hey guys dr gooden here to talk about biomechanics and muscle leverages let's get ready to [Music] learn [Music] as i mentioned in the intro in this video we'll talk about biomechanics i'll first introduce and define that field very briefly and then talk about some of the topics within biomechanics as they relate specifically to strength and conditioning and then we will focus in on types of levers in the human body different joints are set up with different leverage systems sometimes we operate at a mechanical advantage and sometimes from a mechanical disadvantage and we'll figure out some of those classifications in this video so we are in chapter two of the essentials of strength training and conditioning and this chapter was written by jeffrey mcbride okay so biomechanics what is biomechanics well biomechanics refers to the mechanisms through which components of the body interact to create movement so pretty simple right bio meaning life mechanics meaning a machine it's like the machine of life how do we look at the human machine how do we look at that human machine from both a kinetic and a kinematic point of view kinetic referring to the forces encountered and produced by the body kinematic referring to the resultant movement of the body in space so let's examine skeletal musculature and how it creates movement so we have a system of muscles that enables the skeleton to move i think we already all know that well each of these muscles has what's called an origin and an insertion so the origin is the proximal or toward the center of the body attachment and the insertion point is the distal or the away from the center of the body attachment okay so for instance if you think about your hamstrings those muscles the origin is on the ischial tuberosity right your sit bone or your butt bone and the insertion is well two of them insert medially on the knee uh at the pez answering and the other one inserts laterally on the knee and so if you think about the origins and the insertions running from just below your glutes down to the back of your knee then we think of those hamstring muscles contracting concentrically shortening and as they do that they generate tension and they will pull either at the origin or at the insertion to produce movement either at the hip or at the knee joint now we also have a classification system for determining what a muscle's action is during a movement so if a muscle is causing a movement say i'm doing a biceps curl so my biceps is causing that movement it's causing my elbow to flex i would call the biceps an agonist in that movement the muscle most directly involved in bringing about the movement we also sometimes call this a prime mover the antagonist then is the exact opposite it's a muscle that has the capability of stopping that motion or opposing that motion so the antagonist in that movement would be the triceps a muscle that can slow down or stop the movement okay so if i'm doing a biceps curl go ahead and do it with me at home do a biceps curl right this is number 1000 for me today can you guys tell the muscle that could slow down this movement would be the triceps if i co-contract that triceps it slows down the movement and my joint grinds to a halt and if my triceps was overpowering my biceps let's say i now relax the biceps i could pull my elbow into extension assisted by gravity because gravity always goes down with the triceps muscle a synergist is a muscle that assists indirectly in a movement and the key word there is indirectly so in that biceps curl a synergist muscle could be pronator teres right pronator teres pronates at the radial ulnar joint but my biceps brachii not only does it flex the elbow but it supinates at that joint so if i wanted to prevent further supination by biceps brachii as it's going through its elbow flexion movement then i could also be contracting isometrically my pronator teres and in that case it is stabilizing one aspect of my body so that i can complete a movement at the other part of my body another example is your upper back musculature the small muscles of the upper back connected to your scapula right these scapular stabilizers often stabilize the scapula in various positions whether it's completely retracted or sometimes in protraction or sometimes in upward rotation and they stabilize it there so that you can then move at the distal segment at your shoulder joint right so the shoulder girdle stabilizes and those synergistic muscles hold it in place while your shoulder joint moves or performs an action now if we think of these muscles as levers it opens up a lot of possibilities as far as really assessing movement so there are some muscles in the body that do not act as levers these might for example be your abdominal muscles don't act as a lever they don't attach necessarily to two solid moment arms of bone but most body movements directly involved in sport and exercise do primarily act through bony levers of the skeleton now what is a lever a lever is a rigid or semi-rigid body that when subjected to force whose to a force whose line of action does not pass through its pivot point exerts force on an object impeding its tendency to rotate that was a mouthful but essentially what it's saying is that a lever is a rigid or semi-rigid body that when there's a force acting on it anywhere but through the fulcrum right if i put a force right at the fulcrum nothing's going to happen but it will rotate right or it will have a tendency to rotate if i put a force on it on either side in either direction and by putting a force on one side it's causing a force on the other side okay so if you imagine if i had you know i don't know something sitting here on this pencil and i put some force here that pencil is going to rotate this way and put force onto whatever is on top of it right and that's that's a lever okay so here's a picture of a lever and we will talk through the components of it so the lever can transmit force so it transmits the force of a muscle tangential to the arc of rotations so if this is your applied force here and this is the resistive force we're going to turn turn the lever this way and it's going to push the supplied force this way so it's applying force at a tangent to that arc that's a tangent line but let's say that we moved through part of this arc and now the ball is right here sorry i'm drawing over the definitions but now the tangent would be there so you get the picture as this lever moves through its range of motion through this arc the force is being applied in different directions right because it's always tangential to that to that arc so the applied force is here the resistive force is here and this distance between the applied force and the fulcrum and between the resistive force and the fulcrum these are what are known as moment arms now the moment arms are very important because the ratio of one moment arm to the other gives us the mechanical advantage of the lever system so let's talk a little bit more about mechanical advantage mechanical advantage is the ratio of the moment arm through which an applied force acts to that through which a resistive force acts applied force over resistive force so if this number is greater than one meaning that the applied force moment arm is greater than the resistive force moment arm then your muscle will be at a mechanical advantage meaning that the muscle can apply less force than the resistance is applying okay because of the mechanical advantage now if that moment arm for the muscle or the applied force is smaller is operating at a disadvantage and so your muscle actually needs to produce more force than the force provided by that object on the other end of the lever so here is an example of a mechanical disadvantage and this would be a first class lever all right in this example we're thinking about elbow extension against resistance or like a tricep push down okay so you're pushing into