When nerves fire, muscle fibers twitch, or contract, then in a fraction of a second, the nerve stops firing and the muscle fibers relax. Some muscle fibers, anaerobic fast-twitch fibers, have a faster contraction velocity and split ATP faster than their counterpart aerobic slow-twitch fibers. As the names imply, fast-twitch fibers do NOT use oxygen in their metabolic process, while slow-twitch fibers DO use oxygen. Each and every skeletal muscle in the human being and other primates is made up of both anaerobic fast-twitch fibers and aerobic slow-twitch fibers – about half of each type. A muscle may have a slightly varying percentage of fast-twitch and slow-twitch muscle fibers, but never far from that 50/50 numerical split. This fact is disputed by many sources, but the confusion lies in discriminating between primates and other animals.
For instance, the primate gastrocnemius calf muscle has a slightly higher preponderance of fast-twitch fibers, which gives it the capability of very forceful and rapid contraction of the type used in jumping. The soleus muscle just underneath the gastrocnemius has a slightly higher preponderance of slow-twitch muscle fibers, and therefore is used more for prolonged muscle activity such as walking.
Other members of the animal kingdom can have muscles that are all or almost all slow or fast-twitch fiber. For example, a chicken’s breast is all fast-twitch, which is why it is white in color, and a cat’s soleus muscle is mostly long-enduring slow-twitch fibers, allowing the cat to creep in stealth mode without fatiguing. When it comes time to pounce, cats use their gastrocnemius, which is almost all anaerobic fast-twitch fibers producing massive amounts of pyruvic acid, along with a few aerobic fast-twitch fibers to process the pyruvic acid. Cats can creep, pounce, and leap 10 times their height because both their nervous system and their skeletal muscle are different from ours. We primates tend to wear out rapidly when we slink around in stealth mode because all our muscles are about 50% fast-twitch, which exhaust rapidly.
Furthermore, even with this 50/50 numerical split in primates, almost 3 quarters of primate muscle is proportionally fast-twitch, because fast-twitch fibers are 2 to 3 times larger than slow-twitch fibers. This is important to understand.
As stated above, our small slow-twitch fibers are aerobic: they utilize oxygen while metabolizing fatty acids and proteins; and large fast-twitch fibers are anaerobic, using no oxygen in their metabolism of glucose. Gray’s Anatomy states that primate skeletal muscle also contains aerobic fast-twitch fibers, and because Gray’s is sometimes considered the gold standard, many other reference sources follow their lead. But Thomas Griner avows that muscles of human beings and other primates do not in fact contain this 3rd type of muscle, which are actually more like little bags of oxidative mitochondria than muscle fiber because they have almost no contractile myofibrils. These diminutive aerobic fibers are stuffed with tiny mitochondria and myoglobin, and are moderately resistant to fatigue. They use oxygen only to burn pyruvic acid.
Confusion also exists as to whether or not the different types of skeletal muscle fiber use different blood and nerve supply; but because fast-twitch is dependent upon slow-twitch for its metabolism, the 2 types of fiber must co-exist side by side. And because they are intermingled and interface one with another, they live within the same fascicles (bundles of muscle fiber), which means they share the same corridors between their fascicles; therefore they must share and utilize the same nerve and blood supply. However the slow-twitch fiber uses the blood supply differently because its metabolism is aerobic; more capillaries cluster around the slow-twitch fiber, making more blood available to the slow-twitch fiber in order to deliver more oxygen to its aerobic powerhouse mitochondria.
When a skeletal muscle contracts, the controlling motor units (a motor unit consists of one motor neuron and all of the muscle fibers it contracts) govern how each fiber in the muscle takes turn twitching. All fibers hooked to one motor unit fire simultaneously, but not all fibers in one fascicle are hooked to one motor unit, nor is one motor unit even confined to one fascicle; rather one motor unit fires fibers distributed in an arc throughout the muscle; some are slow-twitch and some fast-twitch within that one motor unit. Some books state that each muscle motor unit is mad up of all slow-twitch or all fast-twitch fibers, but this is not true, nor are muscle fibers bundled together into fascicles for any neurological reason, but simply because the corridors between the bundles are necessary to accommodate nerves, blood vessels, and of course muscle spindles.
Small slow-twitch fibers are ½ to ⅓ the size of fast-twitch fibers and have weaker contractions, but they have longer endurance. They carry energy with adenosine: ADP coming to the mitochondria to be oxidized to ATP, and ATP traveling back into the interior of the cell with its energy. Slow-twitch also contain large amounts of myoglobin, an iron-containing protein similar to hemoglobin in red blood cells. Myoglobin combines with oxygen and stores it until needed, then engages in its principal function of speedily transporting oxygen to the aerobic mitochondria energy factories. It is iron-carrying myoglobin that gives slow-twitch muscle fibers their red color.
