While there are three types of muscle tissue (smooth muscles, cardiac muscles, and skeletal muscles) only the skeletal muscles are under our voluntary control. The smooth muscles of the digestive tract and blood vessels, for example, function below our consciousness and the cardiac muscle responds mechanically to the oxygen needs of the body, particularly the energy intensive skeletal muscles. This article will offer some foundational big ideas to gain an understanding of how our skeletal muscles work.
1. Because of their microscopic arrangement muscles are only capable of shortening on their own. To lengthen a muscle requires an external force such as an antagonistic muscle (an example: the biceps brachii extend the triceps brachii and vice versa) or gravity.
2. All muscles have two ends which are drawn toward each other during contraction. One end is arranged in the body so that it is relative stable (the origin) while the other end is free to move (the insertion). An example would be the hamstrings which originate on the relatively stationary ischium of the hip (on the “sit bones”) and insert on the mobile tibia allowing knee flexion.
3. Muscles are arranged in lever systems which result in different effects at the end of the lever. A typical bicep brachii muscle can generate 800 pounds of force at the tendon, but because of the location of the insertion so close to the elbow, the amount of weight able to be lifted at the hand is much less. However, while this may seem like a disadvantage it allows for a large movement at the hand for little shortening of the muscle. This speed/range of motion design is common in our bodies.
4. A general rule for determining the strength of a muscle is its cross-sectional area; the bigger the muscle, the greater the strength. There is a nervous component that moderates this to a degree, for example: much of the strength gained in a beginning strength training program is from improved recruitment and coordination of muscle fiber contractions. While women are generally weaker than men, the primary difference is in the size of muscles not the gender. If we account for the cross-sectional area, men are no stronger than women.
5. While most muscles have a single origin, some muscles branch off creating multiple insertions or heads (L. -cep, head). The biceps brachii, bicep femoris, tricep brachii, and quadriceps femoris are named by the number of insertions they have.
6. Muscles move bones. These movements are described within the framework of our three-dimensional orientation. Flexion and extension of a joint occur in the sagittal plane; abduction and adduction, in the frontal plane; and horizontal adduction, protraction, and retraction in the transverse plane. While the naming of muscle actions is quite tidy when described according to the three cardinal planes, we move in an infinite number of planes allowing the incredible variation among us and our activities.
7. The quality of muscle movements is also determined to a degree by the metabolic characteristics of the tissue. On one end of a continuum we find muscle fibers (fiber is the name we give to muscle cells) that are very powerful, yet fatigue quickly (referred to as fast twitch). On the other we find fibers that have great endurance but are not capable of generating great force (slow twitch). These fibers are distributed within all muscles in a mosaic arrangement, with some muscles have greater percentages of one fiber over the other, as well as some people have differing distributions. Our fiber type distribution is mostly set from birth because a fiber is typed because of the diameter of the neuron that enervates it. However, we can train fibers to be more like one than another, but never approaching a true change.
8. Muscles will also vary according to how the fibers are arranged between the attachment points. If the fibers are arranged longitudinally, end to end, the muscle will show the greatest degree of shortening, to the detriment of its force generating capacity. The sartorius is the longest muscle in the body with this arrangement. Some muscles, such as the quadriceps have multiple feather-like arrangement of fibers resulting in the ability to pack many fibers into a small area thus increasing strength. The trade-off, because of the angled arrangement, is that the muscle shortens to a lesser degree in the same time as the longitudinally arranged ones.
In summary, there is almost an infinite possibility for how muscles move bodies as one characteristic is compromised for another. “Bo can’t know everything.” While all muscles are made up of the same building block, the resulting movements will vary based upon the position in the lever system, the neuron size, the fiber arrangement, the number of heads, the cross-sectional area and the metabolic factors within them.