The nervous system of mammals is very complex. For most major actions in the body the brain must decide what movement or action must be taken, the nerve impulses must be transmitted out of the brain, down the spinal cord and out to the intended receiver. Then when the action is carried out the impulse must return back via the reverse pathway to tell the brain it was completed and start the next process. This is the path for any brain-controlled, conscious, impulses. Although it takes a lot of words to explain, it is really a very rapid process.
There are many processes in the body that do not require direct thought to complete. The heart functions, breathing, metabolic processes, disease fighting and many other autonomic processes happen automatically in the body. The body uses signals to increase, decrease, or maintain many of these actions. If the carbon dioxide levels in the body begin to rise, the autonomic nervous system, through acid/base thermostats, calls for an increase in respiratory rate.
Another automatic response by the nervous system is the reflex. The body reacts in a predetermined way based on specific stimulus. This may be a practiced response or a pre-programmed one. The stretch reflex (or myotatic reflex) is one of those responses.
What is the Stretch Reflex?
The stretch reflex; which is also often called the myotatic reflex, knee-jerk reflex, or deep tendon reflex, is a pre-programmed response by the body to a stretch stimulus in the muscle. When a muscle spindle is stretched an impulse is immediately sent to the spinal cord and a response to contract the muscle is received. Since the impulse only has to go to the spinal cord and back, not all the way to the brain, it is a very quick impulse. It generally occurs in 1-2 milliseconds.
This is designed as a protective measure for the muscles, to prevent tearing. The muscle spindle is stretched and the impulse is also immediately received to contract the muscle, protecting it from being pulled forcefully or beyond a normal range.
The synergistic muscles, those that produce the same movement, are also innervated when the stretch reflex is activated. This further strengthens the contraction and prevents injury. At the same time, the stretch reflex has an inhibitory aspect to the antagonist muscles. When the stretch reflex is activated the impulse is sent from the stretched muscle spindle and the motor neuron is split so that the signal to contract can be sent to the stretched muscle, while a signal to relax can be sent to the antagonist muscles. Without this inhibitory action, as soon as the stretched muscle began to contract the antagonist muscle would be stretched causing a stretch reflex in that one. Both muscles would end up contracting simultaneously.
The stretch reflex is very important in posture. It helps maintain proper posturing because a slight lean to either side causes a stretch in the spinal, hip and leg muscles to the other side, which is quickly countered by the stretch reflex. This is a constant process of adjusting and maintaining. The body is constantly under push and pull forces from the outside, one of which is the force of gravity.
Another example of the stretch reflex is the knee-jerk test performed by physicians. When the patellar tendon is tapped with a small hammer, or other device, it causes a slight stretch in the tendon, and consequently the quadriceps muscles. The result is a quick, although mild, contraction of the quadriceps muscles, resulting in a small kicking motion.
The muscles are attached to tendons, which hold them to the bone. Muscles have tendons at each attachment. At the attachment of the muscle to the tendon is a muscle spindle that is very sensitive to stretch. The motor neurons that activate the muscles are attached here as well. These are considered lower motor neurons. When they are stimulated they can cause the muscle to contract. This frees up the upper motor neurons and other portions of the central nervous system for more important functions.
|Stretch Reflex (Myotatic Reflex) image from
the Anatomy of Stretching by Brad Walker.
The motor neurons travel from the spinal cord to the muscle and back again in a continuous loop. Conscious movement comes from impulses in the brain travelling down the spinal cord, over this loop, and then back to the brain for processing. The stretch reflex skips the brain portion of the trip and follows the simple loop from muscle to spinal cord and back, making it a very rapid sequence.
The diagram to the right shows how nerve impulses triggered by the stretch reflex travel between the spinal column and the muscles.
The gamma efferent cells in the loop work to keep the muscles ready for the stretch reflex, even when inhibited or contracted. This is important because if the muscle is working against a load and shortening during contraction and an additional load is added, the muscle recognizes the stretch immediately and can compensate with a stronger contraction. This also protects the inhibited antagonist muscles from being injured from excessive stretching.
What causes it?
The stretch reflex is caused by a stretch in the muscle spindle. When the stretch impulse is received a rapid sequence of events follows. The motor neuron is activated and the stretched muscles, and its supporting muscles, are contracted while its antagonist muscles are inhibited.
The stretch reflex can be activated by external forces (such as a load placed on the muscle) or internal forces (the motor neurons being stimulated from within.) An example of the former is a person holding an empty tray in their outstretched arm and then having a plate of food set on it. The stretch reflex would kick in to keep the tray at the same height and balanced. An example of the latter would be the shivering of a cold muscle. The motor neurons are stimulated from an internal "stretch" to warm the muscles.
Any abrupt, forceful stretch on the muscle causes the stretch reflex to fire, in a healthy person. Delays in or absence of the stretch reflex are signs of possible neurological or neuromuscular compromise.
What to Avoid When Stretching?
Many people have never learnt how to stretch properly. Maybe you have done this yourself: You watch other people stretching in the gym and try to imitate what you see. But who is to say that the person you are watching is doing it right? Here are some of the most common mistakes made while stretching:
So, to avoid the stretch reflex and potential damage to your muscles and joints, avoid pain. Never push yourself beyond what is comfortable. Only stretch to the point where you can feel tension in your muscles. This way, you will avoid injury and get the maximum benefits from your stretching.
While the recommendations on this page are a good starting point, you'll get a lot more benefit when you include a wider variety of exercises. So to improve your athletic ability, reduce injuries and really take advantage of all the stretching exercises on offer, grab a copy of the Ultimate Guide to Stretching & Flexibility (Handbook, DVD & CD-ROM).
In total, they include 135 clear photographs and 44 video demonstrations of unique stretching exercises for every major muscle group in your body. Plus, over 80 printable stretching routines for 22 sports and 19 different muscle groups.
The DVD also includes 3 customized stretching routines (8 minutes each) for the Upper Body; the Lower Body; and the Neck, Back & Core, plus a bonus CD-ROM that allows you to print out over 80 stretching routines that you can take with you wherever you go.
The Handbook and DVD will show you, step-by-step, how to perform each stretch correctly. Plus, you'll also learn the 7 critical rules for safe stretching; the benefits of flexibility; and how to stretch properly. Check out the Ultimate Guide to Stretching & Flexibility for yourself.