Abstract The human heel pad is considered an important structure for attenuation of the transient force caused by heel-strike. Although the mechanical properties of heel pads are relatively well understood, the mechanical energy ( E tot) absorbed by the heel pad during the impact phase has never been documented directly because data on the effective foot mass ( M eff) was previously unavailable during normal forward locomotion. In this study, we use the impulse–momentum method (IMM) for calculating M eff from moving subjects. Mass–spring–damper models were developed to evaluate errors and to examine the effects of pad property, upper body mass, and effective leg spring on M eff. We simultaneously collected ground reaction forces, pad deformation, and lower limb kinematics during impact phase of barefoot walking, running, and crouched walking. The latter was included to examine the effect of knee angle on M eff. The magnitude of M eff as a percentage of body mass ( M B) varies with knee angle at impact and significantly differs among gaits: 6.3% M B in walking, 5.3% M B in running, and 3.7% M B in crouched walking. Our modeling results suggested that M eff is insensitive to heel pad resilience and effective leg stiffness. At the instant prior to heel strike, E tot ranges from 0.24 to 3.99 J. The combination of video and forceplate data used in this study allows analyses of E tot and E tot as a function of heel-strike kinematics during normal locomotion. Relationship between M eff and knee angle provides insights into how changes in posture moderate impact transients at different gaits.