Question

Why doesn''t the patellar reflex happen when your leg is straight?

Answer 1

The patellar reflex can be defined as the kicking movement of the lower leg which occurs with a sudden twitch when the patellar tendon of a person is tapped. This patellar tendon is present below a person's kneecap. This reflex happens only when the leg is bended. It is impossible for patellar reflex to occur if the leg is straight as the quadriceps muscles are already contracted and a sudden kicking movement of the leg in such state could not be elicited.  The patellar reflex protects the quadriceps  from stretching too much.

Answer 2

The patellar reflex doesn't happen when our leg is straight because tendon will not be flexible.

When our leg is straight, it doesn't happen. It only happens when tendon is flexible.

This reflex helps us maintain posture and balance,
allowing one to walk without consciously thinking about each step.