okay so in this section we're going to talk about uh basically the factors that affect muscle contraction so in terms of generating force of muscle contraction it depends on the number of cross bridges that are attached you know when myosin attaches to actin and this is affected by four main factors uh first of all the number of muscle fibers that are actually stimulated or recruited to be stimulated and the more motor units that are recruited the greater the amount of force that's that can be produced by that whole muscle also what depends on the how much force this produces the relative size of those fibers right the bulkier the muscle the more tension that can it can develop because muscle cells can increase in size or hypertrophy with regular exercise because these individual cells can make more myof fibral we also uh can generate more contraction Force based on the frequency of stimulation so if you increase the action potential frequency going out to muscle this is going to generate more Force because these stimuli are added together you flood the synapse with acetylcholine which means you have more imp plate potential and even more action potentials in the muscle cell so that muscle twitches can start to add together or summate now the degree of muscle stretch also affects how much contraction can be produced so we find that uh if muscle fibers whose sarir are at 80 to 120% of their normal resting length this can generate the most force and if saram mir's uh length is less than 80% of its resting length the filaments overlap too much and the force decreases and it's the opposite too here if sarir are greater than 120% of their resting length the sarir are stretched too much the filaments don't overlap enough so that the force decreases and so just to summarize these four major factors here remember the number of muscle fibers that are recruited can increase contractile Force how large those muscle fibers are that can also increase contractile Force you know if you go from a smaller fiber type to larger fibers through hypertrophy like regular use of the muscles will make these cells get larger because they're going to start to generate more myofibrils well if the cells are larger because they have more myofibrils there's more sarcomeres which means you can produce more contraction also a higher frequency of stimulation can lead to things like uh wave summation or even fuse tetanus and that's going to increase contraction Force Through summation of muscle twitches or depending on the stretch of that muscle that can also increase contract off Force remember if if the muscles are between 80 and 120% of their resting length this is going to produce the most Force because the sarir are in their optimal position if sarir overlap too much or if they're stretched too much the fibers aren't you know going to have as much contact so therefore the sarir don't can't generate as much Contra contractile Force so just to summarize those four major factors remember the number of fibers recruited the size of the fibers the frequency of stimulation and the length of that muscle also can increase contractile Force so what this figure is showing here is essentially the relationship of tension versus the percent resting length of the sarir and you can see here that uh there's a tendency then for sarir length to uh affect tension that can be produced and so if you look here we see that the most amount of tension that the sers can can produce is when they're at a 100 I'm sorry when they're at 100% of their resting length so if um sarir is at their 100% of resting length you find here that meios filament can overlap very well with the actin filament so there's a lot of potential contact sites here which means that that this sarir in this position has the most potential to produce the most uh most tension which is what we see here so uh you know a s at 100% of its resting length will produce 100% of its maximal tension however if sarir are kind of compressed you see there's a general tendency for tension that's able to be produced to go down and this is because when sarcom start to overlap you find that the the the myosin filament has less potential contact sites here now uh with actin because they're overlapping and because of this there's less a available uh binding site which means that less myos heads can actually bind and therefore you're going to produce less tension as a result so a sarir that's at 80% of its resting length can only produce you know about 80% of its you know maximal tension and the opposite goes for The Other Extreme so if sarir are excessively stretched where you might find that the thin filaments barely overlap with our thick filament we see here then that there's even less potential contact here between the meos and heads and and the thin filament which means that at 170% of its resting length we see that the the circum can only produce about 20% of its maximal tension and so what's interesting then is if you're thinking about applying this to regular day-to-day activities it's important to note then that wherever your muscles are before you start to perform an activity is going to depend on how much contraction Force those muscles can produce as a result so for instance if you're going to lift some weights you probably don't want to start then at the most stretched state of those muscles because you're going to really resist and there's going to be a a large struggle than to actually produce enough tension to lift those weights rather it's better to start at a relaxed