this portion will discuss the internal Force when we look at the human movement when we move our body segments uh we can see that uh we move the body segment in the circular pathway uh but when we combine all the involved joint together we we are able to create this kind of linear uh path from carrying an object from A to B so there are several features of uh human movement moving in this kind of rotary or circular pathway so first of all um um in addition to um our body segment move in a circular way uh because we are doing this the orientation um changed uh based on the path so uh uh we can change from pointing up to pointing to the right whereas uh when it is a linear pathway the orientation always uh stay is the same uh when we talk about the external Force if you remember we we talk about the uh uh the second law of Newton uh which is the uh deplacement of uh an object um in the rotary uh movement um the displacement of um an object kind of varies so the distal segment has more displacement than the proximal segment whereas when we produce this linear movement the displacement uh stays equal um we will explore more about uh why our human body uh perform the rotary motion um this way but before we do that I would like to introduce you a term or concept called mechanical advantage from physics mechanical advantage is the tendency of a force to cause rotation related to the spot where the force I'm acting on so using this seesaw as one example the the location of this person sit matters that create this kind of the advantage or leverage um to to move so for um that's assume that this two person have equal weights so it really matters is where the person sit that would C that would create this uh rotation um so the point of uh the application that we call is the leverage or the mechanical advantage the farther this person sit uh we actually um gain leverage um compared to um creating this kind of rotation motion um if this person sit closer then again the the um it it created this kind of um um less leverage to cause the rotation so you can see the relationship here is um the torque which is the tendency to rotate it it is influenced by the distance where the force is acting on right so the distance uh what we call it as a m a moment arm and we will explore that uh later this is one example that I um choose to explain to you about the torque Force the torque is uh the moment of force that uh from the previous uh slide we know the torque equals the force times the moment arm the distance uh uh from the point that the force is acting on so this is the entire set is the torque so we know the distance per perpendicular to the axis is key here um so and we see this as the moment arm so example here this is the brachialis muscle when the elow is in 90° you can see the moment arm is you know 90 degre to the axis right here um it's this um much when um the elbow is ler than 90 de or more than 90° you see that the moment arm actually shortened it is because the ankle uh the the displacement or the angle cannot change even though the muscle Force um that's assume that it remain the same that would produce different um torque because the momentum just changed in another way if we want to maintain the torque because of the different anle of the elbow then the brachat need to generate different amount of force just to maintain uh the same torque so why is this important though this is related to the equilibrium of the torque as the picture shows that this person is holding a down Bell that uh because of the weight of the down Bell that would create this kind of tendency to um extension so when we are trying to hold this stbell in hand and maintain in this position then the biceps need to create this kind of muscle Force to produce this isometric contraction to uh maintain in this u u position if this person is trying to do uh concentric contraction then this uh bicep muscles uh need to produce um a force that would create this tendency toward elbow uh flexion if this person is doing Ecentric contraction um again the the movement direction is uh in the same direction as the dumbell um pool so this person doesn't need um that much tendency that they created for uh the elbow uh flexion and and they can still do the U Ecentric contraction of um using the bicep muscles but why do we need to know U these Concepts it's related to um the liver system that you know from another physics concept that we can apply to our human body so the first class of liver system is where um the pivot point is in between the load and the resistance something like a seesaw so uh in the middle that's the pivot point and we can assume that this is the load or the weight and then this is the force that we want to generate in our daily life we can see that when we're trying to turn a door knob the Axis or the pivot point is in the middle and the two forces acting around this uh pivot point is in a different direction so that we create this kind of rotation move movement when we doing this doorknob turning same thing we can imagine that when we driving a car uh we're stealing uh steering a wheel um it's still the first class of liver system in our human body you we can see that the spine here this um can serve as the Pivot Point the weight of the head and the gravity pole will uh pull this uh the our head down anding forward this is the load and then the resistance can be the nerve extensors to kind of counter um the to uh the weight of the the head so this example is one um um the first class liver system so the second class of liver system is different where uh the moment arm for our force is much longer than the load that we are trying to counter so examples for uh this is when we're using the bottle opener to open the bottle so this is where uh we use uh we modified uh um task for our clients using Assist Technology for here we're using this uh bottle opener so the um pivot point is really right here the weight of the load we doing is right here because we working against the load right here right and our force is actually our uh muscle force is it has much longer moment arm so it's easier for us to open the the bottle right here in our human body we can see that if we're trying to uh stand on our toes um the pivot point is really the toe right right here um and from what we learned about body mass body mass is at the center of uh a segment so we can assume that the the the body mass is in between or in the middle of the the foot right here or here right and um the muscle Force it's at the very far end um compared to the assess of the toll right here so it's uh qualify for um the second class of liver system so we don't don't need that much muscle Force just to stand on our toes the third class lever system is totally opposite from the second class lever system because now the mechanical advantage um goes to the load or preferred um the load compared to the force that we apply and this is H this is very commonly observed in the human body where the muscle is acting as the the force that we do and then we act we constantly um have to work harder to work against the load so take uh the example here um the weight of this body segment is let's say um in the middle of here right and so the moment arm for the weight um uh of the the the the body segment is this long um whereas the muscle in the picture here it looks like a biceps so the insertion of biceps uh is right um here close to the Elbow um joint right here so the moment arm for the insertion is much much shorter than the weight of the body segment that means the bicep needs to produce a lot more Force just to uh move the body segment from A to B and why uh what is the benefit of our human um being designed this way though so that's imagine say um if the insertion of the biceps is actually very close to the wrist joint in this way we are changing um this uh from the third class lever system to the second class L system in this sense the bicep doesn't need to produce uh you know that much force and just to create a counter uh torque uh you know to um the weight of the body segment right here that means there's not much displacement or range of motion produced just to counter death right that means if we trying to eat something we can never eat it because you the muscle doesn't need to produce that much displacement or range of motion to do it so um in our um human body structure the since uh most uh muscles insertion is very close to the movement axis that means we uh are able to produce more range of motion but the downside is we have to produce a lot of muscle Force just to counter um the external Force so so I hope this kind of explain why um the majority of body movement is a rotary movement because that allows us to have more uh joint displacement in other way in other terms that uh increase more range of motion um here are two examples that we can see uh in our human body right here so this is uh the patella this is femur bone and then this is t uh T Bone the patella act um as um you know to increase this um mechanical advantage for the quadriceps right here you can see that with the uh patella the moment arms for the quadricep is much longer compared to um a person who have who has to remove the potato bone so that means uh with without the POA bone this person needs to produce more uh Force from the quadricep just to counter the torque uh another example is uh in our hand the police system is trying to keep the uh hand flexor tendons in place when the pce system is ruptured that we lose that uh advantage to keep uh uh those uh tendon in place then we will lose range of motion and in in the situation like this um we have to educate our client how to remain functional um if they're not doing something uh to repair