i n engineers in this video we're going to talk specifically about the mechanics of muscle or Muscle Mechanics all right so if you guys have watched our videos on skeletal muscle contraction you guys have seen part one and part two and part three and all the aspects of the how the muscle is Contracting and also if you guys have watched the actual muscle fiber types that's also going to help you in this video as well so the main topic of this video is we're going to be talking about graded muscle responses okay what what do I mean by that so what is first off let's write down over here what is a graded muscle response and what does it depend upon so a graded muscle response is basically how the muscle cell responds to two different types of stimuli okay so what do I mean by that so there's two main types of stimuli that can produce these graded potentials these graded responses within muscle cells in other words trying to bring the muscle cell from resting membrane potential to a threshold potential all right so those two stimuli is depending upon the frequency so it depends upon the frequency of neural stimuli because that's mainly the way that these skeletal muscle fibers are stimulated our skeletal muscle fibers are mainly stimulated based upon neural stimuli primarily the sematic nervous system via the the um nicotinic receptors from acetylcholine the other thing that determines the actual graded muscle response is the actual strength of the neural stimuli so the frequency of the neural stimuli affects the graded response and the strength of the neural stimuli is also another important effector of graded responses now before I get into depth about grer responses I'm going to talk about something else really quick and it's they're two uh things that I'm going to talk about are in relation to one another I want to talk about first what is referred to as a muscle twitch what is a muscle twitch okay that's the first thing I want to talk about and then after that we're going to relate a muscle twitch to motor unit okay for right now I'm going to Define what these two terms are meant okay what they mean by these two terms a muscle twitch right is a muscle contraction it's a very very brief and Rapid contraction of a muscle response to a single neural stimulus that is what a muscle twitch is here let's write that down just so you guys have that so a muscle twitch is what it's a contraction rapid contraction of muscle from one okay I'm going to put like a little starring around that one neural stimulus okay that is what is meant by a muscle twitch it's a rapid contraction of a muscle from one neural stimulus um uh the other thing that I wanted to talk about was a motor unit and we'll just we'll talk about these in great detail but a motor unit is by definition I like to make it like a mathematical equation it's a motor neuron plus so a motor unit equals the motor neuron plus muscle fibers that motor neuron supplies so muscle fibers supplied by that motor neuron that together makes up a motor unit so I just wanted to get these terms just the quick definitions and explanations these terms because now we're going to go into depth and explain what all of these things are so again a graded response is dependent upon two things and we can we can say a graded response is a graded potential it just means you're trying to take the resting membrane potential of a muscle cell which is 9080 90 Mt and bringing it to threshold okay and that's about 55 how do we do that it depends upon the frequency of the neural stimulus and it depends upon the strength of the neural stimulus we'll also talk about what's called a maximal threshold okay we'll talk about that too muscle twitch is a rapid contraction of a skeletal muscle in response to one neural stimulus just a really rapid contraction and a motor unit is a motor neuron in all the muscle fibers that it supplies okay now that we got these definitions out of the way let's go ahead and start talking about this stuff first thing I want to talk about is a muscle twitch I want to talk about the phases of a muscle twitch so if you look here you guys probably have already seen the structure of skeletal muscle video we basically took a whole muscle belly broke it up right here and then you have a big old here's the muscle belly cross-section and all of these structures here are fices if you remember that these this big bundle right here that we're just blowing up one this is an example of a fasle right and a fasle is surrounded by a connected tissue we talked about called parium and then that fasle consists of bundles of muscle fibers or muscle cells which each individual one is wrapped by endomysium well we're going to do is we're going to talk about how these muscle fibers are actually Contracting with the response to that twitch let's say I let's okay for example in a muscle cell it has three phases let's mark out those three phases one phase is the latent phase okay the latent phase the second one is the contraction phase and the third one is the relaxation phase or the refractory phase okay so for example the latent phase the latent phase is let's say that I actually if you guys uh remember let's say here that I have my actin okay my all my thin filaments right so here's my actin there's the little pockets right and if you guys remember let's pretend for a second that this is Mein okay here's my thick filament which consists of my myosin heads right if you guys remember whenever these guys the mein and the actin interact with one another that's the crossbridge formation we're going to start the sliding filament theory well let's say for that brief period in time it's very very very brief okay but it can differ based upon different types of skeletal muscle fibers depending upon their location what they're designed to do but generally