I'm engineers so if you guys are back for part two for the muscle mechanics appreciate it what we're going to do now in this video is we're going to talk about again the strength of neuro stimuli how that's affecting the graded muscle responses and then after that we're going to talk about motor units specifically the recruitment principle based upon size okay so we've talked about frequency of neuro stimulus and how that affects these greater responses which can produce you know muscle contraction and generate tension now I want to talk about the strength of the nerve signals let's let's pretend I'm like a little lab nerd I'm a little laboratory nerd and I'm a freak and I got like let's say I have a muscle here let's have a muscle here and I wanna say I have like a little shocker sometimes some type of like defibrillator or something like that and I try to shock this muscle let's say I set the dial up at 10 millivolts whatever okay we're talking theoretically again because I say I started off my first thing is I'm gonna be like I I want to see what happens in this muscle cell if I hit it with 10 millivolts so say I do that let's say a hit with 10 millivolts so I shock this sucker here I shock the muscle nothing happens no response okay that's interesting let's Jack that sucker up let's increase the voltage to 30 millivolts let's hit it with 30 millivolts now let's say I hit it with 30 millivolts for the second time and I notice on a contraction the first observable contraction which is initiated by this first neural stimuli means that I must have reached a threshold so that must be my threshold stimulus or my threshold voltage so thresholds stimuli it means it's the actual minimum amount of voltage that I have to hit this muscle cell with to produce the first observable contraction let me put that under that are right next to it first observable contraction now let's say I decide to be a freak and I jacked a sucker up to a thousand millivolts alright I you know I'm like I you know what I'm going to hit this sucker with a thousand level so let's make it a different color because this should be memorable so I hit it with a thousand millivolts I don't know why I would do this but I do all right I just hit it with a thousand millivolts shock it boom the muscle contracts powerfully I'm talking extremely powerfully and I generate a lot of tension in the muscle shortens excessively so when I have that and I produced this very prolonged and very strong and powerful contraction so let's put here powerful contraction I noticed that okay all right well that's cool let me actually increase it up a little bit more let's say I hit it with like 2000 millivolts for a second so let's say that I follow this one up and for the fourth and final time I hit it with 2000 millivolts but it still produced the same the same powerful contraction so if it still produces the same amount of tension and the same amount of contractile force well then there's nothing different with me shocking it at a thousand millivolts versus mm bolts this must be my maximal stimulus my maximal stimulus so a thousand millivolts let's just suppose is my maximal stimulus for this muscle cell to produce the most powerful force or contraction on this actual muscle cell so let me explain one more time I hit it with very little voltage no observable event occurs that must mean that it didn't reach threshold so it must be sub threshold where's my black there is sub threshold steamed line or stimulus right if I hit it at threshold that produces the first deliverable contraction that must be threshold but if I jack that sucker up to a thousand millivolts and it produces a very very powerful contraction but then I jack it up to two thousand millivolts and I give it the same voltage may give it a different voltage but it still produces the same contraction force then this doesn't mean by maximal Simmons I can't go beyond that point because no matter what even if I increase the voltage it's still going to produce the same amount of contractile force or tension okay let's explain that now taking this concept and applying it to this graph here in this graph okay so first thing we said let's say with the first event let's say here I put two horizontal asthma toads okay let's actually say here is this one here let's say that's my threshold stimulus so this is threshold potential so threshold potential or we can say since we put stimulus there we can also put that we'll put that in like a little parenthesis so this is the stimulus so at that point if you reach threshold that'll produce the observable contraction so let's say that a muscle cell is here at this point in this solid pink line let's say that that's resting membrane potential so I'm gonna put our impe resting membrane potential okay and I hit it with that voltage I hit it with the voltage of about we said 10 millivolts right nothing happened so in other words if we were to draw that one let's say that we do that one with a green I hate it with the actual 10 millivolts and it went up and up and up and up but it didn't hit the threshold attention what will happen to it all right it'll go back down to resting membrane potential but then let's say here with 30 millivolts if I hit it with 30 millivolts what will happen we're going to draw this one now 30 millivolts Oh a reaches threshold potential if it reaches this threshold potential then it's going to generate some tension it's going to generate some contraction okay cool then let's say it comes back down relaxes then let's say here with a thousand millivolts I blast this thing with a thousand millivolts and it goes way beyond the threshold stimulus way way beyond that potential and it goes up to another horizontal asthma - let's say I come across here and this is the maximal voltage that you can hit this cell with so it's the maximal stimulus the maximal stimulus or the maximal potential right so the maximal potential or the maximal stimulus beyond that point I can't go I can't go no matter how much more voltage I hit this sucker with now I'm going to happen so I'm going to reach this maximal potential and then again like every muscle cell it has to go into a relaxation period okay so now that we know that if that's the case then let's see what this corresponds to on this