so this Friday your fifth worksheet is due um remember there's only seven of them um and so this Friday the fifth one is due and then in lab you have a project work week so we don't have any in-person meetings uh you were sent information about this um before spring break so uh make sure you look at that document and then send me everything that's required um under the deliverable section by the end of your lap time so last time we talked about the components of the external moment we discussed manipulating moment arm length to increase or decrease exercise difficulty and then we calculated joint moments in static equilibrium um and we did it by using either the rotary component or by finding the length of the moment arm so in today's lecture we'll talk about stability and equilibrium and then discuss anatomical levers with some examples and then we'll talk about what a mechanical advantage versus a mechanical disadvantage is at the end of the lecture today um we'll have some time for some questions regarding um other components of torqus and moments um so if you have questions please be sure to ask after so stability um it's it's the ability of an individual to maintain static equilibrium so recall from last time that static equilibrium occurs when the sum of all the torqus equals zero so there's no angular motion happening and so stability is the ability of an individual to maintain static equilibrium or basically stay balanced so there's a couple factors there's four factors that influence stability the first two are some things we've already covered mass and friction the heavier someone is the more stable they are and the higher friction is the more stable they are as well now the other variable or the third variable that influence uh influences stability is the base of support and the fourth variable is the location of someone's center of mass so if we start with base of support someone's base of support is the area within the outer edges of the body in contact with the surface and that sounds a little funky and I I get that i don't know how but an iPhone user is drawing on the screen i'm going to ask that you not do that um give me one second to clear that perfect um so biomechanically our base of support are our feet and so if we're standing in this position we have some outer edges that are in contact with the surface so if I draw that for you our base of support is essentially right here it is the outer edges of the body in surface uh in contact with the surface which is the ground right so that's our base of support now if we wanted to increase our base of support we could take a staggered stance and now the area enclosed by the outer edges of the body in contact with the ground becomes our base of support so if we stand with the staggered stance or maybe we stand with our feet wider apart we have increased our base of support so when you know athletes are you know in that athletic position that coaches tell them to get into right they're widening their stance by standing with their feet further apart so they are increasing their base of support the greater the base of support greater the stability now if the weight vector so the vector of his body weight stays within the base of support he's going to remain stable if this vector of his weight falls outside of the base of support then stability is disrupted right and he's more prone to falling over so basically we want to keep our belly button within uh the base of support if we want to be stable let me know when I'm good to move on now the fourth variable is our center of mass location so the higher our center of mass is relative to the ground the less stable we are so the more upright we are the less stability that we have and it has to do with torque so let's say this is an individual right obviously this isn't a person but let's say this is a person and we apply a force to their center of mass well now we have a torque we have this force multiplied by the distance from our feet to our center of mass but if we take again that athletic stance that coaches ask us to get in right we're going to flex our knees and increase our base of support so we're basically going to be squatted down a little bit and if we apply the same force to our center of mass our center of mass is lower to the ground so for the same amount of force there is less torque applied to us so that's why if you want to be stable you're going to bend your knees a little bit and lower yourself to the ground so stability is optimized when you have a low center of mass you have a wide base of support you wear shoes that increase friction and you are heavier so the first three are modifiable for the most part the fourth isn't modifiable modifiable um at least in an acute sense so we're we have some examples and I'm going to put up the pictures and I want you to take a minute you don't have to share with me um if you want to you can but you don't have to if we eliminate mass so if we take the size of the individual out of the equation how do the individuals in the pictures take advantage of the concepts of stability so we have uh Kobe playing defense we have this guy trying not to be taken down and then we have this guy trying to take down the other person so how do these three individuals take advantage of the concept of stability to either maintain stable or disrupt someone else's stability again you don't have to share with me just think to yourself Okay so if we're playing defense we we don't want to uh fall over right like it's one of the most embarrassing things that could happen to a basketball player is you know they fall because of the offensive player and so what he's doing is he's increasing his base of support