whenever a muscle cell is getting ready to contract the cross bridges are activated but the mein is starting to get ready to move the thin filaments but it hasn't started to move them yet there hasn't been any sliding occurring okay no sliding has occurred yet but the mein and the actin are connected with one another that is the key thing there is a mein actin interaction in this Laten phase but there's no power stroke there's no movement there's no sliding there's no shortening of the actual muscle fiber so therefore if you were to look on this graph let's say on this graph we have x x- axis here which is time in milliseconds on the y- axis we have tension what did I say the cross brides are active right but they're not starting to shorten the skeletal muscle so will there be any tension not really there might be a tiny tiny bit of tension but there's not going to be anything significant so in other words we're going to see almost a straight line okay a straight line or a plateaued line which means that there's almost no tension being developed or the tension is remaining constant at zero let's say or we if we want to we can say that it develops a tiny bit of tension let's say it's like five okay so it's like five you know Newtons in this case if we're talking about tension in the form of force right let's talk let's say it's in that case but then what happens in the skeletal muscle you guys remember that we had that sarcoplasmic reticulum let's say say here's our sarcoplasmic reticulum here's another cop plasmic reticulum and you know in between the sarcoplasmic reticulums you have the T tubal and here's your muscle cell membrane right let's say for a second that you stimulate this muscle cell you provide a neural stimulus so here's a neural stimulus neural stimulus activates voltage gated sodium channels sodium starts FL flushing in right when the sodium starts flushing in it causes an action potential across the actual ccma which stimulates the sarcoplasmic reticulum to produce calcium and put it out here into the cytoplasm when the calcium is out here it'll actually bind onto that protein what is that protein called it'll bind on to troponin and troponin will change the shape of the actual tropomyosin and it'll leave to the myosin acting uh binding to the actin that's what's happening it's the latent phase right so that's happening in the latent phase but in the contraction phase what's different now the mein heads are actually moving now it's creating power strokes if it's creating these power strokes what's going to start happening to this actual thin filament it's going to start moving moving moving moving closer to the actual mline remember so you have the M line right down the middle of the ccle mirror these thin filaments if they're moving closer and closer and closer towards the mline aren't they actually isn't the muscle fiber shortening then aren't the eye bands getting smaller isn't the AG Zone getting smaller isn't the Z disc getting pulled closer together so the muscle fiber should actually beginning to shorten if it starts shortening what does it begin to develop tension and that is when you develop this actual high amount of tension so let's say here this is the latent phase but then we're going to do in this color it actually starts to develop this actual tension because now the cross Bridges between actin and myosin are very active and they're starting to move remember just briefly if you pretend I was the uh mein here's the actin so remember whenever ATP binds what happens to the myosin it detaches then it hydrolyzes it then what happens it gets cocked back let's say here's another actant what happens after it actually uh hydrolyzes that ATP into ADP and an organic phosphate it back and then reattaches to that one then what does it do releases the phosphate and creates another power stroke and that event keeps happening in the latent phase it's stuck here it's not moving it it's getting ready to in the contraction phase it's moving it detaching coming back and attaching to another one okay so that's how it's developing this tension so you're going to see a rise in the tension okay this phase is the contractile phase or the contraction phase so we'll say this is the second phase can't even see that two I'm sorry let me fix that guys that is the contraction phase and this is the laden phase okay now if you guys remember remember uh whenever the muscle was done Contracting it reached a peak potential of about positive 30 MTS so let's say that it reaches a maximal tension here right okay and it reaches its point of Maximum voltage so positive 30 molts if it reaches that positive 30 MTS what did that do to the pottassium channels remember it activated the activation Gates and open up the channels for potassium lines to start leaving the cell what else happened do you guys remember that we started pushing some of the calcium back into the sarcoplasmic reticulum and if we start pushing that calcium back into the sarcoplasmic reticulum via the calcium proton ATP Asis or the calcium sodium exchangers we're getting it back in so now the calcium is starting to be released the potassium is starting to go out what's happening to the muscle cell it's starting to relax the cross bridges are becoming inactive if the cross bridges are becoming inactive then what's going to happen to the tension it's going to decrease so now look at this graph I'm going to draw the end part of the graph with green look what happens to it it goes back down and this is the actual relaxation phase or the repol ization phase and again this is the third phase and what is that due to that's due to two things the potassium ions leaving from the cell as well as the calcium going back into the sarcoplasmic reticulum so that the cross Bridges become inactive and then if