The state-resolved reaction probability of CH4 on Pt(110)-(1×2) was measured as a function of CH4 translational energy for four vibrational eigenstates comprising different amounts of C-H stretch and bend excitation. State-specific reactivity is observed both between states from different polyads and between isoenergetic states belonging to the same polyad of CH4. For the stretch/bend combination states, the vibrational efficacy of reaction activation is observed to be higher than for either pure C-H stretching or pure bending states, demonstrating a concerted role of stretch and bend excitation in C-H bond scission. This concerted role, reflected by the non-additivity of the vibrational efficacies, is consistent with transition states structures found by ab-initio calculations and indicates that current dynamical models of CH4 chemisorption neglect an important degree of freedom by including only C-H stretching motion.