Transcription
This tissue that you're looking at right here, muscle tissue, is some of the most adaptable tissue in the human body. We beat the crap out of muscle tissue during some of our workouts, and then two to three days later, it's ready to go again for another workout. This tissue can get bigger, it can get stronger, and it can adapt to contract with more velocity.
So today, we're going to talk about five adaptations that occur in skeletal muscle tissue. What is incredible about this is that you can have a huge amount of influence on the adaptations that you want to develop based upon your exercise choices. This is going to be full of all sorts of anatomical and physiological awesomeness, so let's do this.
Let's start with introducing, or if you saw our previous cardiovascular adaptations video, reintroducing the nine fitness adaptations that you can get with exercise. Generally, you can kind of block the fitness adaptations into nine or ten categories, and there may be some slight variation to this list. But here we have our list, and as you can see, we've categorized them into cardiovascular adaptations and musculoskeletal adaptations.
The cardiovascular adaptations are more systemic, as they are adaptations in the whole cardiovascular system, such as with the heart and blood vessels. As I mentioned, we went into these cardiovascular adaptations in much greater detail in the cardiovascular video that I'll link at the end. But today, we're going to focus on the musculoskeletal adaptations.
These would be adaptations that, for the most part, occur within the actual skeletal muscle tissue. What's important for us to understand is that we will first address these as individual adaptations, like in their own little box, because I think that's the easiest way to start learning them. But keep in mind that a lot of the time, these adaptations exist on a spectrum.
Certain types of exercises or even sports might require some level of muscular endurance and strength. Something else that commonly happens is that you can see people can often get bigger (hypertrophy) and stronger at the same time. But we can also tweak things to express one adaptation more than another.
So let's go from kind of just left to right here and start with muscular endurance. Muscular endurance refers to how long a muscle can sustain exercise, or in other words, the ability of a given muscle to exert a light to moderate amount of force consistently and repetitively over a period of time.
Think of the leg muscles contracting during light to moderate running or cycling, or lifting lighter weights with higher repetitions. When someone's muscular endurance improves, the physiological adaptations that account for this improvement mostly occur within the slow-twitch muscle fibers and even somewhat within the intermediate oxidative fibers.
All these fibers would become more efficient at utilizing oxygen, and this would be through the increased number and size of the mitochondria within the fibers, which we know utilize oxygen to create ATP. You also see an increase in the number of capillaries that feed those slow-twitch fibers.
So now we have more oxygen and nutrients being delivered to the muscle, more mitochondria to process the oxygen and nutrients, which results in more ATP production. People that develop this adaptation may notice that they could run or cycle further with less effort and less fatigue, or they might notice the number of repetitions they could perform would increase.
Like if your push-ups went from 35 repetitions to 50 repetitions. Now, out of all these musculoskeletal adaptations, muscular endurance has some of the strongest ties to the cardiovascular adaptations. People that are interested in endurance sports like running, cycling, swimming, and rowing, which are aerobic-based events, tend to develop tremendous muscular endurance just by following their endurance training plan.
That naturally results in repetitive slow-twitch muscle contractions through all the mileage or time they spend doing that aerobic training. But again, you could also improve muscular endurance with lighter loads of resistance training, like gradually trying to increase your reps each week with, say, push-ups.
Now, this adaptation does not typically result in much of a size increase with the muscle, but there are situations, especially if you started doing a lot of reps or volume with more moderate loads, where you can start to stimulate the next adaptation: hypertrophy.
Muscular hypertrophy is an increase in muscular size. Now, hypertrophy often comes with increases in strength because, in general, a muscle with a greater cross-sectional area can produce more force. But hypertrophy doesn't always have to come with strength, which we'll get into in just a second.
Now, this overall increase in the size of the muscle does not come from a significant increase in the number of muscle fibers, but rather due to the muscle fibers that you already have just getting larger. Now, there are some minor exceptions to this, as there are some cells called satellite cells embedded within skeletal muscles.
Sometimes those satellite cells will differentiate into a mature skeletal muscle fiber or fuse with existing muscle fibers. But again, primarily, the increase in the size of the overall muscle comes from mature muscle fibers just getting larger, and this primarily occurs in the fast-twitch fibers.
This increase in fiber size is due to the muscle fiber creating more sarcomeres and myofibrils, which are the contractile subunits of the muscle fiber. So if this size comes from an increase in the contractile subunits of the muscle, this size increase will also be paired with an increase in muscular strength, and this is referred to as myofibrillar hypertrophy.
This also explains why bodybuilders are often quite strong. However, you can also get a size increase due to the increase in fluid within the muscle fiber, as well as an increased capacity to store glycogen, which is the storage form of glucose that the muscle fibers can use as an energy source.
The fluid within the muscle fiber is referred to as the sarcoplasm. Maybe from a previous biology class, you've heard of the cytoplasm as the fluid found inside cells, and the sarcoplasm is just a similar fluid found in muscle fibers. But this increase in muscle fiber size due to this fluid increase is referred to as sarcoplasmic hypertrophy.
