Okay, let's go ahead and get started. Um, a couple announcements before we start with the lecture. Um, so this week, remember your labs are split, right? So, uh, you got an email saying that, you know, if come at this time, come at that time. So, make sure you come at the appropriate time today and make sure you're on time because if the class leaves without you, right, like then you can't participate in the lab because we're we're going to a place that's not our usual lab meeting. Uh, and then Monday, we're going to do our elective lecture, which the majority of you voted for spine biomechanics and low back pain. Wednesday. So, in a week from now, we're gonna either It depends if we get through the lecture today and then if we get through spine and biomechanics and low back pain, we're not going to meet. But if something happens, um, and we can't finish both lectures, then we'll use Wednesday to catch up. But more than likely, uh, we're not going to have class Wednesday because we've gotten through all the content. Friday, uh, you have your last quiz. So, make sure you take your quiz this Friday. It's the last one of the semester. Next week, the week of Monday the 28th all the way till Friday the 2nd, uh instead of doing a lab 9 where we collect data. Uh some of our equipment isn't functioning very well right now. Um I try to go in and and make sure that it's working correctly and um I've made some progress, but I haven't completely fixed it yet. So to account for that in your projects, we're going to have a movement analysis uh project workday. So, by the end of your assigned lab time, you're going to email me, not Kiana, what you worked on that day and what you have left, plus any questions if you have any. Right? So, just CC your group members. I don't need an email from every single group member. Just one email per group is fine. And this is the information you are going to send me. Right? So, as long as you send me a summary of what you did, what you have left, and any questions, by the end of your assigned lab time, you will receive credit for that lab. Monday, uh, May 5th, we're going to have our exam 3 review. Wednesday, May 7th, we're going to have an exam 3 study day. So, no class, right? And then after that is just your third exam. And then the semester is over after the presentations. Any questions about the schedule moving forward? All right. If not, let's move on. So, last time we covered muscle biomechanics. We talked about architecture and quality. Then we talked about time, length, and velocity. The five things that could influence muscle function. We talked about the five types of mechanical loads. And so, what we'll do today is we'll talk about stress and strain. We'll talk about what it means to be anotropic and visco elastic. We will discuss tissue elasticity and the stress strain curve. And then we'll talk about bone and cartilage biomechanics. So when we talk about stress, right, mechanical stress considers how large of a force we apply, but it also considers the area in which that force acts. So I have this formula for you. I'm not going to put it on the test, but I do think it helps you understand what it is. Stress is how much force you apply divided by the area in which you apply it on. And the unit of measure is Pascals. So stress is how big of a force you apply divided by how large of an area you apply that force on. right? So, like you know, would you rather punch something with the fist or an open hand? And more than likely, you're going to prefer an open hand because given the same amount of force, right? Your hand is bigger than your fist. So, you absorb less stress, right? That force is spread out over a larger area which is your hand than your fist. It's why boxers wear boxing gloves, right? Yes, there's padding in and a cushion, but that force of the punch is applied over a greater area so you absorb less stress. So this stuff it's not on your test. Uh so don't worry about writing down every single detail. Um but it might help you understand the concept of stress a little bit better. So during a loaded squat exercise, right, like a barbell squat, goblet squat, front squat, whatever it is, the patellophmoral joint forces increase as the knee angle increases. So what that means is as you squat lower and lower the patella and the femur are pushing against each other more and more. But at the same time as we go lower and lower into our squat the area in which the patellophmoral forces act also increases. So as we squat lower more of the patella touches more of the femur. Right? So that contact area also increases. So it has the potential to reduce patellophmoral joint stress. Even if force goes up, stress might stay the same or go down because the contact area also goes up. However, it's not enough to offset the forces. The patellophmoral joint stress peaks at about 90° of knee flexion or about halfway down in the squat. And so if individuals have telephoral joint issues, a lot of the times they'll squat with low levels of knee flexion. I'm not saying squatting to 90° or lower is bad. All I'm saying is that the the patellophmoral joint stress peaks at 90 degrees. But it's also unknown how much patellophoral joint stress it takes to actually cause damage. And there's no data on patellofmoral joint stress when you squat lower than 90°. Right? So the whole point of this is that stress considers not just how much force is applied but how big of an area that force is applied on. Now, mechanical strain is how much an object deforms in response to a given force or a given stress. So, I have a diagram that shows this pretty well. So, I'm going to go over that diagram and then I'll give you a chance to write things down. So, again, strain is a measure of how much an object deforms. And this formula that you're not going to be tested on is strain equals change in length divided by original length. So here we have this object within original length. And if we apply a force or a stress to that object, it might change shape. Right? So this this