the cross bridges are inactive they're not having power strokes they're not moving the myof filaments there's no sliding of the myof filaments occurring so therefore tension decreases but then what happens after the tension decreases it goes back into the latent period okay and we start the whole cycle all over again this is referred to as the actual this is going over the events of a muscle twitch this is going over the events of a single muscle twitch okay due to a single neural stimulus so these are the phases of a muscle twitch again the laden phase is the cross bridges are actually you know are happening there is a crossbridge between the Act and the mein but they're not moving contraction phases they are moving the Acton there is sliding of the M fils there is shortening of the muscle there is tension developing and the relaxation phase is when the calcium goes back into the sarcoplasm reticulum potassium goes out of the cell and the cross Bridges become inactive and tension starts decreasing and then it goes into relaxation okay so that's our muscle T but one thing I want to mention to you guys is you know what's really interesting this is on on average we're going to say on average this contraction phase it technically can occur from a range um so from here to here so this is the about the point here let's say I come down here and I make this point here that point there is usually only about 10 it can range from 10 to 100 milliseconds okay this one right here from this point here to this point here so from about that point there to about this point here right this one can range also from about 10 to 100 milliseconds however if you notice doesn't the relaxation period appear longer than the actual contraction phase it is and the reason I said it can occur from 10 to 100 milliseconds for the contraction phase it depends upon the muscle so for example let's say I take my superior rectus muscle you know superior rectus is helps me to lift up my eyeballs and stuff like that helps to elevate them that muscle has to contract very very rapidly and very fast so what do you think his contraction phase would be would a very very short window or very long Long window be a very short window right so for example I'm just giving an example if I were to say the extraocular muscles so the extraocular muscles man I couldn't even spell extra sorry guys extra ocular muscles those would have a very fast contraction phase so fast contraction phase so they' be significantly low maybe only 10 millisecs let's say I take for example the gastrus muscle the gastrus muscle he has to carry a lot of load so it might take a longer time for him to have to be able to lift that load for it to contract that load so the actual contraction phase might be a little longer but now compare that to the Solus muscle the Solus muscle is right near the gastrus but it has to carry a significant amount of load so therefore the contraction phase for it would be a lot lot longer so that's why I wanted to that way some of you might be confused like oh Zach the contraction phase looks like it's a lot shorter than the relaxation phase it is but it depends upon the actual muscle fiber so for example an extraocular this would be very narrow but in something like the Solus if I already give for example the Solus muscle it's a very slow contraction phase so in other words imagine this peak here going a lot longer this way okay so it would actually come a lot longer this way it wouldn't stop here at this point it might even go a little bit farther and then relax okay so just want to explain that so again remember the extraocular muscles contract very fast so they have a very very short contraction phase whereas the soul is a very weightbearing muscle it's going to have a very long contraction phase okay now that we've talked about the muscle twitch let's go ahead and talk about this next thing which is the graded muscle responses so stick with me for a little bit here this is kind of a tough topic but we're going to do the best we can here in engineer science to help you guys out okay so first off let's talk about the frequency of a neural stimulus so let's say here we come over here to this Edge and here I have a muscle fiber just a really crude diagram nothing crazy if you see here we have a skeletal muscle fiber so just a muscle cell okay what happens is this is our neuron so let's say that we have two neurons so we'll call this one neuron one okay neuron number one and we'll call this one up here let's do this one in uh this color here we'll call this one up here neuron number two okay so let's say first thing happens we want to contract this muscle fiber okay we want to contract it this neuron is going to have an action potential and it's going to send these Action potentials down its axon you guys know that it sends it down the axon when it sends it down the axon it triggers the synaptic bulb to release a very potent neurotransmitter mainly that of acetylcholine AC we're not going to go into the whole mechanism here but you guys know that AC stimulates this muscle cell by developing what first it develops in plate potentials right so it activates those nicotinic receptors and causes small small amount of sodium influx less potassium influx brings it to threshold once it brings it to threshold what's the second thing to happen an action potential and then after the action potential what happens during that point there calcium is released from where the cop plasmic reticulum so let's kind of follow this real event really quick let's say here's it stimulates it positive charge is moving along the ccle Lemma and then it moves down the T tubules you guys remember it activates the dihydropyrene receptor which activates the ryanodine receptor and pushes the calcium out what happens to that calcium it goes