This type of hypertrophy does not really come with an increase in muscular strength, meaning you wouldn't see an improvement in someone's one-rep max because we haven't created any more contractile subunits. But due to the increased fluid and ability to store more glycogen, someone may notice that they may be able to perform more repetitions and sets due to sarcoplasmic hypertrophy.
Now, the reality is that both sarcoplasmic hypertrophy and myofibrillar hypertrophy happen together, especially when you are just starting a hypertrophy program. Beginners kind of just get the best of both worlds with those beginner gains. But as you get more advanced, the type of training you perform can drive one of these types of hypertrophy a little bit more than the other.
This is also where we can start to open up the discussion of these muscular adaptations existing on a spectrum. Remember, with muscular endurance, this adaptation didn't result in much size but in the ability to utilize oxygen. This was from repetitive contractions of those slow-twitch fibers at a lower intensity, such as zone two running or high repetitions with light weight.
But let's say you started adding hill runs to your running routine or hill climbing on a bike. You may start to notice that your leg muscles are getting larger, like a lot of cyclists have some decent-sized thighs. Or maybe you start adding more moderate loads to the bar during resistance training and continue to still do fairly high reps.
This could be from 15 to as much as 30 reps. But now that you've added a moderate load and are still accumulating a lot of volume through multiple sets and sessions per week, you would start stimulating some of that sarcoplasmic hypertrophy in your fast oxidative fibers.
You kind of get this transition zone of improving endurance and hypertrophy. But let's say you started to increase the load even more to where you might only be able to do 8 to 12 repetitions. This would start to stimulate more of that myofibrillar hypertrophy and an increase in muscle size, pushing over into strength.
If we continue to increase the weight to say like 80 to 90% of our one-rep max weights, that we can only do like 3 to 5 repetitions with and longer rest periods, three to five sets per exercise, we start moving into that pure strength direction.
So as we are definitely starting to see, muscles are incredible at adapting to different forms of exercise. But this process of muscular adaptation doesn't just come from the stimulus that exercise provides, but also from recovery.
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Muscular strength is a measure of the maximum amount of force a muscle is able to produce, and this is often measured with someone's one-rep max, the maximum amount of weight that someone could lift one time. Now, one of the ways our body improves pure strength is at the neuromuscular level, meaning that your body becomes more efficient at recruiting more motor units.
Now, if you haven't heard of a motor unit before, we have a very detailed video on this topic that I'll link at the end. But let me quickly mention a few key points. A motor unit is the motor neuron and the group of muscle fibers that it controls. Motor units can be extremely small, such as within the muscles that surround the eye, called the extraocular muscles, where you can have one motor neuron only controlling as few as 10 muscle fibers.
Or they can be quite large, such as in some of the limb muscles, where one motor neuron can control a few thousand muscle fibers. But the point is, we have many motor units within each muscle, and this is how your body modulates force by recruiting more motor units in sequence.
For example, if you were doing a squat with just the bar, your body would recruit a fewer number of motor units. But as you continue to add more weight, you would need more muscle fibers to move that heavier load, so your body would recruit more and more motor units.
One of the things that makes you stronger is that through pure strength training, your body learns to recruit more muscle fibers on demand, and we especially start firing into the most fast-twitch muscle fibers called the fast glycolytic fibers.
Now, what is interesting about more efficient motor unit recruitment is that this is an increase in strength without size, which explains how one can get stronger without getting bigger. Something else that is thought to account for increases in strength without size is that pure strength training also improves the speed and efficiency at which calcium is released and recycled within the muscle fibers.
Calcium is really important for muscle contractions, and I'll link another video to that. There is also some evidence that suggests the bond between the contractile proteins gets stronger with pure strength training, also contributing to this improved force production without an increase in size.
Now, I do want to stress this one more time. If we took a beginner and put them on a pure strength building program, high loads of something they could only do three to five reps of, they would also get bigger because, again, beginners get all sorts of adaptations when they first start.
Obviously, strength and powerlifters have big muscles, but as you continue to get more advanced, you can start to see a greater distinction between strength and hypertrophy. Next, we have speed, and this is likely not going to shock you that this is about contraction velocity, or in other words, how quickly your fast-twitch muscle fibers contract, especially those fast glycolytic fibers.
Activities that are great expressions of speed would be martial arts and boxing, like punching or kicking as quickly as possible. Certain plyometric hopping and footwork drills are also great expressions of speed. When you get improvements in this area, your muscle fibers get better at contracting with greater velocity due to increases in the rate of motor unit recruitment, meaning your nervous system sends the signals and activates those fast-twitch fibers more quickly.
Remember, prior to this, we just saw an increase in the number of motor units recruited, which resulted in improvements in strength. So we can get these different neurological adaptations for strength and speed. In a second, we're going to see that there's some major crossover between these two.