state or relaxed position because from that position then you can produce the most Force so uh it turns out that velocity and duration of contraction uh can also vary and so what can influence the velocity and duration of a muscle contraction are factors like muscle fiber type the amount of load that's placed on that muscle as well as the number of muscle fibers that are recruited so muscle fiber type can vary and what's kind of weird here is that not all muscle fibers or muscle cells are the same and they can be divided into several main categories here uh by the speed of contraction and the type of metabolic pathways these cells actually rely on so if the muscle twitches that these cells produce are fast we call them fast fibers and if the muscle twitches that these cells produce are more slow they're slow fibers ultimately this relates to the speed of the meos and atpa and the pattern of electrical activity in these types of cells now metabolic pathways also uh can influence how these muscle cells respond to stimuli like oxidative fibers use aerobic Pathways because they can use oxygen glycolytic fibers rely on glycolysis which are more an anerobic type of metabolism so uh what we find here is that there's uh you know B two criteria that really tell us about about fiber type uh remember if it's slow or fast or whether it uses uh oxidative or glycolytic um metabolic pathways so there's three major fiber types we're going to talk about here we got the slow oxidative fast oxidative and fast glycolytic muscle fibers now it turns out most of your muscles actually contain a mixture of all three fiber types and this results in a range of contractile speed and fatigue resistance across your muscles however there are some muscles that can contain more more of fiber type than another and this is dependent on things like your genetics as well as how you use the muscle over the course of your lifetime so the different muscle fiber types are best suited for different jobs right so slow oxidative fibers are better used for low intensity endurance activities like maintaining posture like just keeping your back upright would require very low intensity but you need to have endurance so that way your muscles don't get fatigued and your posture just slumps down and you fall to the floor so fast oxidative fibers though are good for medium intensity activities like sprinting or walking but not so much that you can run out of oxygen it's get you know sprinting or walking or you know aerobic types of activities where you know the muscle fibers aren't so active that they run out of oxygen and so therefore they can still rely on oxidative metabolism the fast glycolytic fibers on on the other hand rely on more shortterm intense or powerful movements you know examples of this would be like hitting a baseball or if you're a sprinter you know then you'd have more fast glycolytic fibers and those muscles that would promote those types of activities so what we find here then is like let's say let's give an example here where uh if you want to have you know a high contractile velocity what you're going to want then are a lot of the fast glycolytic fibers however these are more fatigable so you don't want all of the muscles in your body to only have fast glycolytic fibers because that means we would only be able to move in very quick short bursts where we fatigue very rapidly opposite to this you know uh if you want to have a long contractile duration what we have then are slow oxidative fibers which are much more fatigue resistant but their onset of maximal tension is much more slow and this also wouldn't make sense for all your muscles because although you can have a long contractile duration you also would wouldn't be able to move very fast so let's say for example we need to have an A something in between right like if you're going to bicep curl this ice cream cone with like strawberry ice cream in it you know you might want to get that to your face quickly which means you're going to want to have a lot of fast glycolytic fibers right that way you can lift it up fast and then get it close to your face but you also want to be able to hold that ice cream cone there in the long term that we can lick the ice cream and actually enjoy it and in that regard you're going to want to rely more on the slow oxidative fibers which are more fatigue resistant so in this regard it turns out muscles like your Bice BPS are going to have a mixture of these fiber types that way it kind of you know promotes the things that we do on a day-to-day basis like eating ice cream cones so just to compare these three fiber types remember we have slow and fast oxidative as well as fast glycolytic fibers remember if they're oxidative then we're actually relying on oxygen uh metabolism like aerobic Pathways and if it's glycolytic we're relying on anerobic metabolism like glycolysis so for the slow and fast varieties this ultimately refers to the speed of the meos atpa so the myosin atps is just the speed of the myosin head and so if they're slower that's what going to make it a slow fiber because have slower twitch and if those my npce is faster that's what makes this a fast oxidative I'm sorry fast fiber so the speed of contraction is dependent on these myosin atpases now the primary pathway for ATP synthesis uh for the slow oxidative fibers is just aerobic metabolism like like olysis kreb cycle oxidative phosphorilation this produces a large amount of ATP but it's about two and a half times slower than if you relied on glycolysis which