over here and assists in the crossbridge formation right to activate the crossbridge to allow for the muscles to slide to produce tension so the third event is crossbridge activation right and then what happens muscle contracts and if you know there's a there's the term load then there's the term resistance you know let's say that I have a load let's make this really really simple let's say I have a load I'm trying to be able to curl dumbbells right I'm trying to be able to curl dumbbells if the dumbbells the load it actually has more resistance than I can provide Force so force is I'm trying to contract the muscle I'm trying to shorten it right tension if the load is really really heavy and it's giving me a lot of resistance to lift it will I be able to lift that load no if the if the load is heavier than the amount of force that I can exert I won't be able to lift the load okay so now in this situation if that happens if the load is greater load or the resistance is greater than the amount of force that my muscles can exert to move the load the muscle will not shorten but let's say for example that the load is a lot less okay the load is a lot less and it doesn't give as much resistance and my muscle can contract and generate enough Force to actually shorten the muscle now that would lead to this actual isotonic contraction right so isotonic is whenever you're shortening it so it's concentric if it's lengthening it's Ecentric and they have isometric whenever the load is way way way too heavy and if the load is way too heavy and you aren't able to actually shorten the muscle it's called an isometric contraction we'll talk about that in another video just wanted to introduce the concept there so in General the muscle will contract the fourth step would be contraction but again the type of contraction depends upon the load and the resistance and this term Force right or tension sometimes you might hear it as tension a lot so tension is force so now if that happens that's great what would we see on the graph if that was the case let's say that we we actually come here and this neuron it actually activates this guy what would we see on the graph neuron number one wise so let's say here neuron number one tries to stimulate this guy so let's say here we have a time let's say at time 10 so 10 milliseconds right I provide a stimulus from neuron number one if I provide a stimulus for neuron number one what's going to happen let's just make it simple well first off where is the muscle cell generally generally when you're looking at a muscle cell it's usually a resting membrane potential right but if I hit it with a stimulus let's say it actually brings it to threshold if I bring it to threshold potential what will happen it'll generate an AC action potential if I generate an action potential what will happen to the muscle it'll contract so let's say here I actually see this event I actually have this point here let's say here is the I make like this actual horizontal asmode here and that dotted line is maximum tension you can't go beyond this point that's the maximum amount of tension that this muscle fiber can exert okay so this is the maximal tension Point okay so now if I deliver a stimulus that's the first electrical stimulus say say this is from neuron one okay neuron number one if I deliver that stimulus and it reaches causes an Inplay potential leads to an action potential the action potential causes the crossbridge formation right to be active and then leads to contraction it's going to generate tension as long as the actual no matter what it's going to generate tension whether it's actually going to be isometric isotonic but look what happens let's say the first one it actually doesn't reach any anywhere near the maximal tension point so it gets up to that point and then it starts getting ready to repolarize so here again what happens here this is let's say they have a Laten phase here the Laten phase and here is the actual contraction phase and then it starts getting ready to repolarize so what is happening during that repolarization period the calcium starts getting pushed back into the sarcoplasmic reticulum okay so this is the neuron stimulus number one okay so let's come back to this muscle cell for a second so first thing that happened we stimulated this muscle cell to begin to contract right and now it's at the point now where is it in that actual point here we're at the point now we're pushing the calcium back into the sarcoplasmic reticulum and on the membrane you have these potassium channels right let's say that the potassium channels are still closed yet okay so they haven't been activated yet we're not at that point where the potassium channels have opened let's say they were at this point where some of the calcium is getting pushed back into the psychop reticulum let's say at that very moment that brief Moment In Time neuron number two decides to fire if neuron number two decides to Fire and it releases more acetylcholine and acetylcholine activates this muscle cell generates a inlay potential generates an action potential and calcium is released from the sarcoplasmic culum what will happen Okay so let's say it generates a little bit of a action potential here and what it does is it pushes out a little bit more calcium so it Squires out a little bit more calcium so some of the calcium is getting pump backed in what he decides to do is slow down that process and push some of the calcium out so if some of the calcium is going out again and very little of it is actually getting pushed back in what's going to happen well you're going to have crossbridge activation but we already have crossbridge activation slightly right it hasn't gone into a it hasn't repolarized it it hasn't relaxed yet so there still is a minimal amount of tension that we