Little slow-twitch fibers are loaded with large aerobic mitochondria lying close to the oxygen-bearing capillaries around the periphery of the cell, as well as small anaerobic mitochondria deep within the cell. The aerobic mitochondria near the outside edge of the cell support high levels of aerobic metabolism, and make slow-twitch fibers easy to identify. They contain cytochromic oxidase, which allows them to phosphorylate creatine and ADP. Fatty acids are stored near these mitochondria for energy. Slow-twitch fibers contain no glucose and therefore no lactic acid, a by-product of splitting the glucose molecule (glycolysis).
While slow-twitch fibers make fewer contractions per second, they are able to adapt rapidly to changes in their environment, and therefore don’t react to nerve potentials not sustained for a certain period of time. As an example, in an electrical power system, a 20-amp slow-blow fuse can temporarily take “transients” or sudden pulses of voltage over 20 amps without blowing. That sudden spike would blow an ordinary 20-amp fuse, but a slow-blow fuse will adjust to that sudden transient spike – even one of 50 amps – providing the spike doesn’t last very long. In order for a 50-amp transient to heat the element enough to blow a 20-amp slow-blow fuse, it must sustain for a period of time.
Much in the same way, slow-twitch fibers can rapidly adapt to short spikes of nerve current coming down the axon because they adapt rapidly; they won’t contract if the nerve stimulation is fast or short enough, even if it comes in rapid repetition. Any stimulation must sustain for a period of time in order to make a slow-twitch fiber contract. Again, remember, this refers to speed of nerve potentials rather than their strength.
Large Fast-twitch fibers twitch 3 times faster, and are 2 to 3 times larger than slow-twitch fibers; this extra size gives them greater strength of contractions, but because they use no oxygen and twitch faster, they exhaust rapidly. Fast-twitch fibers carry energy with creatine phosphate. There is no myoglobin in fast-twitch fibers, but they do store glycogen (starch) throughout their interior for energy. Remember that the body’s sugar – glucose – is stored as glycogen in both the liver and muscles until it is needed.
Fast-twitch fibers have extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate contraction. Because they function only in glycolysis, they have large amounts of glycolytic enzymes for rapid release of energy by the glycolytic process. But they do not contain large complex aerobic mitochondria, only tiny anaerobic mitochondria, and they constantly produce lactic acid as a metabolic by-product. Remember that volume-wise our muscles have almost 3 times the number of fast-twitch as slow-twitch fibers, so some lactic acid is being produced at all times, even during sleep. Lactic acid is present in blood drawn from a person at total rest, proving that at least a few fast-twitch fibers fire during basal metabolism. Fast-twitch carry their energy with creatine rather than adenosine; they cannot deal with 18 wheelers because they don’t have time to wait for them, and also because fast-twitch energy carriers must cross over to the slow twitch fiber – must penetrate all those membranes and walls, a job for the little creatine molecule.
Although fast-twitch fibers twitch faster and exhaust faster than slow-twitch fibers, they adapt more slowly than slow-twitch fibers; it takes a high number of nerve pulses per second to trigger them. Although they squeeze more contractions into less time, they are slower to adapt to changes in their environment and will fire with short spikes of stimulation. This is important because it is what allows NeuroSoma to stimulate the flower spray nerve endings coming from the fast-twitch intrafusal chain fibers within the muscle spindle, while slipping under (not firing) the annulospiral nerve endings coming from the slow-twitch intrafusal bag fibers. This difference in adaptation rate is our ‘in’, and is why we can avoid activating the stretch reflex mechanism by stroking across the grain of muscle fibers.
Additional Interesting Tidbits:
• The small primate slow-twitch fibers are 3 times bigger than feline aerobic fast-twitch fiber
• Although primate fast-twitch fibers fire 3 times faster than slow-twitch fibers, slow-twitch muscle fibers still fire twice as fast as primate cardiac fibers; heart muscle, the miracle of love and electromagnetics, doesn’t work as hard as skeletal muscles
• Some medical doctors claim it is possible for us to convert some of our fast-twitch anaerobic fibers into fast-twitch aerobic fibers with certain exercises. To claim one type of muscle fiber can be converted into another is controversial, but to say we can convert our muscle into a type we don’t have in the first place is lacking good judgment.
• Human skin has “multi-motor unit smooth muscle” that is neither skeletal nor smooth muscle, but ‘in-between’. This muscle gives us goose bumps and makes our skin creep. You’ve heard someone say they have a “creepy feeling”? Our skin really does creep, and when we’re cold, our skin is actually shaking before we begin to shake. Although these are the same type muscles that allow horses and cows to jerk their skin, we can no longer consciously control them; instead, they are controlled by our emotions, which can still twitch, or jerk our skin.
Recap:
• Primate Fast-twitch fibers
o Are anaerobic, are larger, have stronger contractions, wear out rapidly, adapt slowly; they store glycogen, burn glucose, produce lactic acid, and carry energy with creatine phosphate. They have small anaerobic mitochondria.
• Primate Slow-twitch fibers
o Are aerobic, are smaller, have weaker contractions, have great endurance, adapt rapidly, contain cytochromic oxidase and large amounts of myoglobin, store fatty acids for energy, and carry that energy with adenosine. They have large aerobic mitochondria close to the cell surface.