This is an excerpt from Motor Learning and Control for Dance by Donna Krasnow & Mary Virginia Wilmerding.
The gamma efferent system explains how a dancer can be much tighter in movement than while passively stretching, but this system also affects alignment. When examining the problem of hyperlordosis (swayback) in the lumbar spine, the explanation is often given as tight hip flexors (possibly psoas or rectus femoris), tight lumbar extensors, and weak lumbar flexors (abdominals). These muscle imbalances may in fact be present. However, there may also be a neurological contribution to the swayback alignment.
If this swayback alignment is one that is familiar to the body because it has been patterned over a long period of time, then gamma motor activity to the various muscles is set so that the muscles are preset to maintain their various lengths. Dancers can override the nonconscious messages to the hip flexors and lengthen them to move toward neutral pelvis. One way to accomplish this change would be to increase abdominal or gluteal activity with conscious thought, bringing the pubic bone forward and reducing anterior pelvic tilt and hip flexion. This increased length also serves to lengthen all the muscle fibers in the hip flexors, including the intrafusal fibers, putting the gamma sensory nerves on stretch. As soon as the conscious mind goes to another movement task, such as relevé or chaîné turns or jumps, the conscious effort to use the abdominal or gluteal muscles diminishes. At that point, the stretched gamma sensory neuron fires, causing a contraction of extrafusal fibers that exist in the hip flexors and returning them to the shortened, habitual length. A similar analysis can be made for the lumbar extensors, which return to their shortened, habitual length (hyperlordosis of the spine) and the abdominals, which return to an elongated state and allow the anterior pelvic tilt. Each time the dancer consciously contracts muscles to try to place the pelvis into neutral alignment, the gamma system acts to return it to the anterior tilt (swayback) that has been patterned for years. This is why it can take months of vigilance, imagery, quiet awareness work, and sensations of being in what at first feels like the wrong alignment for a time, before the brain's programming is literally rewritten, and gamma motor output to the intrafusal fibers in the key muscles is reorganized and adjusted. Stretching and strengthening the appropriate muscles may not be enough. This proposed situation of stretching and strengthening being insufficient to create alignment changes has not been documented in the research. However, teachers are familiar with dancers who do corrective exercises and yet do not change habitual patterns, and given what is known about the gamma efferent system, it is reasonable to hypothesize a neural contribution to changing habitual patterns.
The pathways of the gamma efferent system are illustrated figure 6.13. Messages from areas of the brain (1) go to the gamma motor neuron (2), which sends messages to the intrafusal fibers in the muscle (3). The gamma sensory neuron (4) wrapped around the intrafusal fiber is stretch sensitive, and it signals length, velocity, and change in the muscle. It reaches its threshold and fires on the alpha motor neuron (5), which then tells the extrafusal fibers in the same muscle (6) to contract, thus regulating how much or little that muscle can stretch at any given time. It is no wonder that dancers can become frustrated trying to make changes to alignment. However, once they understand the complexity of the gamma efferent system, they can be more attentive and perhaps even more patient.
The gamma system is a complex organization that regulates alignment and muscle tension. Messages go from the brain (1) by way of the gamma motor neuron (2) to the intrafusal muscle fibers (3). The gamma sensory neuron responds (4), and it sends its message to the alpha motor neuron (5), which contracts the extrafusal muscle fibers (6).
Muscle Spindles and Golgi Tendon Organs Work Together
Along with the reticular activating system and various interneurons, the muscle spindles and the Golgi tendon organs act together to create smooth, coordinated movement. The muscle spindles are excitatory or facilitatory, while the GTOs are inhibitory. Like a dance between two partners, these structures (in association with brain areas such as the cerebellum and basal ganglia) constantly interact to ensure that muscle length changes are fluid, not jerky, and movement is coordinated. This relationship is at a nonconscious level, and it is one of the wonders of the neuromuscular system.