developed if you remember from the graph we haven't gone to four repolarization because if you hit repolarization the muscle cannot be stimulated you have to remember that I'm going to repeat that one more time if the potassium channels open the pottassium starts leaking out repolarization begins no matter even if that neuron provides a stimulus you cannot take it out of repolarization you have to obey the refractory period but if it's pushing the calcium back into the SR and the potassium channels haven't opened up yet and you haven't pushed the potassium out you can provide a second neural stimulus and push more calcium out if more calcium is pushed out it'll actually generate an enhanced an enhanced crossbridge Activation so let's say for example so again it'll increase your actual it'll enhance the crossbridge activation and it'll increase the contraction which will increase the tension we'll talk about that in a second think about it like this let's say that I have the myosin and the actin heads right they're activated so the myin and the ACT heads are actually moving right so let's say that the myosin and the actin again let's pretend I'm the masas and this is the my hand is the acum let's say that again they're Contracting they're Contracting really hard so again what happens ATP I detach I it when I hydrolize it I remove into the next actant and slide that one and I keep sliding it let's say that I start to actually reach this point where my muscle is going to relax getting ready to it hasn't relaxed yet and I give more calcium if I give more calcium what's going to happen it's going to enhance that crossbridge formations even more and I'm going to have more activation I'm going to keep moving that act and across the actual me right and what's going to happen the muscle's going to shorten even more if the muscle shortens even more doesn't it develop more tension yes so think about that whenever you're trying to be able to lift something like a heavier load your neuron neurons can provide very very frequent and consistent excitatory stimuli subsequent stimuli and if it does that it can cause the contraction the second stimulus can cause an even more powerful contraction than the first so let's come over here and explain it on this graph now okay so it's at this point it's at that Peak tension point for that that stimulus but then it's getting ready to it hasn't repolarized yet but it's getting ready to and what did we do let's say here at and we're just hypothesizing these times let's say it's at uh happens to be at about 30 milliseconds whatever okay and it comes over and you give this second stimulus so this is from neuron number two okay what will happen Okay well then this guy he's going to enhance the actual tension from that point because that's what we said we said he'll cause the action potential he'll cause more calcium to get released he'll cause more cross Bridges to get activated so it'll increase the crossbridge activation and enhance the contraction which will increase the tension so what should we see we should see this curve going up even more but then let's say that this one again it doesn't reach maximal tension it starts getting ready to drop also and let's say for example just for the heck of it I have neuron number three and it releases acal choline and if it releases acetylcholine it stimulates this guy causes the action potential if it causes the action potential causes more calcium to get released the more calcium that gets released the more crossbridge activation is enhanced and the more contraction is enhanced and the more tension we develop it's so simple right so again at this point here what will happen let's say that this is at time 50 milliseconds and this is going to be a stimulus from neuron number three what will happen it'll start getting ready to repolarize but the pottassium channels didn't open repolarization hasn't begun only calcium starting to get pushed back into the SR but we blast more that that actual what calcium out and enhance the crossbridge formations and enhance contraction which increases tension and let's say that we reach that maximal tension point and then at that point let's say that the muscle has reached maximal tension it doesn't want to contract anymore there's not any more neural stimulus and it has to relax what will happen it'll go and into its relaxation State and eventually wait for another neural stimulus and when it receives another neural stimulus it'll again perform the same action okay so in this graph over here to the actual left what we did the first one that was just one single muscle twitch due to one single neural stimulus but if you provide very very frequent neural stimuli what can happen so in this one look at this one here we only provided one neural stimulus so it only had one contraction one relaxation phase but what we did in this one is we hid it at that point before the potassium Channel started opening all that was happening is some of the calcium was starting to get pushed back in so when we hit it at that point again we allowed for it to ride on the shoulders of that first action potential so that's what's happening and whenever you see this summation of waves huh that's a special word I want you guys remember special summation of these waves we call this event here temporal and they do you even can call it or wave summation okay so temporal or wave summation is this activity whenever you're having again you're providing multiple frequence stimuli causing the acual potential to occur in one stimulus the second stimulus will ride on the shoulders of that first stimulus you give a third stimulus it'll ride on the shoulders of that second stimulus and help to be able to reach a maximal tension Point whenever you see it like this these increasing waves like greater and