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Josh Avila, M.S.


When sitting on a couch late at night watching infomercials featuring super fit, muscular people, you might wish you could look like them because you’d be really popular and happy and look great in a swimsuit. Rarely do people think “Man, I wish I had as many capillaries to my muscle fibers as those guys do!” But those inward adaptations are critically important, too. While the overall outward change is the change we tend to focus on, changes invisible to the outside are occurring all over especially within the muscle.


In the adult body, skeletal muscle increases in both size and strength with resistance exercise. This type of adaptation occurs as a result of the individual muscle fibers increasing in size, and not as an increase in the total number of muscle fibers within the muscle. Endurance training, while maybe not as visible, also has an effect on muscle. Endurance exercise improves both the muscles’ aerobic capacity for work and also the composition of the muscle itself. It’s important here to lay a foundation for your next paragraph, which becomes technical and requires a much higher level of reader knowledge, which can’t be assumed. You can create a bridge by taking a word from the previous sentence (recommend “composition”) and using some form of it in the next; for example, “Muscles are composed of different type of fiber cells, which have different functions or purposes” and go on to explain in more detail (fast twitch, slow) so the reader will be following in the next paragraph.


Resistance and endurance training performed together lead to, over time, the creation of new blood vessels, a process known as angiogenesis. Additionally, in muscles that have a mixed composition, skeletal muscle fiber type transformation occurs. This dual adaptation occurs as quickly as four weeks, as demonstrated in studies using electrical stimulation in the leg muscle as a controlled substitute for physical activity. The study showed that the number of capillaries within a cross sectional area doubled compared to a control muscle. This brings more oxygen to the muscle, allowing the muscle to work for longer periods of time. There was also a shift in the muscle fiber type from a fast twitch to a slow twitch muscle. This chronic stimulation of the muscle converted a muscle designed for short-term, high-energy, anaerobic activities to a muscle that resists fatigue for longer-lasting aerobic activities.


Individual response to exercise varies highly. Some individuals may not respond to exercise at all; therefore it becomes important for researchers to determine what the type, intensity, and duration of stimulation needed to shift the muscle fiber type and start the process of exercise induced angiogenesis within skeletal muscle. Additionally, identifying any genetic factors causing the high degree of variation within responders and non-responders of exercise could provide great insight on the mechanisms of chronic diseases that are correlated with low levels of physical activity and fitness.


So the next time you are exercising, think of more than just the big picture. Imagine the many pieces to that big picture happening inside your muscles!


For readings related to this topic:

  • Exercise: what’s really happening


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