But increased contraction velocity, and therefore improvements in speed, is also due to improvements in the muscle fiber's ability to utilize ATP more rapidly. This means that the myosin ATPase, which is this enzyme on the contractile protein myosin that breaks down ATP to release energy for the muscle contraction, can increase its enzymatic activity, thereby increasing the rate of ATP use and resulting in improvements in contraction velocity.
Now, notice I placed power slightly above in between strength and speed. The reason for this is that power is the expression of both strength (muscular force) and speed (contraction velocity). One way of thinking about this is the faster you can express the strength that you have, the more power you will produce.
This is often referred to as the rate of force development. Activities that are expressions of power are things like Olympic weightlifting, weighted jumps, and regular jumps. Sprinting, all are activities where you are trying to move a weight or your body as quickly as possible.
You can probably see that these activities also exist on a spectrum, meaning some of these activities have a higher contribution of one more than the other. Like the power produced during Olympic weightlifting tends to have a greater contribution from strength than it does from speed.
But something like sprinting has a greater contribution of speed than it does strength. The idea is if you increase one of these, strength or speed, your power should go up. If power goes up, this would result in a higher vertical jump, greater sprint speeds, or even higher loads moved during Olympic lifts.
Now, it's a little more nuanced than just improving strength or speed in isolation. Because if all you did, for example, was focus on strength and never did any specific speed training or never practiced jumping or sprinting, you're not going to train your body to express that strength very quickly, and therefore leave power gains on the table.
On the flip side, if all you did was focus on, say, speed and never did any strength training, yes, you would express the strength that you do have quickly, but you wouldn't have very much strength to express overall, also causing you to leave power gains on the table.
This can get quite detailed, with people getting into force-velocity curves, and power athletes love this stuff. But the summarized version is this: people that are involved in power sports or want to increase their sprint speed or vertical jump spend time working on both strength and speed.
They may divide their training into more focused phases or training blocks. For example, they may spend a few weeks to a couple of months focusing more of their sessions on strength, but they don't fully neglect speed. They just might do fewer sessions.
Then they may flip-flop to a more speed-focused phase, where they do fewer sessions of strength training to just maintain their strength, but then more of their sessions would focus on speed, such as plyometrics, weighted jumps, and even Olympic lifts.
Now, our last adaptation that we'll discuss is motor learning. The reason I put a little asterisk next to this one is because this adaptation hedges much more towards a nervous system adaptation than it does a musculoskeletal adaptation. But the result of this motor learning ultimately affects your skeletal muscles and can result in improved performance.
What we are talking about here is essentially coordination and learning movement patterns. When you initiate a skeletal muscle contraction, that actually starts up in this largest portion of your brain called the cerebrum, specifically in this area here called the motor cortex.
That signal will move down the spinal cord and eventually go out of a nerve that connects to a skeletal muscle, causing the muscle to contract. But the motor cortex isn't alone in this. This beautiful structure of the brain called the cerebellum will also get involved by helping to coordinate skeletal muscle contractions.
It does this by evaluating how well movements initiated by the motor cortex are actually being carried out. For example, when movements initiated by the motor cortex are not being carried out properly, the cerebellum detects these inconsistencies and then sends inhibitory feedback signals back to the cortex.
These feedback signals help correct the errors, smooth the movements, and coordinate complex sequences of skeletal muscle contractions. This is what allows you to perform skilled muscular activities such as playing an instrument, speaking, catching and throwing a ball, and even performing complex weightlifting movements.
As you continue to practice these skills through more repetition, your cerebellum gets better at this, and you become more coordinated in your movements, which can improve performance. For example, someone's vertical jump can improve just through practicing jumping and learning to coordinate the muscle contractions more effectively.
Even with weight training, just learning to properly execute the movements, like practicing a squat, for example, often results in someone being able to lift more weight independent of improving strength, speed, and power.
One last thing I'll leave with you: as you continue to practice complex movements, the cerebellum stores this information to the point where the movements don't require as much conscious effort. This means that the cerebellum starts to coordinate your movements at a subconscious level, which translates to you not needing to watch your hands while dribbling a basketball or while playing an instrument.
It almost becomes instinctual, where you can just decide to move in a certain way, and the cerebellum takes over the details of coordinating that movement. If you've ever heard the phrase, "It's like riding a bike," this is referencing these stored motor pathways that you tend to hold on to at some level, even if you take a break from practicing those movements, which is sometimes referred to as muscle memory.
But there are some differences in what people actually mean by muscle memory. Like some people refer to muscle memory as your muscle's ability to quickly reacquire hypertrophy, strength, and speed gains that you obtained in the past. But that is coming up in our muscle memory video.
Yes, that was a lot, but hopefully, you learned some useful information about these incredible musculoskeletal adaptations. Thanks for supporting our channel, everyone. If you want to take a look at our cardiovascular adaptations or our muscle fiber types videos, we'll link those here, and we'll see you in the next video.