Computational modeling of neuromuscular activation by transcutaneous electrical nerve stimulation to the lower back.

Objectives.Transcutaneous Electrical Nerve Stimulation (TENS) to the lower back is an established electrical therapy for acute and chronic back pain. The efficacy and mechanisms of lower back TENS depend on the penetration depth of electrical current. We compare the intensity and spatial extent (depth) of current flow in the body during TENS with varied electrode positions/shapes on the human back.Materials and Methods.A high-resolution MRI-derived anatomical model of the back was developed, considering major tissue compartments, including skin and muscles. TENS with upper and lower back electrode positions and varied electrode shapes (square, circular, rectangular) were simulated. An exemplary 50 mA current was applied under quasistatic approximation and quasi-uniform electric field assumption of 6.15 V m^-1(low), 12.3 V m^-1(mid), and 24.6 V m^-1(high) neuromuscular activation thresholds were considered.Results.Under all simulated TENS conditions (50 mA), electric fields at the skin exceed the high threshold (consistent with peripheral nerve activation) and at least some muscle regions exceed the mid threshold. Muscle activation was influenced by the anatomy of muscle in the medial-lateral direction and upper-lower back. The electrode shape had minimal effect on deep tissue current penetration.Conclusions.Our simulations indicate significant current penetration into back tissue (electric fields above low threshold) to >8 cm in all TENS conditions simulated, consistent with nerve and muscle activation.Significance.Anatomically precise models of upper and lower back TENS show current penetration to deep muscle, supporting direct muscle stimulation driving clinical benefits.