ELI5 – Why is it important to check the reflexes of the knee with a small hammer when we are kids, but never as an adult?

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It IS used outside of pediatrics. It’s useful diagnostically sometimes to verify proper nerve function.


They’re testing your forced shortening reflex. Your leg muscles are forced to stretch by the hammer, and your body says “hey now, I didn’t do that, let’s flex this muscle to regain control”. If this reflex is absent In a child then it could indicate that their Nerves are not developing properly, which could indicate a plethora of potential developmental issues.


Current medical student here Reflexes are a great way to test a simple circuit in your leg. The reflex hammer hits your patellar tendon (right below your knee cap) which activates your muscle spindle’s stretch reflex which causes your quad to contract (and your leg to kick up). For all of this to happen, you need the nerves sensing the hammer tap to transmit their signal to your spine, where they connect with the motor neurons in charge of your quad. Those neurons send the signal back down from your spine to your leg. So with a simple test, we see how this circuit is functioning which helps determine if there is anything wrong with your spinal cord at the L2-L4 levels. This is particularly important in developing children as nerve damage can lead to long term problems. If you are getting regular check ups as an adult, your PCP should still be checking your patellar reflex but it depends on your physician.


I’ve had my knee reflex tested at every physical I’ve been in across different hospital systems in different states. I’m not a child…well maybe maturity wise but I’m a 33 year old adult


Generally, the knee reflex test is used often on children to make sure that their nervous system is developing properly as they grow older. However, these tests are also commonly tested on adults as the older you get, the more likely you’ll start to have problems with your body and nervous system in particular. If an adult does not exhibit this reflex, then there is something wrong neurologically speaking.