High intensity eccentric exercise is a potent stimulus for neuromuscular adaptation. A greater understanding of the mechanical stimuli afforded by this exercise will aid the prescription of future eccentric training regimes. This study sought to investigate the mechanical characteristics of supramaximally loaded eccentric exercise when using a custom-built leg press machine. Using a within subject, repeated measures design, 15 strength trained subjects (age 31 ± 7 years; height 180.0 ± 6.8 cm; body mass 81.5 ± 13.9 kg) were assessed under three different conditions; LO, MOD and HI which were equivalent in intensity to 110, 130 and 150%, respectively, of peak force during an isometric leg-press at 90° knee flexion (IMVC). All loading conditions demonstrated a similar pattern of mechanical profile, however, the variables underpinning each profile showed significant (p < 0.01) load dependent response (LO vs MOD, MOD vs HI, LO vs HI) for all variables, except for average acceleration. Average force associated with each loading conditions exceeded IMVC, but equated to a lower intensity than what was prescribed. Repetitions under higher relative load intensity stimulated greater average force output, faster descent velocity, greater magnitude of acceleration, shorter TUT and a decline in force output at the end range of motion. This research provides new data regarding the fundamental mechanical characteristics underpinning supramaximally loaded eccentric leg press exercise. The information gathered in the study provides a foundation for practitioners to consider when devising loading strategies, and implementing or evaluating supramaximally loaded eccentric exercise when using a similar exercise and device.