his center of mass is low to the ground and basketball shoes have higher friction than running shoes on hardwood so he's increasing his stability so that he doesn't fall over now this guy um he's trying not to be taken down by him so he doesn't want his stability uh disrupted and if you notice his feet are really really wide apart his center of mass is lowered to the ground now if you have any experience in like a grappling sport or like like wrestling one of the first things that you learn is if you don't want to be taken down what you have to do is you have to do something called sprawl right that's basically when you uh widen your feet and then you drop your uh center of mass or your belly button as low as possible um and so it's harder for the opponent to take you down and basically you're being an amateur biomechanist you are increasing your base of support and lowering your center of mass so that stability goes up and then here this individual is trying to bring this individual to the ground and again one of the first things that you're taught is when you're trying to uh take someone else down you want to get your arms and wrap it around your opponent's knees and if you wrap your arms around your opponent's knees and you squeeze right you are reducing their base of support and then you're making them more upright so that they're less stable and they're more prone to falling over so on an exam I'm not going to give you pictures and you know tell me how they're taking advantage of stability you are very likely going to get a question that sounds like this which of the following are or which of the following are not ways instability is optimized right so maybe put a star or highlight this part uh this bullet point here because that's a very very likely quiz and or exam question all right so before we move on to levers any questions about stability and equilibrium okay so what levers are uh lever systems have a couple components they have a rigid bar and this rigid bar rotates around an axis of rotation and then a force is applied to the lever to move a resistance so it looks like this we have this rigid bar that rotates around an axis of rotation and we apply a force to one side to basically move something on the opposite side so it looks like this right that rigid bar moves now anatomically our joints are levers so our bones act as our rigid bars and our joint center is the axis of rotation i don't um I don't have your notes in front of me can someone let me know um are these given to you or uh do you have blanks is anyone looking at the blank versions by chance okay there are blanks okay so uh again anatomically our joints are our levers so our bones are these rigid bars and then our joint center like our elbow joint center or our knee joint center is our axis of rotation and then our muscle force is the part that causes the rotation we can have resistive forces so things like external objects or the antagonist muscle force those two things can act as resistance so here we have our axis of rotation we have our muscle force and then the force of the dumbbell would be our resistive force so we can have three possible arrangements of the axis of rotation the muscle force and the resistive force that cause three different types of levers that we have in our body so again axis muscle force and resistive force some of your slides might say motive force muscle force and motive force are are synonymous um I use muscle because we're talking about um anatomy but if we talk about just regular levels people call it a motive force so if you see motive force just know that it's the same thing as muscle force so we can have a first class second class or a third class lever and our axis of rotation is A our muscle force is M and our resistive force is R and so the axis muscle force and resistive force can be arranged three different ways to create three lever classes so in a first class lever the axis the A is in the middle so it doesn't matter where the the muscle force or the resistive force are if the axis is in the middle it's a first class lever so we could have muscle force axis resistive force or the opposite resistive force axis muscle force it doesn't matter as long as the axis is in the middle in a secondass lever the resistive force is in the middle and in a third class lever the muscle force is in the middle a little trick that I use to memorize um the lever classes is the acronym A RM so ARM so if I just kind of draw that for you or write it for you it's A R M in a first class lever the axis is in the middle so A in a second class lever the resistive force is in the middle so R in a third class lever the muscle force is in the middle m a R M arm right so that's how I So if you ask me like in a third class lever what's in the middle i won't have it memorized but I'll think a R M m is the third one so in a third class lever the muscle force is in the middle right so that could be a little trick to help you on a quiz or on an exam on a quiz or an exam um I would expect you to know in each type of lever what is in the middle and I would also expect you to know an anatomical example of each of these types of levers so for a first class lever it's where the axis is is in the middle again the order doesn't matter now if we want to have a practical but not anatomical example of a first class lever this is a classic seessaw i believe you are old enough to at least know what a seessaw is right the resistive force is on one end so maybe it's your buddy who's sitting and you want your body to go up so you sit on the other side and provide a muscle force anatomically we can have agonist or antagonist pairs or the atlano oicipital joint so if we take