The Miracle of Muscles

Our muscles are capable of amazing, precise and coordinated force; from lifting a teacup to explosive movements such as sprinting and power lifting. Muscles allow for tremendous function so that we can explore our full physical potential. Approximately 40% of our body is composed of skeletal muscle but there are several other types of muscles including smooth and cardiac muscle (Guyton and Hall 2006). There are almost 700 individual muscles in your body and they come in all shapes and sizes (Tortora and Grabowski 2003). The smallest skeletal muscle in your body is located in your middle ear and is called stapedius. This muscle is only 1 mm long and 1 mm in diameter but has an important job stabilizing one of the ear’s small bones responsible for hearing (Guyton and Hall 2006).  By contrast, the largest muscle of the body is your quadriceps; a powerful muscle that is over five hundred thousand times larger than stapedius yet is comprised of muscle fibers that are as small as ten micrometers.

It is widely accepted that muscles have four main functions: move, stabilize, store/transport substances, and produce heat. Some of these are fairly intuitive; climbing stairs or giving your friend a high five, stabilizing positions such as standing or crouching. What is remarkable is how essential our muscles are to storing and moving different substances throughout the body. While it is easy to identify the heart as a vital muscle for delivering blood and oxygen, our body also depends on skeletal muscle to transport lymph as well as return blood from our lower limbs back to the heart for recirculation. Generating heat by way of thermogenesis is another important part of muscle function to help maintains your core body temperature. Shivering in particular, is your skeletal muscles undergoing involuntary contractions for the purpose of generating rapid increase heat to stave off cold and hopefully keep us out of hypothermia (Tortora and Grabowski 2003) .

Our skeletal muscles are able to generate a tremendous amount of contractile force but maximum output is not needed for every task. It is impressive how the body is able to produce the correct amount of force for the right situation. In the example of the teacup, could you imagine how far your tea would fly if you were to lift the cup with maximum force?  This is due to the physiological structure of muscle and how it response to nervous innervation. Skeletal muscle is comprised of hundreds to thousands of muscle fibers woven together by connective tissue, fueled by blood vessels and activated by motor neurons that are woven throughout the muscle fibers (Tortora and Grabowski 2003). Each motor neuron connects with muscle fibers distributed throughout a muscle, allowing for greater control of muscle fiber recruitment and precision of movement. Some muscles have over 3000 muscle fibers and the total contractile strength is influenced by how many muscle fibers comprise each motor unit and how many motor units are activated at the same time. This is what keeps you from launching your tea into the air – your body recognizes the weight of the teacup and how much force is needed for controlled movement.

The way our muscles adapt to physical stress caused by exercise is extraordinary. You don’t have to be a body builder or a power lifter to partake in resistance training. In fact, resistance training is something that, when done properly with attention to form and limitations, is healthy and encouraged throughout life. Age related changes to muscle are well documented and start at about 30 years of age where gradual loss of skeletal muscle mass begins and is replaced by adipose (fat) and fibrous connective tissue, which can decrease maximal strength, reflex speed and flexibility.  This gradual change is due to a number of factors including inactivity, atrophy, hormone changes as well as lowered response to leucine – the amino acid that stimulates muscle protein synthesis (Charette, et al. 1991) (Katsanos, et al. 2006).   Resistance training is correlated to improved recruitment and synchronization of motor units and is important to maintaining muscle mass and motor control. This is why there is potential value in proper supervised training for seniors looking to feel their best, reduce and even reverse the rate of age related changes (Milner-Brown, Stein and Lee 1975) (Tortora and Grabowski 2003).  It has been demonstrated that resistance training in post-menopausal women can be done safely and significantly improve strength with additional benefit to posture, gait and bone mass (Charette, et al. 1991).

Our muscles keep us moving and maintaining their health throughout life is a significant part of reducing the magnitude of age related changes that can affect quality of movement, posture, and strength. Whether it is stretching via yoga or passive movements, or more dynamic activities such as weight lifting, it is important that we make sure to move it so we don’t lose it.

Works Cited

Charette, Susan L, et al. “Muscle Hypertrophy response to resistance training in older women.” Journal of Physiology, 1991: 1912-1916.

Guyton, Arthur C, and John E Hall. “Contraction of Skeletal Muscle.” Chap. 6 in Textbook of Medical Physiology, by Arthur C Guyton and John E Hall, 72-84. Philadelphia, Pennsylvania: Elsevier Saunders, 2006.

Katsanos, CS, H Kobayashi, M Sheffield-Moore, A Aarsland, and RR Wolfe. “A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly.” American journal of physiology. Endocrinology and metabolism. 291, no. 2 (Feb 2006): 381-387.

Milner-Brown, H.S., R.B. Stein, and R.G. Lee. “Syncrhonization of human motor units: possible roles in exercise and supraspinal reflexes.” Electroencephalography and clinical neurophysiology, 1975: 245-254.

Tortora, Gerard J, and Sandra R Grabowski. “Muscle Tissue.” Chap. 10 in Principles of Anatomy and Physiology, by Gerard J Tortora and Sandra R Grabowski, 273-303. John Wiley & Sons, Inc., 2003.

About The Author

BSc, ND, AOR Medical Advisor

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