In past editions of Functional Anatomy, we have primarily focused on muscles and muscle groups. Several other structures are addressed and affected by bodywork, and I want to highlight some of these in the next several columns, beginning with the muscle spindle.

Proprioception
Proprioception is an overall awareness of body position. It serves as an important safety mechanism and adaptive strategy for muscles and joints. This aspect of positional awareness is independent of vision and critical in preventing injury and creating efficient movement. The nervous system communicates with muscles, tendons, and joints through different proprioceptors to sense and alter body position.

Muscle Spindle Anatomy
Muscle spindles are proprioceptors distributed throughout skeletal muscle tissue that monitor changes in tissue length. A muscle spindle includes specialized skeletal muscle fibers called intrafusal fibers surrounded by a coil of sensory nerve endings. The sensory nerves, or afferent fibers, monitor the rate and magnitude of stretch within the muscle.  
If a stretch is strong or fast enough to potentially damage tissue, the alpha efferent fibers prompt the surrounding extrafusal fibers to contract and shorten the muscle, thus protecting it from harmful overstretching. This response is called the myotatic 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 at the front of the knee. This action usually prompts the quadriceps muscles on the front of the thigh to contract. Your leg kicks out, telling the doctor that your muscle spindle is working correctly. All skeletal muscles contain muscle spindles and are protected by the myotatic reflex.
In addition to the immediate response of contracting a muscle that is in danger of being injured, muscle spindles recalibrate themselves in response to that stimulus. As the extrafusal fibers adjust their length to protect the muscle, gamma efferent fibers adjust the tension of the muscle spindle to maintain its length-monitoring function. This creates a more reactive muscle spindle that is triggered more easily when length changes occur. If you have ever felt that shaky, spasm sensation when stretching a muscle, you are experiencing the activity of muscle spindles.

Muscle Spindle Function
What’s critical here is to understand the relationship between speed of movement and activation of muscle spindles. Rapid stretching will trigger muscle spindles and prompt a contraction of that muscle. Additionally, repeated activation of 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. Stretching must be slow and gentle enough to convince the muscle spindles that you can be trusted not to cause injury.

Application to Bodywork
When applying passive range of motion, Swedish massage, or other bodywork techniques that include moving or positioning the client’s body, keep muscle spindles in mind. Rapid or excessive lengthening may provoke muscle guarding, hypertonicity, and constriction of motion. If the goal is to improve mobility and lengthen tissues, first convince the muscle spindles that what you are doing is safe. Go slowly and allow the proprioceptors time to recognize what is happening, determine the safety of the movement, and become less reactive. Once that is accomplished, you will be able to stretch the actual tissue without interference.

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.