area here that's the change in length. This object, if we apply a force or a stress, might only deform that much. And so that white area is our change in length. So strain is just a measure of how much something deforms in response to a given force or stress. It could lengthen, it could shorten, it could twist, whatever it is. It's just how much an object deforms. I'll give everyone a minute or so to write that down and then catch up. So for the same amount of stress or force if there is a little bit of strain then the tissue is considered stiff. So if it doesn't deform a lot it's considered stiff. But for that same amount of stress or force, if strain is high, then the tissue is considered less stiff. So it might be considered something like um compliant or flexible. So we could say here that object one it's more stiff than object two, right? For the same amount of force or stress, it deformed less. There is less strain. Now, I believe if you're following along with uh the student version of the notes, your next slide looks something like this. Okay, I accidentally put this out of order on your notes. The slide that we're actually going to do is the next one that we see. and then we'll come back to this uh when when it's time. Right? So, what I want us to do is I want us to go to this slide. It should be the next one, the one that says strain and uh energy return. So, when force energy or stress is applied to a tissue via impact, it'll deform and energy is stored as strain energy. So, we talked about this in linear kinetics, right? like when your foot hits the ground, your Achilles tendon stretches and stores energy and then it'll return that energy to the system. So human tissue can absorb high impacts because it stores strain energy really well. So our tissues will lengthen and deform or shorten and deform and then store that energy as strain energy and then when that stress or force is unloaded the energy is returned to the system. This is the basis of our stretch shortening cycle or plyometrics. It's why you could jump higher if you squat down fast and then come up versus squatting down, holding, and then going back up. However, there's this kind of flowchart, right? This, you know, this ability to store uh strain energy. It can be compromised. So, let's say we do strenuous exercise, right? Maybe we maybe this is an acute sense or a chronic sense. We do a really tough workout or we work out for um you know extended period of time without taking adequate rest. We get fatigued tissue. So, maybe it's our ligaments or tendons or cartilage or even muscle, right? we have some fatigued tissue and if we if we continue to exercise or move on fatigue tissue there's a reduction in the shock absorbing ability. So these tissues that deform and store energy well right it won't do it as well and so that could lead to abnormal joint loading or a stress distribution which has the potential to cause injury. This is one of the mechanisms in which we get hurt when we're fatigued. Now we can load materials and tissue to see a given deformation. We can take a pair of shoes and apply a force to it and see how much it deforms. If a shoe deforms a lot, we probably have a really soft shoe, right? like we have a very cushiony shoe and if it doesn't deform very much it's probably pretty bouncy or the sole feels really really hard. We could see how much bending a bone can take before fracture or how much tension a ligament or tendon can take before failure. So what we're doing is we're basically measuring that object's stiffness. And so again for the same load, stiff objects deform less. Less stiff or compliant or flexible objects will deform more. Now the next term is called elasticity and elasticity is measured via the modulus of elasticity. Right? It's also a lowercase k. So the modulus of elasticity is the ratio of stress and strain. So if we have a high modulus of elasticity that object is stiff. If we have a lower modulus of elasticity that object is compliant or flexible. So stiffness is measured by uh dividing stress by strain. But instead of you know worrying about that what I would do is I would pay attention to this graph right here. On the vertical axis we have stress. On the horizontal axis we have strain. And so what we can do is we could say okay this object we applied this amount of stress and we want to see how much strain they have. And so we would draw a line. So what this is saying is for tissue one we could apply a lot of stress right but it'll it'll only undergo a little bit of strain or a medium amount of strain. The slope of the line is basically the modulus of elasticity. This slope here or tissue 2 for only a little bit of stress there seems to be a lot of strain. So even if we only apply a little bit of stress it'll deform a lot. So stiffer objects have a steeper slope. So this tissue one is a lot more stiff than tissue Let me know in the chat what human tissue do you think is really stiff or what human tissue do you think is more compliant or flexible? Let me know in the chat. I'll wait for a couple answers and then we'll move on. right? So, I'm seeing an answer that skin is compliant and I would probably agree, right? If you just poke yourself in the calf for only a little bit of stress or force, you get a lot of deformation. Or if if you pull on your skin, right? Like I don't know if you can see if I pull on my skin it deforms a lot. And then I'm seeing two answers that say bone tissue is more stiff. And I would agree with that as well. With bone we can apply a lot of force and a lot of strain but it's not going to deform very much. So those are all excellent answers. So every tissue will have its own individual stress strain curve. And so what this one does is it provides a general explanation. So remember again the slope is the modulus of elasticity. Steeper the slope, the more steep the uh the more stiff the object is. As you can see we have a couple different regions. And so let's talk about this area, the plastic or the elastic region. I apologize. So, what