greater and greater wave increase until it reaches maximum tension this is a specific type of contraction we call this type we call it incomplete or unfused tetanus okay and incomplete or unfused tetanus is the more common I'm going to star that because that's important this is the more common type of contraction that we exemplify in a regular day basis incomplete or unfused tetanus is a very sustained and quivering contraction okay it's a sustained and quivering contraction and it is the most common types of contractions that we exhibit on a daily basis there is another type but it is not as common it doesn't happen in everyday life unless you for okay so let me explain what this next one is the next one is called complete tetus and the reason why this one doesn't happen on everyday life basis is because it requires very very excessive and frequent stimuli I'm talking like consistent this one had a little bit of gaps in it this one is consistent one after another after another after another and because of that this is going to be trying to lift extremely heavy loads like for example you know how the like the super moms they lift a car off of a baby or something like that that's the kind of activity that involves complete tetanus and we don't do that every day if we did that our muscles would become fatigued very quickly and what is that going to do for us so we need this type of complete tetus to be very very uh utilized in situations where it requires excessive amounts of uh energy and excessive amounts of strength and activity so for example let's say that I take that same muscle fiber but instead of me stimulating it like you know at a in a different like second like you know for example this was 10 to 30 so that was a 20 millisecond uh time difference this was 30 to 50 that was a 20 millisecond time difference let's say I do it one after another after another after another I don't give any time breaks in between that so let's say that I have here let's say I stimulate this muscle here okay then I stimulate it here and I stimulate it here and I just keep doing that I keep stimulating it at multiple points and if I keep providing very very frequent stimuli look what happens in the beginning all you have here okay you'll have that Laten phase but then it's going to start Contracting it's going to start Contracting right and you keep giving multiple multi multiple multiple multiple stimuli so what's going to happen in this muscle fiber there's going to be very very frequent Action potentials extreme amounts of calcium released into the coplas um I'm sorry into the piroplasm extreme crossbridge formation and activation extreme contraction which is going to generate an Exane amount of force what does that mean then let's say again here's maximal tension let's say we bring over here maximal tension Point okay so this is the maximum tension no matter how much more frequent stimuli you give it's never going to past that point it never can go beyond that tension point so this is the maximum tension so let's say that I reach let's say I actually stimulate this so much let's say one two three four and I get to this fourth point and I by the fourth point I reach that maximal tension okay so one two 3 four by that fourth point I reached the maximal tension so this is stimuli one stimuli 2 stimuli 3 stimuli 4 stimuli 5 stimuli 6 by the fourth one reach maximal tension okay during the contraction phase but I keep giving stimuli even when it's at its maximum tension do I go beyond that no it stays at that point of maximal tension even though there's consistent and more frequent stimuli it's not going to change it stays at that same maximal tension you can't generate any more Force the muscle can't shorten any more than it already is at that point maximal tension is released and it starts plateauing all of these imagine it like this imagine all of these waves summating and fusing together forming one complete wave instead of an incomplete wave that's called complete fness so all of the Waves fused together in form a very sustained and very powerful and very smooth contraction and this is going to look like this it'll plateau and then eventually every muscle cell has to reach a point where it actually no longer can maintain that tension and it starts to relax and then it goes back into its Laten phase and waits for another stimulus this right here my friends is called complete tetus or fused tetanus so it's called complete or fused tetus and again like I said it's not the more common one that we utilize on a daily basis unless you're lifting up piano single-handedly every day I am not so more commonly we're utilizing incomplete deadness where in certain situations ation which is you need to you can utilize complete or fuse tetus but again the reason why we don't utilize that as much is because it can easily fatigue our muscles very quickly because it requires a lot of tension a lot of energy and a lot of very very frequent stimuli okay so that covers temporal wave summation that covers maximal tension concept with the completer fuse tetanus and we talked about maximal tension there okay so what we've done so far guys we talked about muscle twitch you know with the rapid contraction of the muscle muscle you know from one neural stimulus we talked about the graded muscle responses but only thing that we've really talked about so far is the frequency of the neural stimulus what I'm going to do right now is we're going to stop this video we're going to go into a second one and in the second video and what's going to be on Muscle Mechanics part two we're going to talk now about the strength of the neural stimulus and how that affects muscle tension and then we'll finish up by talking about motor unit and size principle recruitment okay engineer so I'll see you in a little