There are many causes of muscle pain and dysfunction, including both hypertonicity and chronic ischemia. One strategy for addressing these underlying issues is selecting treatment methods according to the dominant type of skeletal muscle fibers in the area or region of discomfort.
Skeletal muscles are made up of two distinct types of muscle fibers that vary in both structure and function. The primary variable that differentiates these fiber types is how each one produces the energy needed to drive muscle contractions. Remember, energy is released in the body when a single phosphate bond is broken from the adenosine triphosphate (ATP) molecule. These molecules are formed within the muscle cells either aerobically (in the presence of oxygen) or anaerobically (without oxygen) through a process called glycolysis.
Energy Production Methods
Aerobic energy production is extremely efficient, producing a large number of ATP molecules from a given amount of glucose or fuel. Delivery of adequate amounts of oxygen and fuel to the mitochondria of muscle cells is essential for continual aerobic energy production, as is the removal of carbon dioxide, a metabolic by-product of aerobic glycolysis. This is the preferred method of energy production in the body, and the one utilized for most activities of daily living. The process continues indefinitely, as long as fuel and oxygen are delivered to the mitochondria and carbon dioxide is removed.
Sometimes muscles need to perform activities that are fast and forceful for short periods of time. Skeletal muscles are capable of exceeding the force production offered through aerobic means, but with less efficiency and only for short periods of time. Anaerobic energy production occurs without oxygen or mitochondria. This process occurs in the cytoplasm of the cell and only yields a couple of ATP molecules per unit of glucose. Since it does not utilize oxygen, there is an increase in acidity in the surrounding intercellular environment, as the phosphate bonds are broken and energy is released. The subsequent change in pH as well as rapid depletion of stored fuel make this type of energy production short-lived, lasting between 12 seconds and 2 minutes.
Fiber Types
There are two distinct types of fibers that make up skeletal muscles: slow-twitch, also called Type I fibers, and fast-twitch, or Type II fibers. Twitch refers to the amount of time it takes a motor unit or group of fibers to develop force and relax.
Fast-twitch fibers are divided further into Type IIa and Type IIb. Each fiber type has unique characteristics:
All skeletal muscles have a combination of fiber types, with the ratio and distribution of these fibers being genetically determined. This explains why some individuals have a slighter build with longer, leaner muscles, naturally excelling at long duration activities (higher ratio of Type I), and others are more densely muscled with shorter, thicker muscles and excel at activities requiring strength and power (higher ratio of Type II).
In addition to fiber-type dominance differences between individuals, there are also fiber-type dominance differences between muscles. Large, superficial muscles like the latissimus dorsi, pectoralis major, and gluteus maximus have a much higher ratio of fast-twitch fibers. Smaller, deeper muscles like the supraspinatus, quadratus lumborum, and piriformis are dominated by slow-twitch fibers.
Variations in fiber-type dominance indicate a difference in function between muscles. The large, superficial ones, dominated by fast-twitch fibers, should be at rest most of the time and recruited for short-duration, powerful movements. Smaller, deeper muscles dominated by slow-twitch fibers are postural muscles, optimally positioning the joints for long-duration and low-intensity activities. Muscles with a balanced ratio of fiber types offer variable function.
Contribution to Dysfunctional Patterns and Pain
Muscle pain and dysfunction can occur when large, superficial muscles dominated by fast-twitch fibers are recruited for long-duration activities. For example, when the head is positioned slightly forward, the upper trapezius is recruited to maintain that unbalanced position. The fast-twitch fibers fatigue quickly and the chemical remnants of anaerobic energy production increase the acidity of the tissue. More fibers of this muscle are then recruited, creating a positive feedback loop of pain, hypertonicity, and dysfunction.
Issues occurring in the small, deep muscles dominated by slow-twitch fibers are of a different nature. If recruited to perform quick, powerful movements, they are slow to react and suffer damage, resulting in muscle strain and potential acute injury to surrounding structures. Also, if the superficial muscles in the area are hypertonic, these deeper ones may become ischemic, lacking the necessary fuel and oxygen needed for aerobic energy production. This ischemic condition is equally painful and may result in muscle inhibition and eventual atrophy.
Addressing Fiber Type with Bodywork
Understanding the different fiber-type structure and function helps direct appropriate application of bodywork. Hypertonicity and resulting discomfort in the large, superficial muscles can be effectively addressed using neuromuscular techniques to confuse proprioceptors and reduce excessive tone. Once inhibition of these hypertonic muscles is achieved, apply methods to increase circulation in the deeper, slow-twitch dominated muscles in an effort to increase capacity for aerobic energy production. The combined effort begins to restore proper function in terms of energy production to the various muscles.
While this strategy may provide immediate relief, it is critical that clients address chronic postural issues and body mechanics to alter dysfunctional recruitment strategies. It is essential that large, superficial muscles be retrained to remain relaxed unless called upon for short-duration, powerful movements. The small, deep muscles are reactivated as posture improves, deep circulation increases, and optimal joint mechanics are reestablished. This may require a combined effort between multiple disciplines.
Christy Cael is a licensed massage therapist, certified strength and conditioning specialist, and instructor at the Bodymechanics School of Myotherapy & Massage in Olympia, Washington. Her private practice focuses on injury treatment, biomechanical analysis, craniosacral therapy, and massage for clients with neurological issues. She is the author of Functional Anatomy: Musculoskeletal Anatomy, Kinesiology, and Palpation for Manual Therapists (Lippincott Williams & Wilkins, 2009). Contact her at functionalbook@hotmail.com.