I'm going to do is I'm going to describe the bullet points in the elastic region and then I'm only going to ask you to memorize or understand a couple for the exam. So, in the elastic region, this slope, right, it's linear. So, there is a linear response between stress and strain. For x amount of stress, there is x amount of strain, right? They have a they have a linear relationship. What I do want you to know, you don't have to necessarily remember that first one, but I want you to know that in the elastic region, any change in length or shape of our tissue is temporary. So, as long as we remove that stress right in the elastic region, the shape of that object will return to its normal shape. Right. So just some examples, eccentric muscle lengthening. Right? When we undergo, you know, um our muscles in the eentric phase, our muscles lengthen and then when we let that load go, it'll just go back to its original shape. Ligament stretching, right? Our ligaments stretch and then they go back to original shape. So, as long as we're in this elastic region, any change in length or shape is temporary. When you're ready for me to move on, just give me a thumbs up. If you want me to go back to a different slide, feel free. This would be a good time to go back if needed. So, just let me know in the chat what you need. Give it another 30 seconds to let me know uh if you're good to move on or not. If I'm good to move on or not. Um, and then I'll assume I'm good. Okay. Now, you know, this slide was the elastic region. Once we exceed something called the elastic limit, what we're doing is we're entering the plastic region. So again, I'm going to read a couple bullet points and then I'll let you know which ones are important. So it's entered if we don't remove our strain before the elastic limit. So if we get to our elastic limit and we and we don't let go of that uh that stress, right? This should say stress, not strain. I apologize. If we don't remove our stress uh before the elastic limit, we enter the plastic region and deformation is no longer linear. A little bit of stress can lead to a lot of strain. But what I want you to know is just this last bullet point. Any change in length or shape, it's permanent now. It's never going to go back to its original shape or length. right? That's what plastic means, right? If you if you think about plastic, like if you take a plastic uh spoon or a fork, right, and you bend it, right? It'll never go back to its original shape, right? That's what it means for something to be plastic or have plasticity. A good example would be like a hair band or a rubber band. If you take that rubber band or hair band and you stretch it, if you let go of it, it'll go back to its original shape. But if you keep pulling and you exceed the elastic limit, right, it's going to stretch more. But when you let go, right, it's lengthened and it's it's more it's it's less taut, right? It'll never go back to its original shape. That is the plastic region where that change in shape it's permanent. So what I want you to do, I want you to take about two minutes and I want you to tell me, do you think any kind of human tissue has a plastic region? Um, think in the acute sense, not the long-term or chronic sense. Does human tissue have a plastic region? And if so, what kind of tissue does? Right, take two minutes um and then let me know in the chat. You can let me know publicly or privately. I'm going to wait for a couple answers and then again we'll move forward. Right? So, take your time to think and then let me know in the chat. Public or privately, up to you. Don't don't just give me the type of uh structure. Um, maybe tell me why as well. So for the most part, human tissue doesn't really have a plastic region, right? So if you if you think about, you know, let's say tendons, ligaments, and muscles, right? They lengthen and then once you let go it goes back to its original shape. If human tissue had a plastic region then that length the the change in length when it lengthens that change in length would would then become permanent right so your ligaments that stretch if it had a plastic region right it would permanently be stretched to that length or your tendons would be permanently stretched to that length but we know for a fact that doesn't really happen. So what actually happens is if we exceed something called the ultimate stress which is the maximum amount of stress a tissue can handle we might hit the failure point and at the failure point the tissue becomes ruptured and or damaged. In other words we get hurt. So we get ligament tears or tendon tears or muscle tears. So what it seems like is humans have this elastic region but we don't have a plastic region and we basically skip to ultimate stress or the failure point. Right? If we stretch a tendon too far and we don't let go that change in length isn't going to become permanent. It's just going to tear. It's going to fail and then we need surgery to go back in and repair it. So ultimate stress and failure point they can basically be the same point it can be different. Sometimes at ultimate stress our tissues begin to uh fail at the microscopic level and then we see whole tissue damage at the failure point. I'm not too concerned about that. Basically know what the failure point and ultimate stress are by definition. Let's take a quick like a really really quick two-minute break. Um we'll reconvene at like 9:04 and then we'll talk about um tissue anisotropy and visco elasticity. Okay, let's talk about anosotropy and visco elasticity. So, what I'll do is I'm going to again talk and then I'll let you know when you can start writing things down. I think this it's one of those things where it makes more sense if you write um after I give the explanation. So human tissue is anotropic. And so what that means is that it has different responses depending on the direction of loading. And so last time we talked about the five ways a tissue can be loaded, right? Tension, compression, torsion, bending, and shear. And so