A hypertonic or spasmodic muscle may be made to relax by employing either a rapidly applied stretch, which provokes the golgi tendon organ to reflexively inhibit muscle tone, or by a long duration stretch that permits a viscoelastic lengthening of a muscle. The latter of these two is, by far, the most employed by physical therapists, trainers, massage therapists, and sports enthusiasts. This stretch should be held for a minimum of 10 seconds to a maximum of 30 seconds.

While using the BackSystem3, numerous muscles are involved. During full flexion, the muscles being stretched or extended include the erector spinae group, sacrospinalis, ilocastalis, ongisimus and spinalis. Deeper muscles include the quadratus lumborum and lattisimus dorsi. Pelvic musculature includes (pelvic extensors) the gluteus maximus, medius, minimus, biceps, femoris, and semitendinosus. The ligamentous support structures and the thorac-lumbar fascia are also engaged when stretching on BackSystem3. While returning to the upright position, the user consciously bears down, increasing intra abdominal pressure. The rectus abdominus, internal oblique, and external oblique are in an eccentric contraction to facilitate stability throughout the full range of motion.

In broad terms, anteriorly, the abdominal muscles pull upward on the pelvis, balanced by the hip flexors which pull it downward. Posteriorly, the sacrospinalis muscles pull the pelvis upward, balanced by the hip extensors, which pull it downward. Thus, four groups of muscles, two anterior and two posterior, mutually determine the sagittal plane carrying angle of the pelvis.

Very few devices exist that let a person passively stretch the lower back and pelvic musculature in an unassisted, controlled, safe manner without simultaneously contracting the antagonist musculature.