Objectives: The main aim of the study was to investigate whether there is a difference in the characteristics of posterior root muscle reflex (PRMR) of the cauda equina between a group of healthy subjects and a group of patients diagnosed with autoimmune polyradiculoneuropathies. The secondary aims were to determine the latency and area values of PRMR in healthy subjects and to examine their association with demographic characteristics. Study design: The study was designed as a cross-sectional prospective study. Participants and Methods: The study included thirty healthy subjects, without known peripheral nervous system diseases, and ten patients diagnosed with acute or chronic polyradiculoneuropathy. Subjects underwent transcutaneous electrical stimulation of the cauda equina roots. PRMR was recorded in four groups of lower extremity muscles bilaterally: TA – tibialis anterior, GC – gastrocnemius, Q – quadriceps femoris, and BF – biceps femoris. Results: The PRMR was successfully elicited in TA (96.70%), GC (100.00%), Q (93.30%), and BF (96.70%). No statistically significant difference was found in the intensity of stimulation, latency, or area of PRMR in the same muscles of the left and right legs of healthy subjects. The area of PRMR showed significant variability between the same muscles of the subjects, while the latency values varied within a small range. Body weight and abdominal circumference showed a significant positive correlation with the intensity of stimulation and a significant negative correlation with the area of PRMR. Older age showed a significant negative correlation with the success of eliciting PRMR and the area of PRMR. The height of the subjects showed a significant correlation with PRMR latency. In 90.00% of the patients' muscles, PRMR was not elicited or the area of PRMR was below the expected values in healthy subjects. An unelicited response or small PRMR area was recorded in 5 – 8 out of 8 muscles of each patient. Latencies within the expected range in healthy subjects were recorded in only 8.75% of the patients' muscles. Prolonged PRMR latencies were recorded in 5 – 8 out of 8 muscles of each patient. Of the 80 muscles of the patients, PRMR with normal area and latency values was registered in only one muscle. Conclusion: PRMR is a non-invasive, successful method for selectively eliciting reflex responses of the posterior spinal roots. The described methodology and the results obtained in healthy subjects can be used to assess the functional integrity of the lumbosacral spinal roots and proximal parts of the peripheral nerves of the legs. The methodology demonstrated high sensitivity for detecting multiple lesions of the lumbosacral spinal roots. The usability and value of the PRMR in confirming the diagnosis of autoimmune polyradiculoneuropathies have been proven, avoiding the disadvantages of standard electrophysiological tests.