graph because these two we're just going to take okay let's see how voltage the strength of the stimulus affects tension now because that's what that's all we're doing here okay so let's say here's our tension and let's say that this point here we come back here and we make this horizontal Osmocote here this point here corresponds to this point here so it's not maximal potential it's maximal tension so it's the most force that the actual muscle cell can exert so it's the maximal tension okay look what happens here this is super cool now let's say for example though I hit it with the Nike I've kept hitting it with you know the 2,000 millivolts whatever if I were to show that next one look what happens over here now I'm going to show this tension aspect let's say that this one the tension it comes and arises here so rise has a little bit of tension I'm sorry no well let me rephrase this if this is sub-threshold will it produce any tension no because it didn't reach threshold so if it did not reach threshold it won't activate an action potential sorry about that so this should be a straight line no tension developed here but then in the next one we hit it with enough voltage to get it to threshold what should happen okay what hits threshold generates an in plate potential generates action potential muscle contracts it to generate tension so I should see a little peak here so what should happen now I should see a peak at this point it doesn't reach maximal tension but it does increase in tension on the y-axis and then what happens it eventually goes back down to relax then let's say I hit it with the next one so this was the sub-threshold this is the one where I reach threshold but then let's say I hit that sucker with a thousand millivolts what's going to happen it's going to rise to that point it rises to this point here and reaches maximal tension and then it drops back down but watch what happens I want to explain one more thing let's say for example I use that last example and I hit that sucker with two thousand millivolts even though I hit it with two thousand millivolts which might go beyond you'll hit it with that potential there let's say I go a little bit beyond that point I hit it with as much voltage as possible if I hit that sucker with as much voltage as possible what's going to happen so the voltage is a little bit higher it might even be a little bit higher than the maximum potential the maximal stimulus but I told you we cannot go beyond this tension point we can't so even though I'm increasing the voltage guess what's going to happen to the tension it's still not going to go beyond that tension point so what's going to happen here even though it's a stronger voltage it's still going to produce the same tension okay so let me explain this one more time and we're going to go into this last thing with the motor unit again if I take this muscle cell here if I take this muscle set here and the first thing I do is a hit with ten millivolts it reaches sub-threshold it will not produce an observable contraction no tension will be generated second thing I hit with enough voltage that it actually is going to generate the first observable contraction that's threshold stimulus then I increase my actual voltage and I go from that ago 2,000 millivolts if I go to a thousand millivolts and I hit this muscle cell with a thousand millivolt and it produces a very very powerful contraction generates a lot of tension but then I hit it with a voltage even greater than that one so if they hit it with two thousand millivolts and it still produces the same powerful contractile force even though I hit with a thousand millivolts it generate a very powerful contraction let's just say for example 100 Newton's that was the amount of tension that it generated but then I hit with 2,000 millivolts and it still generates 100 Newton's if it generates 100 Newton's when it hit gets hit with a thousand millivolts vs. mm well that's interesting right and that's because maximal potential right if you hit this machical times even if you go above it even if you get hit it with more voltage than normal it still will not change the maximal tension point maximal tension is released I mean is hit whenever you reach the maximal stimulus if you go beyond the maximal stimulus or the maximal potential it's not going to matter because it's still going to hit this threshold point for maximal potential once you go beyond that it doesn't matter so if you hit it with more voltage than is necessary it's not going to matter because all you need is that maximal stimulus to produce this maximal tension okay all right so that's that part so we covered the strength of the neural stimulus now one more thing I got to talk about the strength internal stimulus here and it's related to this motor unit and what do we say a motor unit was it's a motor neuron and all the muscle fibers that is being supplied by it let's say I take a look at these FASTA khals now we apply this concept with these fascicles this is a whole muscle alright and let's say for example that it's the muscles of my arm and get the muscles of my arm let's say first thing I want to do is I want to be able to let's say I have I have a friend named Vincenzo ok a friend named Vincenzo and he's got to give me my money ok and if I come over to him I say hey Vincenzo you got my money and he says yeah I got your money I'm gonna give a pat on the cheek in the patch again a good job Vincenzo right thanks now if I'm doing that I'm just giving them a tap on the cheek am i utilizing a ton and ton a ton of motor units no because motor units so let me write this down this relationship here the greater the motor units like the motor unit recruitment the greater your motor unit recruitment the greater the actual tension or contraction okay so this is for being able to carry out more profound in heavier types of loaded reactions if you have less motor unit recruitment this is very very weak contractions okay so this is for weak contractions or fine movements so it's for very low tension and low contraction or very weak interaction very strong contraction okay another thing is this is for very low fine-tuned movement and this is for very high tuned for high tuned for high fine-tuned movement so for example like I was