a look at the atlano oipital joint right that's basically where the base of your skull meets your first cervical vertebrae so that is a joint and that is our axis of rotation the weight of the skull is our resistive force so if you've ever you know like obviously this would never happen in in my class but if you're in a really boring class and you fall asleep right we as instructors know because when you sleep your muscles aren't very active and so when you sleep you look like this right you fall forward the weight of your skull for the most part makes you flex your neck our muscle force from our cervical extensors prevent that from happening and so this would be an example of a first class lever now something I want you to notice you don't have to write this down because we're going to talk about it later but just just kind of notice that if we have an axis of rotation and we have a force there's a distance between the force and our axis of rotation right there's some type of a distance here and then there's some type of a distance here so the muscle force has a moment arm and the resistive force has a moment arm right so just keep that in mind we'll talk about that in a couple slides am I good to move on to a second class lever or do we need more time here okay I'm going to move on um just keep but keep in mind if uh you don't write something down or if you miss something you can always access my PDFs later in a secondass lever the resistive force is in the middle right between the muscle force and the axis so two examples could be our foot during planter flexion not our ankle joint but our foot or a push-up so here right when we go on our tippy toes and do planter flexion we don't yes it occurs at the ankle joint but our axis of rotation basically becomes our foot our body weight is our resistive force and our muscle force is on the other side so we have a secondass lever the resistance is in the middle in a push-up we have our axis of rotation as our feet right where the foot meets the ground our body weight is our resistive force and where we push into the ground is where we apply our muscle force also keep in mind um or hopefully you notice between the axis and the resistive force there's going to be a moment arm and between the axis and our muscle force there's also going to be a moment arm in a third class lever the muscle force is in the middle right between the resistive force and the axis so our elbow joint our knee joint and most joints in our body are third class levers so here we have a knee joint the axis is our knee joint center our muscle force comes from let's say the quadriceps and our resistive force right is uh on the other side of the muscle force the same thing occurs for the biceps or the elbow joint I'm sorry So we've gone over the arrangement and anatomical examples of the three types of levers that we can have in our body so just something that really kind of confused me when I was a student taking the class is when when you think about lever systems um I I remember asking my professors like well what type of a lever system is a golf swing what type of lever system is a is a basketball shot um and I really overthought things when we think of lever systems don't think about whole body movements think about your joints and how the joints are moving right like in a multi- joint movement like a golf swing or a baseball swing right the the action itself yes can be a lever but don't think about the whole action think about the joints right the the knee joint could be acting as a third class lever but you know your wrist joint could be acting as a first class like whatever it is don't look at the whole movement just look at it by joint that's the easiest way to think about levers now again what I what I kind of called out earlier was I brought to attention that there is a moment arm for the motive uh the muscle force and there is a moment arm for the resistive force and what we can do is we can compare the length of the two moment arms so we can compare the moment arm of the muscle force to the length of the moment arm of the resistive force so there's a formula you're not going to have to know this but if we divide the length of the muscle force moment arm and divide it by the length of the moment arm of the resistive force we're going to get a number right so if this is like I don't know 2 m and the moment arm of the resistive force is 1 m we're going to get a ratio of let's say two so let's say we do the math and we get a ratio that's greater than 1 we have something called a mechanical advantage this won't make too much sense until we go over an example but if we have something called a mechanical advantage it means we need less muscle force to overcome a larger resistive force so if our resistive force is let's say 500 newtons and we want to move 500 newtons we can produce less than that um to move it but the disadvantage is that we don't have very much range of motion if we have a mechanical advantage again this will make a lot more sense when we go over an example if the ratio is less than one so we divide moment arm of the muscle force by the moment arm of the resistive force and we get something less than one we have something called a mechanical disadvantage it means the muscle force we create to overcome the resistive force our muscle force has to be greater than the resistive force it's the opposite of a mechanical advantage but we gain range of motion so I'm going to move on because this information is repeated in uh the the next slide so in a mechanical advantage right just know that all secondass levers have a mechanical advantage the the muscle force moment arm is longer than the moment arm of the resistive