makes sense why youd want to have this type of metabolism in slow oxidative fibers now the fast oxidative fibers rely on aerobic metabolism but also a little bit of anerobic glycolysis just depending on you know what you're doing now the oxidative fibers because they rely on oxygen also have high amounts of myoglobin and globin is a is a protein that helps you to store oxygen in that muscle and it makes sense that if you have muscle fibers that rely on oxygen for their source of metabolism and ATP synthesis you know you want to have a lot of this myoglobin which actually helps to you know hold on to extra oxygen to help Supply metabolism to these muscles now myoglobin has uh kind of a reddish darkish color to it so that the muscles that are redder and darker have more myoglobin content and um that's what we see here like with the color of these fibers they're going to be more red or red to Pink and this is dependent on their myoglobin content now glycogen is also a way for our muscle cells to store Sugar for later use and we find that oxidative fibers store you know a moderate amount low to an intermediate amount of glycogen you know they're not they're not particularly relying on glycolysis for ATP so they don't need a whole lot of glycogen stored here now the slow oxidative fibers are actually recruited first and because they're fatigue resistant the fast oxidated fibers are recruited second and this is because they're moderately fatigue resistant and the fast glycolytic fibers are actually recruited third and they're they're more of a fast twitch but they fatigue more rapidly and because these fatigue more rapidly it makes sense that you want these other fibers to already be activated that way once these fatigue and the twitch goes away you know you still have other fibers that are uh you know activated and Contracting that way your muscles don't just relax all at the same time suddenly so in terms of what these types of fibers are good for um you know slow and fast oxidative fibers are good for sort of you know moderate to medium types of of activities you know like slow oxidated fibers are good for endurance activities like running a marathon or maintaining your posture you know the fast oxidated fibers are good for like sprinting or walking whereas the fast glycolytic fibers you know these rely on anerobic glycolysis which means that they um don't use a whole lot of oxygen here for ATP synthesis uh they're fast because their myosin atps is fast however because they rely on Anor robic Pathways they don't need a lot of myoglobin so their myoglobin contents low which makes these fibers kind of white or pale in appearance their glycogen stores are high because they need a lot of stored sugar to power glycolysis and they're more fatigable which means that these things don't last very long you know they're good for quick burst of energy which is why the these fast glycolytic fibers are are better for short-term Intense or powerful movements like hitting a baseball or sprinting now in terms of the structural characteristics you know the fiber diameter here of the fast glycolics are intermediate or so they have very few mitochondria because they rely on anerobic metabolism they have very few capillaries or blood vessels because again they don't need a whole lot of of oxygen delivery and the fibers are going to be kind of white in uh in appearance their color is white because they have a lot of myoglobin or mitochondria so think of like white meat that's going to be fast glycolytic fibers which have more glycogen now the fast oxidative and slow oxidative fibers are darker and more red because of lots of capillaries because of lots of mitochondria and because of their uh you know their uh myoglobin stores now the fiber diameter is also different here where the slow oxidative fibers are smaller and it makes sense that these are actually recruited first because remember we talked about the recruitment order was based on the size principle where these smaller muscle fibers are actually recruited first so in terms of load and recruitment we understand that that load uh is basically where you know it's a sort of um the amount of force that a muscle needs to resist so that muscles actually contract the fastest when there's no load added that makes sense like there's no if there's no load on the muscle they're going to be able to contract very very quickly now the greater the load the shorter the distance of the contraction and the greater the load also the lower the contraction because you know you have to pull quite a bit of force now the recruitment also depends on the motor units that are activated and uh this is actually going to promote different types of contractions so what we find here then is for heavy loads you have a you know a very short distance of muscle contraction and um you know the muscles are going to fatigue more rapidly uh in terms of uh lighter loads like's let's say no load at all like you just lift your arms up but they're not resisting uh anything but their own weight you know they're going to have a really uh quick uh contraction that's also a very short distance that they're going to move and this can actually happen longer because they're more fatigue resistant now we also see that uh increasing the load changes the velocity of shortening so if there's almost no load muscles contract very rapidly if there's a really large load muscles will contract very slowly because they're having to pull really strongly against that low that they're sort of you know resisting