The seat was too deep for only 5% of the students at school A. In this case, and to avoid the compression on the posterior surface of the knee, the students will place their buttocks forward on the edge of the seat (Pan- agiotopoulou et al., 2004), causing kyphotic postures due the wrong use of the backrest (Khalil et al., 1993; Orborne, 1996; Pheasant, 1991).