In this paper, the assembly line balancing problem when ergonomics principles are taken into account is considered. Two different approaches are presented and discussed with a numerical example to provide new insights on the field and to illustrate their use on a didactic example. The first one applies a multi-objective model based on the energy expenditure, used to estimate the ergonomics level. The second one transforms the energy expenditure rate in a rest time in order to reduce the multi-objective problem to a single objective one. The two methods are alternatively usable in practice and a numerical model demonstrates their foundation.