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Electrotherapy and neurostimulation universally employ the rectangular (square) waveform as their standard stimulation signal. This article demonstrates that this choice constitutes a fundamental error of physical, mathematical, and neurophysiological nature, perpetuated since the mid-twentieth century through three converging factors: insufficient signal theory training in medical and paramedical curricula; technological drift toward ever-steeper wavefronts perceived as progress; and inadequate spectral disclosure by medical device manufacturers. We recall that the founders of electrical neurophysiology—Du Bois-Reymond (1843) and Helmholtz (1850)—stimulated with smooth-envelope signals, involuntarily close to membrane physiological requirements. We analyze the technological stratigraphy that progressively established the square wave as the unquestioned norm, and identify two erroneous assertions in the French foundational literature (Dumoulin & de Bisschop, 1987; Crépon, 1994) as crystallization points of the error in clinical practice. We present spectral and energetic calculations demonstrating the inadequacy of the rectangular signal relative to the biological bandwidth of the excitable membrane: for a 600 μs rectangular pulse at 50 Hz, Fourier harmonics extend to 81,650 Hz, wavefront components exceed 5 MHz, and the calculated peak power reaches 7.75 × 108 W, against 6.1 × 105 W for the equivalent sinusoidal signal. We propose an optimal biomimetic signal described by a parametric Bézier curve whose inflection points correspond to the conformational time constants of voltage-gated ion channels as described by the Hodgkin-Huxley model (1952). This zero-mean signal respects the natural opening and inactivation kinetics of sodium and potassium channels, concentrating its energy within the physiologically relevant bandwidth. We discuss documented clinical consequences of the fundamental error: peri-electrode fibrosis in deep brain stimulation (DBS), progressive impedance drift, and the relative inefficacy of consumer TENS devices. This work is published open access under Creative Commons CC-BY 4.0. All parameters of the optimal signal are fully described herein, establishing permanent publication priority and excluding subsequent patent filing on this concept.