The Biological Architecture of Aging Movement: A Mechanistic Expansion of Barrier 2 of the Dual-Barrier Model

An older athlete can retain exceptional performance in a practiced task without demonstrating preservation of a young-adult neuromuscular system. Building on the previously proposed Dual-Barrier Model of motor specificity and biological aging, this review provides a mechanistic expansion of Barrier 2, Biological Aging (Ruggia, 2026). It examines central motor organization, predictive control, sensory integration, descending input, spinal neuromodulation, motor-unit organization, neuromuscular transmission, muscle composition, and contractile function. Human evidence supports age-associated differences at multiple levels, but does not establish a universal sequence of failure or a single dominant lesion. Lower indirect estimates of persistent inward currents, altered rapid motor-unit discharge, and impaired muscle contractile properties provide complementary explanations for reduced force-time performance. Important counterevidence includes preserved implicit motor adaptation, retained modulation of spinal excitability, training-associated improvements, and successful functional reinnervation. Regeneration of a downstream component therefore cannot be presumed either sufficient for integrated rejuvenation or incapable of useful recovery. The expanded model proposes that the expression of age-associated limitations depends on interactions among biological capacity, compensation, task-specific expertise, and movement demands. Its principal prediction is that constrained preparation time and multidirectional reactive demands will reveal differences that successful performance alone may obscure. This prediction remains untested as an integrated mechanistic model in Masters sprinters. A research agenda combines matched movement tasks with sensory, electrophysiological, force-time, and tissue measurements, while specifying outcomes that would weaken the model. The contribution is a testable systems framework, not evidence that training is ineffective, regeneration must fail, or an immutable biological ceiling has been established.