if you think about the definitions of each type of load, right, it's just they have different directions. And so a vertical load is called compression. or if it's a vertical load the opposite way it's tension, right? If it's horizontal, it's shear, things like that. And so what this basically means is that human tissue could be really strong or stiff to compression or weak or compliant to bending. Right? These aren't, you know, facts. These are just options, right? Some tissues would love compression. Some tissues hate compression. It's due to the arrangement of collagen, right? But that's what it means to be anotropic. It has different responses to the type of load you apply to it, right? So you go ahead, write that down and then we'll move forward. human tissue it's also visco elastic so again uh I'll let you know when you should uh start writing what visco elastic means is that it has both elastic and viscous properties if I didn't give this to you don't have to necessarily write this bullet point But it mean visco elastic means it has different responses to the rate of loading or how fast we load. So for a given tissue if we load it fast versus loading it really slowly that tissue might exhibit different properties a really good way to think about this is water. So, if you're if you're, you know, let's say there's a like a pool in front of you and you take your hand and you slowly put your hand in the water, right? Let me know in the chat how does that water react or what happens to your hand if you slowly put your hand in the water? Does the water feel soft or does it feel hard? Right? The water feels soft. Your hand can enter the water. But on the other hand, if you were to take your hand and slap that water really, really quickly, right, it would hurt your hand and that water, it would no longer feel soft. It would feel hard and there would be surface tension. Well, human tissue is very very similar because human tissue has a lot of water, right? So sometimes if you uh load it really really fast, that tissue becomes really stiff. But if you load that same tissue slowly over a long period of time, right, it might deform a lot more. That's what it means to be visco elastic. So take a minute uh and then write down whatever you need to write down. Let's talk about bone. So we all know that bone protects organs, provides support and is an attachment site for muscle. It's made up of calcium which provides its stiffness, right? It's why people say, "Hey, like make sure you take your calcium. It's for bone health and collagen. It provides bones with tensile strength. There are three types of bone cells. We have osteoblasts which build cells and osteoclast will take away bone cells. And then osteocytes transport metabolites between uh blasts and class and controls the activities of the osteoblast and the osteoclast. Now bone is constantly being remodeled. So what remodeling means is that osteoclass will remove older bone and then an osteoblast or multiple osteoblasts will come in and build new bone. So the bone that you're born with isn't necessarily still the bone that you have. It's been remodeled. This process it happens naturally in response to some kind of stimuli or it'll happen because there is no stimuli. That's called Wol's law. So Wol's law basically says in if there is demand bone strength, density or mass will increase. If there is no demand, bone strength and density or mass will decrease. So if there's no stress or forces applied to the bone, your osteocytes will basically tell it tell the osteoclass, hey take away bone, we don't need it. But if there is demand then the osteocite will have more osteoblasts come and build new bone. So what this tells us is if we want really good bone health and bone strength, we have to create demand and the demand is done via exercise. So not just the amount of exercise but the form of exercise is crucial if you want good bone health. So we want to do high impact sports or high impact activities like basketball, track, volleyball, right? weightlifters, not people who lift weights, but Olympic weightlifters, they have probably the strongest bones because they are loading their bones really, really rapidly with heavy loads. Athletes who might be in really good shape are swimmers, right? they have a lot of cardiovascular demand, but if they don't do any running or lifting weights, right, they might have poor bone health because the water holds up their body weight. So there really is not too much load on the bones. So the amount and the type of exercise both matter. So bone is anotropic. It's really strong in compression. In fact, it's 52% stronger in compression than tension and three times stronger in compression than shear. You don't have to know these last two bullet points. All you have to know is the first that bone is strongest in response to comparison or in response to compression. Bone is also visco elastic. So bone actually responds better to fast loading like rapid high impact loading, right? It's really stiff when you do that. But when you apply loads over a longer period of time slowly, bones are a lot weaker. This is why bones prefer high impact loading because that is how they grow. They are really really stiff when you apply loads quickly. The next tissue that I chose is cartilage. And I chose cartilage because I think cartilage is kind of poorly understood by the general population. So I I kind of want to help that. So articular cartilage is thin and it surrounds the ends of long bone. So this smooth egg looking thing right here, right? This is a bone. But the white part is the cartilage that surrounds that bone. Articular cartilage provides joint stability, distributes forces, allows for smooth movement between bones by decreasing friction. It's 10 times more slippery than ice. So this image here, it looks really glossy and shiny. Right? Because articular cartilage it's really slippery. Did I give this to you in your student versions or do you need more time to write this down? I don't have it in front