Strength training helps your muscles grow stronger and capable of generating more force. In response to consistent strength training, your muscles become stronger and capable of generating more force. Of course, it takes time, patience, and the proper kind of training to gain strength. You won’t keep getting stronger unless you increase the challenge on the muscles you’re working. Too often, people reach a plateau because they don’t increase the stress and challenge on the muscles they’re working.
To build strength, the formula is to work with heavier weights and do fewer repetitions. In contrast, if you’re trying to build muscle endurance, lighter weights and higher repetitions is optimal. With either approach, you need to fatigue the muscles you’re working.
Why do lighter weights and higher repetitions work best for building muscle endurance? You have two main types of muscle fibers: slow-twitch fibers and fast-twitch ones. The slow-twitch muscle fibers are designed for endurance. The slow-twitch fibers can’t generate as much force as fast-twitch ones, but, like the energizer bunny, they have the ability to keep going long after the fast-twitch muscle fibers are exhausted.
In contrast, fast-twitch muscle fibers can generate lots of force and do it quickly. Being optimized for strength and power, they’re the fibers that allow you to lift a heavy barbell or swing a heavy kettlebell. When sprinters sprint and jumpers jump, they’re calling on their fast-twitch fibers. However, these forceful fibers also poop out quickly. So, they’re specialized in their function.
Most people have roughly equal quantities of fast-twitch and slow-twitch fibers. But gifted endurance athletes tend to have a higher ratio of slow-twitch to fast-twitch fibers while the best strength and power athletes often have more fast-twitch fibers. For example, long-distance runners have more slow-twitch muscle fibers while sprinters are fast-twitch dominant.
The Impact of Training on Muscle Fiber Type
Suppose you focus mainly on strength training and spend little time doing endurance exercise. What happens to your muscle fibers? The fast-twitch muscle fibers hypertrophy or become larger and the slow-twitch fibers stay the same or become smaller. That’s because your body is responding to the type of stress placed on it. You’re pushing lots of heavy weight, so you need larger fast-twitch fibers to help out and you don’t need the endurance capabilities that slow-twitch fibers offer.
But do the under-used fibers change in other ways in response to training? A new study shows that gains in fast-twitch fibers may come at the price of slow-twitch or endurance fibers. How is this possible? When you lift heavy weights to build strength, your muscles release short strands of proteins called myokines. One of these myokines is called brain-derived neurotrophic factor or BDNF. In turn, BDNF alters the junction between motor neurons that cause muscle contractions and the muscle itself. In response to myokine release during strength training, motor nerve cells that trigger muscle contractions change their relationship with the muscle they connect to. BDNF also affects the type of muscle fibers that form.
In fact, the researchers found that BDNF produced during strength training causes more fast-twitch fibers to form at the expense of endurance fibers. Some muscle fibers that were optimized for endurance now take on the characteristics of fast-twitch or strength-building muscle fibers. So, at least in mice, you see a shift in muscle fibers. It’s another example of how your body adapts to the stress you place on it and the type of exercise you do. Of course, we’ll need more research to confirm that these changes also happen in humans.
However, researchers believe these findings may explain why endurance takes a hit when individuals devote most of their training to lifting heavy weights and building strength. It also reaffirms the importance of training specificity. If you want to develop strength or power, lift heavy and do power training where you move a resistance at a high velocity. In contrast, if you want to run a marathon, it would be counterproductive to spend long hours lifting heavy weights. Instead, you would focus on running long distances where your slow-twitch muscle fibers do most of the work. Gear your training toward your goals.
Do Myokines Have Other Health Benefits?
BDNF, a type of myokine, may have the capacity to subtly shift the characteristics of a muscle, but there’s also evidence that myokines have other health benefits. For example, myokines alter the signals within cells in such a way that may improve metabolic health. Studies suggest myokines may improve insulin sensitivity and how cells handle glucose and boost fat oxidation. This would make them favorable for weight control. For example, BDNF reduces appetite and food intake in obese mice.
BDNF is also of interest because it may explain some of the benefits exercise has for the brain. Studies link increased production of BDNF with better cognitive health and improvement in the symptoms of anxiety and depression – and it all happens when you start vigorously contracting your muscles. Such is the power of exercise! In fact, muscles are endocrine organs, just like glands like your thyroid, since they produce myokines, proteins that alter metabolism. It’s one reason exercise has such wide and diverse effects on the body.
The Bottom Line
Myokines, like BDNF, change the communication between motor neurons and the muscles they connect to. Preliminarily, it looks like this change in communication, in response to strength training, may boost the fast-twitch fibers at the expense of the slow-twitch ones. So, know your goals when you train and choose your training strategies based on those goals. Whatever you do, keep exercising! Not only do your muscles get stronger in response to training, but they release myokines that may impact your metabolic health and the health of other organs, including your brain.
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