force so we can move really large resistive forces with smaller muscle forces so let's say you want to move a resistive force of 500 ntons we can move a resistive force of 500 newtons with maybe only 300 newtons of muscle force so let's take a look at an example we're going to look at the example of a push-up because the push-up is a secondass lever i'm going to use numbers to help explain this point because it actually does help most students um cement this idea but you don't have to write down any numbers so we're going to say the resistance of his body weight is 735.75 newtons i just made that number up well this resistive force is some distance away from the axis of rotation so there's a moment arm and we're going to say the moment arm is 1 m so his resistive force is 735.75 newtons the moment arm is 1 m so if we multiply those two to get a torque our resistive torque is 735.75 Newton m now let's say he wants to maintain static equilibrium he wants to stay in this position well if he wants to maintain static equilibrium he has to produce a muscle torque that's equal to 735.75 Newton m so to maintain static equilibrium the muscle torque must equal the resistive torque so the combination of his muscle force multiplied by the moment arm of the muscle force has to equal 735.75 Newton m now without even looking at numbers let me know in the chat what has the longer moment arm the muscle force or the resistive force right so if I maybe I can make it a little bit more obvious um where's the pen so right here in yellow that is the moment arm for the resistive force here in yellow that's the moment arm for the muscle force so the muscle force has a lot longer of a moment arm so let's just say it's 1.6 m now to figure out how much muscle force he has to apply to maintain static equilibrium we could divide the resistive torque by the muscle force moment arm and to maintain this position he only has to create about 460 ntons of muscle force this muscle force is a lot less than this resistive force that's the advantage of a secondass lever to move large resistive forces or to maintain static equilibrium our muscle force can be less than resistive force because the moment arm is a lot longer now in a mechanical disadvantage it's the exact opposite all thirdclass levers have a mechanical disadvantage and so the moment arm of the resistive force is a lot longer than the moment arm of the muscle force so now we have the opposite situation the muscle force needs to be greater than the resistive force to maintain or to move something so in this example the resistive force moment arm is long but the muscle force moment arm is a lot shorter so again the moment arm of the resistive force is a lot longer than that of the muscle force so to to maintain static equilibrium our muscle force has to be greater than resistive force this next slide you don't have to write anything down because it's actually a slide from U torqus and moments part I want to say two in this situation we solve for static equilibrium and we saw that in order to deal with a dumbbell that has a a weight of 70 newtons our biceps had to produce 700 newtons right we had to produce 10 times the amount in muscle force versus resistive force so that's a mechanical disadvantage and so what does this mean anatomically well because most of our joints are thirdass levers muscles that act on thirdass levers it they have to be strong enough to produce muscle forces that are much larger than resistive forces right just to deal with 70 newtons our biceps had to produce 700 ntons of force so our muscles especially ones that act on thirdass levers they have to be strong enough to deal with resistive forces because the muscle force has to be so much greater now what I think is kind of interesting is individual moment arm lengths may vary right and I don't mean the moment arm length for the resistive force i mean for the motive force if we look at this right we're not going to do any math but you know maybe you have a friend who's the same height as you and for the most part it seems like you have the same length arm or the same length legs and maybe your buddy right they're just as muscular as you are like you look at him or her and you're like "Yeah we're very very similar but maybe one of you is a lot stronger than the other for whatever reason." It's very possible that one of you has a longer internal lever arm for the muscle force than the other person so we know that between individuals anatomy is it's it's really really variable so yes uh for the like let's look at the quadriceps everyone's quadriceps insert on the tibial tuberosity it's like the little bump on the top of your tibia you could feel it that's the tibial tuberosity but you know person A's tibial tuberosity could be a lot lower than person B's tibial tuberosity right anatomy is really variable and so people of the same height they could have different moment arm lengths for different muscles so those with longer internal moment or lever arms they're going to produce more torque for the same amount of force and it's just genetic so when people talk about like muscle strength um I think there's a default to think about how much force someone produces but we have to consider the length of everyone's internal moment arm so really when we talk about strength we're not interested in how much force they could produce we're interested in how much torque they could produce however in a thirdass lever we get more range of motion and more angular velocity so that's the advantage of a mechanical disadvantage so let me know when I'm good to move forward we are almost done here okay so my a question I want you to think about and I'm going to look