of me. All right, I'm just going to assume we're all good to move on. So, articular cartilage is comprised of zones and layers. So, this is fair game for the exam. The thin or superficial zone is the most superficial layer. It's about 20% of the cartilage volume and it's the first line of defense against shear. So it it enables resistance to shear tension and compression. It's not that cartilage doesn't like this, right? It's just that this zone of the cartilage is protecting right the deeper zones from them. The middle or transitional zone which is about 40 to 60% of the volume is the first line of defense to compressive forces. While the deep zone which is about 30% of the volume provides the most resistance to compression. Now articular cartilage is both anotropic and visco elastic in that it responds really well to compression particularly the deep zone. If you load articular cartilage fast it becomes really stiff like water but if you load it slowly over long periods of time we get a lot more deformation. What's interesting about cartilage is that it receives no blood supply and it doesn't have any intervation from nerves. So this is important because human tissue when it's injured, it relies on blood, right, to remove waste and to supply it with nutrients so that it could heal. But articular cartilage, it doesn't receive blood supply. So there is no blood flowing to your cartilage to remove waste and supply it with nutrients. Articular cartilage is really thin and it stores synovial fluid. And so in order for the uh the cartilage to receive nutrients, synovial fluid has to diffuse in and out of the cartilage. So synovial fluid will diffuse out, remove uh waste, bring in nutrients and diffuse back in, right? So that the cartilage can get the nutrients it needs. If this does not happen, if this diffusing in and out via compression does not happen, the cartilage will die. So, think of cartilage like a sponge. If there's a a wet sponge and you compress it, water comes out. Once you let go, the water goes back in. Articular cartilage is exactly the same way. So the question becomes how can we compress cartilage? It's not like we can go in and just compress our cartilage to make the synovial fluid go in and out. It has to happen externally. And the way we do this is via exercise, especially high impact exercises. Our cartilage can withstand high cyclical loads without taking damage. So, one of the ways that we can exercise at high impacts and and you know apply cyclical loads is via running. Now this is where there's a lot of misinformation and misconception about articular cartilage is there's this thing where someone might say running is bad for your knees because of the wear and tear, right? Or they might say I used to run a lot and my doctor said I have osteoarthritis now and and I have knee pain or my knee joint is bone on bone because I ran too much. Right? I heard this from my friend the other day, right? Like he was he was talking to me about running and how he won't run because he doesn't want to damage his cartilage. Well, if we go back to this slide, remember if we have no um synovial fluid diffusing in and out of your articular cartilage, right, it it's going to die. And so it's not that we're born with a certain amount of cartilage and that we have to preserve it for the rest of our lives. That's not what happens. Excuse me. Research actually shows that sedentary individuals have very thin articular cartilage compared to more active individuals. So if it's not getting the the nutrients that it needs, it's going to die. So it's not this fragile piece of tissue that eventually gets worn down over time. Some of it might happen just naturally due to aging, but cartilage adapts like every other tissue that we have and it can grow and it can get thicker and more healthy. So research shows again that running is not associated with X-ray evidence of neoa and it could actually protect against knee pain. marathon runners, they actually have less prevalence of hip and knee pain associated with knee osteoarthritis. So this study, I thought it was an interesting study. It was an observational study over I think two two three years maybe 10 somewhere in that range and they looked at you know X-ray readings of the both of both groups pain levels things like that and they found that after it was 10 years after 10 years the group that didn't run had more knee pain more osteoarthritis and thinner cartilage than the marathon runners. So even though these marathon runners are, you know, applying like wear and tear to their cartilage, they have healthier cartilage and healthier knee joints. So running is not associated with knee osteoarthritis. In the short sense, after you go for a long run, your cartilage volume might be less, right? But after a short amount of time, the cartilage will expand back to its normal shape. Um earlier when I was doing research in runners um we measured how much cartilage they had via cartilage thickness. We could use ultrasound to do that. And so we had them run but we also after their run had them lay down for 30 minutes to an hour so that their cartilage volume could return to its normal shape. So it's common to associate short-term cartilage volume loss with long-term cartilage volume loss. So as a wrap-up, right, you should know what the difference is between stress and strain and also how they're related. What does it mean to be anotropic and visco elastic? What is the modulus of elasticity? And how can we tell if an object is stiff or not? You should be able to explain all components of the stress strain curve. You should be able to explain how bone is anotropic and visco elastic and how we can improve bone health. And you should be able to do the same for articular cartilage. Remember, if you have uh lab this week, if if you haven't had it already, uh we are splitting times. So, make sure you come at the time the email told you to. Um if you have questions, stick around. If not, I'll see you later this week. If not, have a good weekend.