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The tendon reflexes are lower level "stimulus -> reaction" effects. This means that the reflexes occur below the level of the brain. Each muscle group contains stretch-receptors that, when stimulated quickly cause the muscle to contract. The stretch signal goes up to the spinal cord and the signal to the corresponding muscle to contract proceeds right back down from the spine. The brain does not get involved.
Large muscles come together at one end in the form of a tendon that then passes across a joint to attach the muscle to a bone. Examples that you can actually see in your body are the large patellar tendon that attaches at the thigh end to the quadriceps and passes over and around the patella (kneecap) to attach to the top of the tibia (shin bone). The patellar tendon is the main mechanism for straightening the knee. Another large one is the Achilles tendon which gathers the calf muscles in and then travels across the ankle to attach to the underside of the heel. The Achilles tendon is the way you are able to point your toes downward. Another is the biceps tendon which passes in the bend of the elbow. You can feel the biceps tendon like a hard cord in the crook of your elbow when you flex your biceps.
When the tendon is tapped briskly, it quickly pushes in on the tendon and places a tiny, but quick stretch on the muscle attached to that tendon. The Patellar tendon is the easiest to see and describe. You always do a tendon test when the muscle is slightly stretched, so usually this is done while sitting with knee bent at 90 degrees and the lower leg able to swing freely. When the tap is done, just below the kneecap, the normal reaction of the quadriceps is to contract. When it does it causes the knee to extend (the lower leg to swing out). The nerve loop that controls a tendon reflex is a stretch receptor that travels up to the spinal cord and immediately loops over to a motor nerve that travels down and activates the proper muscle.
The reflexes may be dulled by depressant medications, low metabolic states like low magnesium, severe depression, or hypothyroidism. Also in low thyroid states, the reflexes not only will be lessened, but they will be delayed. Reflexes may be accentuated by things like stimulants, caffeine, anxiety, or hyperthyroidism. They also may be "reinforced" by either slight, active contraction of the muscle/tendon being tested or by having the patient contract another set of muscles forcefully. The usual maneuver for reinforcement is to have the patient grasp the fingers of both hands together in front of the chest and to pull tightly. (Try doing this. First tap on the tendon without grasping the hands. Then do it with the hand-pull. You will normally see a significant increase in the force of the tendon reflex) Reinforcement is only used when there is a problem seeing any reflex action at all.
The evaluation of the reflexes is done on a numercial scale, but it is subject to the interpretation of the examiner, so it is not quantified. The more experience, the more reproducible the test will be between examiners, but it is still really a subjective measurement. If it were to be quantified (measured in exact numbers) it would be by electromyography and would be a research test
The most common rating scale of tendon reflexes is as follows:
0 – Unable to elicit any movement or contraction of the muscle
1+ - Little movement or only seen with reinforcement - lower end of normal
2+ - Normal (what "normal" looks like is learned through doing this 100's of times with hundreds of "normal" people)
3+ - Brisk (more than is normally seen, but not causing injury to the examiner, lol) this is also considered within the normal range.
4+ - Maximum movement and associated with some limited, extra repetitive movement(s) called clonus. This is level and above is abnormal, or hyperactive.
5+ - Major kick with sustained, repetitive movements (sustained clonus). Also considered here and in "4+" is if other muscle groups also react with contraction, called "spreading reflex." Clonus is most often seen at the ankle. The doctor will press suddenly up on the toes, dorsiflexing the ankle. With clonus the toes will repeatedly and rhythmically bounce downward if light pressure is kept on the toes. Some doctors use a 0 to 4+ scale and use 4.5+ (sustained clonus) instead of 5+.
Clonus is normal in newborns who all have immature nervous systems. You can elicit clonus in newborns by taking a finger and pushing up quickly under the ball of their foot. This will cause the foot/toes to rhythmically move up and down. (taken from "Fun and Games to Play With Your Newborn” - Activities for Bored Parents, lol). In babies this is normal, but disappears within a couple months as their nervous system matures.
0, 4+, and 5+ are abnormal and indicate neurological problems.
Hyperactive reflexes usually indicate a problem (lesion) in the spinal cord. You may hear this being called an Upper Motor Neuron Sign. Or the doctor may find hyperactive reflexes and diagnosis Myelopathy (disease of the spinal cord). This is one of the most freqently abnormal tests on the neurological exam in a person with MS.