for a couple answers why do taller individuals have a hard time lifting weights right if you see really tall like basketball players lift weights they they almost look unathletic they they look a little bit awkward so why do taller people have a hard time lifting weights right um you can tell me publicly you can tell me privately you could just turn off your mic and tell me um I'll give you about 2 minutes to think about it and then we'll reconvene so I'm getting some Am I muted or I think you can No you can hear me um so I got some answers um and I would say those are all accurate those are good answers um and so the reason why taller individuals have a hard time lifting weights I'm just going to phrase it in terms of uh torque or levers so typically taller individuals have longer body segments right they have longer arms longer thighs uh longer lower legs and things like that so all right like this this is not the best angle but let's compare Kevin Hart who's like 4'2 and The Rock who I believe is like 6' 3 let's just take a look at the length of their arms right the Rock has much longer arms than Kevin Hart so longer segments means longer external lever arms so even if the force stays the same torque is increased if the longer if the lever arms are longer so let's say they're both holding a weight that is 50 newtons well for the rock that moment arm for the five the 50 or whatever number I said is longer so for the same amount of force he has to deal with greater external torque so some of you might be wondering well if someone's taller wouldn't their internal lever arm for their muscles also be bigger and you'd be absolutely correct their internal muscle moment or lever arms they're longer but it's not enough to overcome so yes they have longer internal lever arms but their external lever arms are so much longer that it doesn't work out historically this is one of the most more I don't want to say popular but it's one of the questions that I'll ask free response pretty frequently so just put a big star by this even if I don't ask it as a free response question you're going to get a question about it for the multiple choice true false and things like that now people with the same height can also have limb length discrepancies as well so I'm about 5' n i have a friend who's 5'7 um I make fun of him cuz he's shorter than me and I'm not I'm not very tall but his arms are a lot longer than my arms right so think about powerlifters or weightlifters or people who lift weights for a living they're not very tall and if they're tall they're not very lanky right it's because those genetics that make them relatively short compared to taller people right they have shorter external lever arms so they don't have to deal with a lot of torqus so if you look at like really good weightlifters or powerlters they are not tall people they are average height or they are short so just to kind of show you um bullet point two right let's let's say this is a shorter individual right to deal with 70 newtons of force they have to produce with their biceps let's say I don't know 1,50 newtons I just made these numbers up don't worry about the math but what I want you to notice is that let's say Kevin Hart's uh bicep lever arm is 02 m now if we look at the rock his bicep lever arm is longer than that of Kevin's it's 0025 m but to deal with the same 70 newtons he might have to produce 1,120 newtons so even though his internal lever arm is longer the external lever arm is so much longer that this little advantage here it doesn't matter it's still harder for him and you don't have to write these numbers down i just use math to explain this bullet point here so that you get a little bit better of an understanding for a free response question um there's a question so it might take someone who's taller a longer time to hit a maybe not a PR because PR or personal records are relative but let's say you know I am competing with someone who's like 6'5 and we want to see who can squat more um I might be able to lift more because I have much shorter levers um it might just be harder for them uh to lift weights in general because of their external lever arms um don't think about it in terms of like who's going to get stronger faster think about it in terms of how how e how much easier it is for the shorter person uh just to lift weights in general because they have shorter external lever arms right because personal records are relative so let's do our little mini quiz wrap-up what determines stability and how can we manipulate those variables right we talked about those four variables what are the arrangements in the three types of levers and what are anatomical examples of each type of lever system what is a mechanical advantage with anatomical examples and what is a mechanical disadvantage i want you to provide anatomical examples and I want you to tell me about the influence of height so that concludes today's lecture um but if we think about the content that's on the exam you have torques and moments which have three lectures then you have angular kinematics which have two so your questions are going to be distributed um accordingly so you're going to get a couple more questions from torqus and moments than we are from angular kinematics so this lecture I I recognize is short um so what I did was I actually made a review game on Cahoot um so we'll play that in a minute um but before we do that are there any questions about Tors and Moments if there's no questions what I'm going to do I'm going to just double check that my questions make sense um and then we'll play a review game so if you want to like maybe open your internet to get ready to play Cahoot uh that'd be a good idea