explaining I can use my same hand the same hand that I can give men chairs oh nice little pad on the cheek forgiving the money if he doesn't give me my money what can I do what bah I'm in a slap right that same hand I could give a pat on the cheek but I could also deliver a very powerful below that's utilizing different motor units so if I only utilize the very little motor units it's going to be for very weak fine tuned movement but if I want to be able to deliver a very powerful blow I'm going to have to pull up a lot of motor units a lot of motor units will recruit more muscle fibers more muscle fibers contracting means a greater force and more tension to one path than chin so make sure never eat that never happens again right that's the whole concept here so if we comply if we compare this right here let's say that you actually the actual tension is increasing right and the tension is increasing what's happening to the actual muscle fiber recruitment so you start off let's say that in this one here this one right here and we generate from Libman attention not a significant amount of tension but it generates some tension let's say that in this muscle this whole bundle I only recruit this many ok just that many but then I decide ok I'm going to hit this sucker with more tension I'm going to have provide more voltage right and more voltage I'm gonna have more maximal stimulus and have more maximal tension what's going to happen here aha I'm going to activate tons in tons of actual muscle fibers ok that's the whole conservationists hold this whole fascicle here carrying out a very weak activity only recruits very little muscle fibers but if you're carrying out a very very heavy activity or something that's require a lot of force a lot of contraction a lot of tension it can recruit many many muscle fibers and that's what we're going to talk about how we do it individually because this goes based upon was called the size principle let me write that down real quick last thing we'll talk about is the size principle okay so let's say first off I have this here incline and tension on the y axis again let's say for the first activity I only want to you know tap in jindo give a little tap right if I only want to give them a little tap let's say that I do this in this maroonish color here what you're going to notice is is you're going to notice it's going to increase right it's going to increase and then it's going to reach this actual plateau phase right and then it's going to come back down right so let's say it's the first one so let's say that this is the one where you're actually going to exert just a little bit of force not nothing crazy you're not maximal tension but your your theory increasing this is the first order recruitment right this is going to recruit your smallest muscle fibers smallest muscle fibers now I want to talk about why but I will in just a second okay so recruit your smallest muscle fibers okay okay so let's say for a second I'm just going to raise this tiny little part here I want to add in another little peak point here okay so let's say that I come down here a little bit with this one obviously it's not perfectly symmetrical but that's okay and I add in another one but it's a little bit higher it's right here it's not a maximal tension but it's it's still increasing it's a little bit higher than this one so let's say that this was number one and this is number two and this is the third one that will pull halt on and it's would still be number three here also okay now watch what happens first thing I did was I rude the smallest muscle fibers right for very very weak contractions so this would be smallest muscle fibers recruited for me for very weak contractions for very fine-tuned type of movement like giving a little back to vincenzo on the face or saying good job it is right that kind of thing now first thing I want to ask you is why is this smallest muscle fibers recruited first besides it's being utilized for weak contractions because they don't generate a lot of force why though it's because these ones have the most excitable motor neurons so these have the most excitable motor neurons in other words their threshold potential that it takes to stimulate these guys is a lot lower so they have a very low threshold potential if they have a low threshold potential they'll reach threshold a lot quicker right and they'll contract a lot quicker so these contracts very quick right so these are very quick it being contracted so these are recruited first vert well for one reason is they only can generate a little bit of force second reason is they're most easily excitable because the motor neurons are super super excited because they have a very low threshold potential so in other words if you were to compare let's say just for a quick second here had to get a chair guys sorry my knees were killing me all right so let's let me erase this real quick so we can have a little bit more room here to explain the swap because I wanted to say a low threshold potential not explained I want to just give a little quick explanation of it so let's say here I have a graph right let's say here's voltage and we'll put it in millivolts and here's time in milliseconds right and let's say here's resting membrane potential at negative 90 millivolts and let's say that I have here this point here is my threshold potential okay normal threshold let's say it's approximately about negative 55 millivolts okay and let's say what I say I said we decrease the threshold potential so let's say I take here in green and I change it now and I make another horizontal asymptote and I have new threshold potential a lot lower than negative 55 which one would it be easier and quicker to get to would it be quicker to get to this one or quicker to get to this one that's an easy question right I mean it obviously is going to be quicker to get to this so if it gets to this one quicker wouldn't it reach threshold lot quicker and therefore would contract a lot faster yes okay so that's one reason why these muscle fibers are recruited first okay one smallest muscle fibers are recruited because not only do they generate very weak contractions but also the motor neurons innervating them are very very excitable they have a low threshold potential okay