What injury is indicated by a popping sound followed by an inability to extend the knee for a straight leg raise?

The presentation of a popping sound followed by an inability to extend the knee during a straight leg raise suggests a significant injury to the knee, particularly involving the quadriceps mechanism. In this case, a rupture of the rectus femoris – one of the four muscles that make up the quadriceps – can lead to an acute loss of muscle function.

The rectus femoris is responsible for both knee extension and hip flexion. A complete rupture would severely compromise the ability to extend the knee, particularly impacting activities that involve straight leg raises. The popping sound typically indicates an acute injury to the muscle, which may occur during activities that involve explosive movements, such as sprinting or jumping.

In contrast, injuries like an ACL tear or meniscus tear may present with different clinical signs. An ACL tear often presents with instability or a feeling of "giving way" but may not immediately result in an inability to extend the knee. A meniscus tear could also cause locking or catching sensations but usually does not immediately impede the ability to initiate extension, especially to the degree described. Similarly, a patellar tendon rupture might prevent knee extension, but it typically presents with distinct physical examination findings such as a palpable gap at the patellar tendon site and an inability to extend