Why do we have the stretch reflex?
In general, reflexes exist to help us stay alive and avoid injury. In fact, the neurons carrying the stretch reflex messages back and forth from the spine are among the most heavily myelinated (insulated) in the body. This means that their messages travel faster and are more important to our survival than the sensations of pain, touch, and temperature.
One critical function of the stretch reflex is that it helps us stand upright. If you suddenly lean to the right side, the postural muscles on the left side of your vertebral column are stretched. When the muscle spindles in those muscles sense that they’re being lengthened, they automatically send the message to contract to correct your posture. We’re rarely consciously aware of how the stretch reflex automatically maintains our balance and keeps us from falling over—but we sure would notice if it wasn’t working properly.
The stretch reflex also prevents us from tearing our muscles, tendons, and ligaments. The knee-jerk reflex is a great example of the stretch reflex. When the doctor taps your patellar tendon just below your knee, it stretches your patellar tendon, your quadriceps tendon, and your quadriceps muscles. The muscle spindles in your quadriceps sense the sudden increase in length and automatically send the message to contract your quadriceps to prevent injury and overstretching. When your quadriceps contract, your foot kicks up. Absence of this reflex could indicate a possible a neurological disorder, like receptor damage or peripheral nerve disease.
When we practice static stretching (the type of stretching traditionally taught in athletic training), the voluntary and involuntary parts of our nervous system are battling each other, trying to achieve opposite results. Our brain is sending the voluntary message to manually stretch our muscles by pulling on them. But despite all our efforts, our stretch reflex is automatically kicking in, contracting our muscles to prevent us from overstretching and tearing our muscles, tendons, and ligaments.