Ergonomics is a growing concerned in production systems. Companies aim to reduce work-related illnesses and the induced expenses (absenteeism, occupational accidents,...). The working conditions are mostly determined by the production system design, so this objective must be included early in the system design. The assignment of the tasks to the workstations plays a key role for ergonomics. The simple assembly line balancing problem (SALBP) is both one of the simplest and one of the most studied variants of this problem. In this work, we propose a solution method for the SALBP, introduced in [1], considering two objectives. The first one is related to the cost (by the number of stations) and the other is an ergonomic measure of the fatigue and recovery of operators. This last objective is based on the idea that the fatigue of the operator at the end of a cycle depends on the duration and difficulty of the tasks executed and on the recovery time. The method is an extension of the work presented in [2], integrating the ergonomic objective which is non-linear. Experimental results compare the performance of the bi-objective branch-and-bound to an epsilon-constraint.