we're going to tip towards so again this is widening our base of support and you have two people there the same weight um two examples it's the same person same weight one with wider legs one with the narrower legs and you can see from where the center of gravity hits the ground to the edge of the base of support is bigger with the wider legs so if somebody's pushing me it's harder to move move my center of gravity over to the edge in the first one than it is in the second one and i can even increase that by moving towards which direction that force is so in all three cases here they have the same stance but in one case they move their center of gravity to one edge of their base of support and in the other to the other side of the base of support so in the middle one they've gone to this side and if the force comes this way it doesn't take much to tip over right oh sorry there's just not much to move you there it whereas if i am if the force comes the other direction so if i'm here i'm on this side and the force comes from this direction i have all of this distance um that i can move okay and so i am i if i move my center of gravity towards the side of the base of support that the force is coming from then i have a lot greater motion to be able to handle so that is the b portion of the uh the base of the distance between your the projection of your center of gravity on the ground and the edge of the base's support next one is w so remember b times w is the torque created by the center of gravity by your weight itself the torque created um by your weight around the edge of the basis support that you're going to tip around i want to maximize b times w i can maximize w and or b we've just talked about ways to maximize b i could also maximize w i can increase the weight so something that is heavier is going to be harder to tip because it maximizes that piece if all else is equal if everything else about a situation is equal but one so they're the same shape the same height as the center of gravity same base of support and the weight is greater than one object than another um one person then another and it's going to be harder to tip over the um the object or the person with the greater weight reducing the height of the object well that has several factors number one is so all being equal i.e they have the same basis support the shorter object um is going to be one that doesn't allow as high a push remember we want to minimize the push force we don't always have control over that if that's coming from somebody else but the other thing we can minimize is where that force is actually pushed on us and for something that's shorter or if you crouch down like this then the highest that force can really be is this level i suppose it could push on your head but you would hope that isn't the case whereas if i'm standing upright they're pushing here so it's easier to tip over here than it is here um be tipped over by a external force the other thing is that the shorter situation so if you're in that crouch your center of gravity is going to be lower right your center of gravity is lower and as i will show you later on that has implications in terms of potential energy and makes it harder to tip something over so if something has most of its weight down near the bottom then it is typically harder to push over than if the weight is up high and that's again because of this minimizing the potential energy potential energy is the energy associated with position off the ground it's an energy associated with gravity so if i have something here and let it go it's going to gravity is going to pick it up and increase its velocity and give it energy it doesn't have any kinetic energy when i'm holding it here but as soon as i let go it will increase and it will have kinetic energy that'll keep getting more and more kinetic energy being the energy of uh velocity in motion so we have um potential energy which is the energy associated with the position off the ground and it is equal to mass times the distance off the ground times g 9.81 the closer the center of gravity of the object is to the earth the less potential energy it has and to raise it requires energy so if i raise something from here to here i have to actually give it energy because it has so much potential energy here and it has more potential energy here because it's higher so when you top on an object it requires that center of gravity to go up and over and then comes down just think about that in a box and i'm going to show you some pictures of that so the yellow one is something that is a low center of gravity and imagine that i'm pushing on it from the right side you see the box sitting there and when i push on it from the right side it's going to rotate around the left corner and as it rotates the the radius the distance from that corner to where the center of gravity is is that line there that you see is affected by you know that affects that dh and as i rotate it around because my radius is smaller the height difference between when it's lying flat and when it's tipped over on its side or on its corner is that's going to be that change in d l l being the low center of gravity okay now when my center of gravity is higher the radius between um where that uh um where it is on the corner and where the center of gravity is actually going to be longer and to be honest when you tip it over you are not going to have as much of a change in the height anymore so that's that delta dh so that's the change in the height of the center of gravity and it takes energy to raise the height of the center of gravity so when you push on the low box you have to raise the center of gravity more than you do if you push on the high center gravity box and with that one you only change that center of gravity a little bit in order to topple it over in other words it does not take as much energy to topple the one where the center of gravity is high as it does for the one where the center of gravity is low all else being equal and finally yes we're not static we can move we can change where our center of gravity is we can change our base of support um and it's a huge area of research it's an absolutely huge area of research that the effect of posture and balance and things that affect that you can stand on a force plate and see how much we sway and how that is affected by having your eyes closed having vestibular problems different types of of perturbation so are you pushed are you leaning against something and then it's released there's a number of different ways we can perturb the body and we learn a lot about how our brain controls the both takes all the stimulus in and then controls the muscles in order to modify our position and allow us to remain balanced big area of research so if we come back to our original problem what can a wrestler do to prevent being uh knocked over well let's start with if you think through that that list that we talked about um let's actually start with that one at the bottom increase the mass can we increase the mass well no we can't increase the mass on a instantaneous in the moment scenario when we're in the actual competition our mass is our mass certainly you all know that uh there are weigh-ins and that different elements different people of different weights compete together and they have a weigh-in to decide which weight class you are in what you want to do is you want to be at the top end of whatever weight class you are so let's you know if you are normally and this is this is actually a a problem not really biomechanically but it could can be a problem with uh um people with uh food control issues but and certainly we know that there's a number of a large amount of fasting that can occur just before the weigh-in so that you drop your weight to just under the top end of the weight class so that you are in you don't want to be the one at the bottom of your weight class just unfortunately the way it works um biomechanically to to give you the most advantage but once you're already in a weight class there's not much you can do about the mass piece at that point it's it's a done deal so now what kinds of things we can do we can crouch lower we can crouch lower which have two effects and one of them is that it lowers our center of gravity which as we've talked about makes it more difficult to tip over all everything else being equal it also means that whatever hand holds your opponent can get on you to try and flip you are going to be lower because if you're standing up high they can grab something high but if you're down low it's it's going to be down low um we can increase the base of support in the plane of the force application now that's important and and you might not know that's the one thing that is problematic do you know are they going to pull you or push you so that would be in the frontal plane or are they going to try and tip you over sideways often you'll see something like this where you have a little bit that is forward backwards and a little bit that sideways but on the other hand if they are you do open yourself up there for something that is a pull on the angle okay so you have to sort of predict which way they're going to pull you based and which hand they're going to grab you with and then you're going to need to lean in to the direction of force application so if you are going to be pulled then you want to be back okay if they're going to pull you forward then you want to be back because that gives you a lot more space here for your center of gravity before you're over the edge if you don't know which way it's going to be then you have to be in the center okay because that's the that gives you the most in both directions so just to review we have stable and unstable systems and unstable really means that our center of gravity gets outside that base of support and understand those factors that are going to affect stability that width of your basis support the location of your center of gravity within the base of support the height of the center of gravity and the height direction of the potential push pull forces the mass and then the ability to control [Music] things you

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