Average acceleration of the key front, 〈A〉, as a function of the initial weight force F applied through the mechanical arm approximately 2 cm from the key front. Excitations are obtained for 0 < F < F_I at an excitation inertia M_I, where F_I and M_I respectively reproduce the effective weight and inertia of the engaged human limb playing unit (Tab. 1). The engaged playing unit (hand only, hand plus forearm, whole arm) is indicated by the increasing size of markers for the experimental points. Experimental data is approximated by three linear regressions, depending on the engaged inertia M_I, passing through the point (F₀, 0), with F₀ being an estimation of the minimum force to accelerate the key (the standard deviation of the estimates as uncertainty on the least significant digit is reported in round brackets). The values A_I = 〈A〉(F_I, A_I) are reported on the ordinate.