Anatomy of a Muscle

Muscle pain, injury or tension are the most common motivating factors when clients seek out massage services. These tissues are responsible for movement of the body and when they are compromised, it can be a real pain. Before we dive into why massage is so vital in keeping these tissues healthy and working properly, let’s investigate what they are and how they function.

Each muscle comprises multiple tissues, including blood vessels, lymphatics, contractile muscle fibers, and connective tissue sheaths. The outermost sheath of connective tissue covering each muscle is called epimysium. Each muscle is made up of groups of muscle fibers called fascicles surrounded by a connective tissue layer called perimysium. Multiple units of individual muscle fibers within each fascicle are surrounded by endomysium, a connective tissue sheath. While these words may seem confusing, the takeaway is that each layering of tissue is bound and protected to keep it from harm.

There are three main types of muscle: cardiac (found in the heart), smooth (found in the walls of hollow visceral organs), and skeletal (connected to the skeleton). Our main concern for this article will be the skeletal muscles which are striated and under voluntary control.

Almost every skeletal muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of that lengthening. The CNS uses this information to calculate the body’s position in space to control motor function, maintain posture and create a stable gait. This process is known as proprioception. Imagine a muscle spindle cell spiraling around the fibers in the central aspect of a muscle. As the muscle lengthens, it pulls on these threads causing them to lose their spiral shape. In response to this stimulus, a signal is sent for the muscle to shorten, thereby regaining its original spiral shape and protecting it from injury. This process is called the myotatic, or stretch, reflex.

If you have ever had a physician test your reflexes, you have witnessed the myotatic reflex. A reflex hammer is used to tap and quickly stretch the patellar tendon as it crosses over the kneecap. This action usually prompts the quadriceps muscles on the front of the thigh to contract causing your lower leg to kick outward.

Rapid stretching or repeated activation of the muscle spindles will create increased sensitivity to stretching, activating the myotatic reflex with less stimulation. This creates increased muscle tension and “trigger-happy” muscle spindles that limit mobility and create discomfort and dysfunction. Desensitizing the muscle spindles is one strategy for decreasing muscle hypertonicity and improving active range of motion. Muscle spindles become less reactive when they monitor and recognize movements that provide comfortable and safe magnitudes and speeds of tissue lengthening. Providing passive movements or engaging in active movements that slowly, gently, and comfortably lengthen muscle tissue will recalibrate the muscle spindles and decrease neuromuscular limitations in mobility.

Muscle tissues also contain golgi tendon organs (GTO) - bundles of braided collagen in the musculotendinous fibers (where the muscle tissue transitions into tendon tissue and connects to the bone). GTOs are sensitive to changes in the amount of tension and the rate of tension exerted on a muscle. They are responsible for notifying the brain as soon as they sense an overload via a process called the inverse stretch reflex.

This is an important concept to understand when working out. Building muscle tissue via lifting weights increases the threshold of force for any given muscle group. If the muscle can tolerate a greater force, it will take longer for the inverse stretch reflex to be activated, which means the muscle can accomplish more, i.e. move more weight.

The inverse stretch reflex is also an important factor during static stretching. When a low-force stretch is held for more than seven seconds, the increase in muscle tension activates the GTO, which temporarily inhibits muscle spindle activity (thus reducing tension in the muscle), and allows for further stretching.

In short, if the muscle spindle system is considered a feedback system that monitors and maintains muscle length, then the Golgi tendon system is a feedback system that monitors and maintains muscle force.