elbow extension this this is a first class lever and it's a lever for which the muscle force and resistive force act on opposite sides of the fulcrum all right so here is the fulcrum right there where the joint axis is and based on the insertion point of the triceps it's about five centimeters from the fulcrum to the triceps where it inserts but it's 40 centimeters from the rotation point of the elbow to wherever that resistive force is okay so let's say in our hands we're holding the handle of a triceps push down or a rope or something like that attached to a cable machine and so that's where most of the force will be in reality we also are moving our forearm as well and so it might not be all the way at the hand it really depends but let's just imagine that most of the force is here and so we can estimate now if we put 5 over 40 that gives us 1 over 8. and so our triceps is working at a at a 1 to 8 mechanical disadvantage you have to produce eight times as much force as is resisting you in order to move or at least even stay even against that resistance if you want to move the resistance you have to produce more than eight times that force a second class lever is when the muscle force and resistive force act on the same side of the fulcrum let's identify the fulcrum in this picture here it is right at the ball of your foot and you can see that most of your body's weight is going to be coming down right here right through your ankle joint about even with your medial and lateral malleoli and gastrocnemius is providing a force up here from your calcaneus as it shortens due to the due to the longer moment arm of the muscle so from here to here is longer than from here to here we're operating at a mechanical advantage so in this case your gastrocnemius has to exert less force then your body is exerting downward because of gravity in order to raise you up onto your toes this is the same principle that is at play when you are carrying a wheelbarrow you could put a ton of weight hundreds of pounds in a wheelbarrow and because you're lifting it way out at the handles and the fulcrum is forward at the wheels and the weight is in between those two you don't have to lift with as much force to actually elevate the wheelbarrow off of its two supports and then you can push it now another example in this case would be push-ups if you think of the whole body as a lever and your arms not just your a single muscle now but your arms through a multi-joint movement are moving your whole body up and down but it's really not your whole weight it's not the weight of your whole body because you are operating at a mechanical advantage when you're doing push-ups now a class lever this is a lever for which the muscle force and resistive force act on the same side of the fulcrum but with the muscle having a shorter moment arm and so this takes us back to our classic biceps curl exercise because your biceps inserts down here on the radius and ulna instead of way out here where the resistive force is because they insert here you're operating at a mechanical disadvantage okay so the distance from here to there is much shorter than the distance between here to there so you're operating at a mechanical disadvantage now another example of leverages at play in the human body is when we consider the knee and the role that the patella plays in the leverages for the quadriceps muscles now in picture a we have a knee with a patella and this increases the mechanical advantage of the quadriceps by maintaining the quadriceps tendon's distance from the knees axis of rotation all right so as these quads shorten and it will pull this way and this patella is actually pushing that tendon out a little bit farther not much but a little bit farther so that when it pulls at the insertion it has a better mechanical advantage around this axis of rotation whereas if you didn't have that patella instead of being out here now we are about that much shorter which even though it's a small distance it's magnified because we are talking about ratios right so that small change in the mechan in the moment arm of the quadriceps yields big big force production differences during knee extension now something that's really interesting is that for most movements if you consider the full range of motion of that movement the mechanical advantage or the ratio of the moment arm of the muscle to the resistive force it actually changes through that range of motion so if we look at this picture we have a biceps curl happening and then just look at the size of m throughout the picture now it doesn't change much but you can tell that in these first three images the moment arm is the longest here in c it's in between and b and it's the shortest in a i don't know why i went backwards through those and then it gets shorter again well let's call this d actually and finally again it's short at e so as far as leverage goes the biceps brachii is is mechanically stronger when you're right at 90 degrees because that insertion point is farther away from the axis of joint rotation now that doesn't mean that necessarily the length of the muscle is the strongest at that point theoretically that happens when the most myosin and actin cross bridges are forming okay so at the top and the bottom of that movement namely here and here your biceps brachii has less mechanical advantage than when you are right here in the middle furthermore as you're lifting the weight the distance from the weight to the joint rotation or the moment arm of the resistive force that changes as well and so if we start at the bottom at a it's small at b it's medium and at c it's the greatest and then it gets smaller again as we go up through d and e so there's really kind of a trade-off even as your muscles moment arm is the longest so is the moment arm of the resistive force and this is all just for a first class lever it will change a little bit in the second and third class leverages but just know that the range of motion through which you're working greatly affects your muscles ability to produce force through the lever that it is acting so key point though in the human body most of the skeletal muscles operate at considerable mechanical disadvantage this means that during sports and other physical activities forces in the muscles and tendons are much higher than those exerted by the hands or feet on external objects or the ground so when you have an athlete and you're measuring their foot contact with a force plate let's say during a jumping movement or a cutting movement and you measure them at four to five times their body weight of force into the ground that's just the force that their foot is exerting but in their musculature those muscles are operating at a disadvantage and so they're producing even more force than what you're measuring in your force plate yes external force measurements are a great way of assessing an athlete's strength or their ability to generate force quickly but if we actually consider the muscle tissue and the tendons they are subjected to much higher forces which has implications for how we prevent injury and how we treat injury in our athletes okay so that was an introduction into biomechanics as it relates to strength training and conditioning and we focused in this video on leverages first class second class and third class levers if you had any questions about this let me know in the comments below and i'd love to get back to you the next video is going to be all about biomechanical characteristics of strength and power in humans so what is strength what is power we'll start with definitions and then we'll get into how do biomechanical factors affect power and strength in individuals okay so that video should be appearing somewhere on the screen go ahead and click on it and i will see you in the next video a synergist muscle could be pronator teres right pronator teres pronates um pronates at the radial ulnar joint