now let's say I want to generate some more tension I'm going to recruit in the next muscle fiber which we have here in purple now let's say that I take this one here from this graph here from the tension and I correlate it to this one here for a single muscle fiber okay now you'll notice once I come from this point here and I generate I go up a little bit more actually we're doing since we're doing this in time since we're doing this in time let's carry this one off over here so let's carry this one off over here let's say this is happening in a timed timed manner and we notice ok the tension is increasing alright cool so the tension increases and then again it goes back to this point here okay well if I first want to recruit if I'm trying to be able to again how are we comparing this the first one is our given will have been changed a little pat then let's say I give them a little you know good slap okay doesn't require excessive amount of tension but it's going to hit them pretty good right and again where my horizontal asymptote be about let's say it's about right here that horizontal asymptote is representing maximal tension okay if I try to hit him with a decent amount of force I'm going to recruit next thing I'm going to have the second recruitment order right now I'm going to recruit more of the medium-sized muscle fibers so now I'm going to recruit the medium-sized muscle fibers right and what is that going to do that's going to help to generate a little bit more contraction force or tension right so in this case it generates some moderate contraction what else did we say not only is going to generate some moderate retraction these ones are not going to be as excitable as this one so it's going to get it's going to get paw recruited later or second because this threshold potential of the neurons innervating this has is a little bit higher okay has a little bit higher threshold potential so if it has a little bit higher of a threshold potential it's actually say moderate threshold potential just for the sake of a moderate threshold potential TP right then it's going to reach threshold a little bit later than this one so that would reach the actual threshold in a weber Gertrud get recruited second then let's say I'm mad at Vincenzo and I'm going to giving Vincenzo of the lap of his life I might get the glove all right and I like that I smack him I have to generate a lot of tension a lot of force let's say it's the maximum amount of tension that my arm can generate to be able to do that what will we see here we see it right here so let's say we follow this time axis here and we go up and reach the maximal tension point okay this would be the third one to be recruited and they sense it's the third one to be recruited what would you what would you expect for this one well this one was the smallest muscle fibers this one's the medium-sized muscle fibers this one has to be the largest muscle fiber size so this one has the largest muscle fibers also it can generate the greatest contraction and this should be self-explanatory right you this should make sense and the last thing is its threshold potential might be a little bit higher than normal now we've applied that concept of motor unit recruitment right so this is multiple what we call this whenever we're some mating all this activity here these maximal potential stimuli with the maximal tension when we do that we're recruiting multiple motor units and we're some eating them to produce a very very powerful contraction right so they call that recruitment or multiple motor unit summation so we can call this let's do this in black multiple motor unit summation or recruitment all right so you can call recruitment or multiple multiple motor unit summation and again this multiple motor unit summation a recruit miss emission is due to the strength of the neural stimulus the variation in the strength of the neural stimulus okay and if the neural stimulus is very strong it's going to produce maximal tension if it's very weak it'll produce very small tension and again if you're trying to have tension very low tension generated you'll recruit the smallest muscle fibers first because they have a low threshold potential and easily excitable neurons and they also generate the weakest amount of force then if you're trying to do something that's going to require a little bit more force a little bit more tension recruit the second or right and that will be some of the medium-sized muscle fibers which have a moderate threshold potential and they generate a moderate amount of contractile force a moderate amount of tension but then if you're trying to lift something very very very very heavy for example I'm trying to be able to I'm trying to get my gains I'm trying to curl a lot of weight and I'm trying to be able to lift something very very heavy I'm going to recruit the largest muscle fibers okay so if I'm trying to lift the weight what's the first muscle fibers that are recruit first won't be the smallest then I'll recruit the moderate size for the medium size and the last one that worker is the largest size because the largest ones generate the most tension but they're the least excitable neuron so the neurons are the least excitable because they're threshold potentials a little bit higher than normal okay and again come taking that concept and applying it to this whole muscle if your this was just taking one fascicle I could apply that concept to all of these fascicles so again if I'm trying to generate you know let's say I'm trying to generate a minimal amount of tension I could activate these guys right but then I'm trying to generate more tension as I try to generate more attention what am I going to do they activate some of the moderate sized ones and then if I'm trying to generate maximal tension I'm going to generate I'm going to activate the largest ones and that's going to happen with every single one of these fascicles okay which are consisting of multiple muscle fibers all right so ninja tears we cover a lot of information within this muscle mechanics I really hope it all made sense I hope you guys did really enjoy it I hope it helped I really do if it did please hit the like button comment down the comment section and please subscribe all right ninja nerds until next time