BACKGROUND: Questionnaires are a common tool to assess people's opinion on a large scale or to sound them out about their subjective views. The caretakers' opinion about animals' "personality" has been used in many studies. The aim of the present study was to assess whether the owners' subjective evaluation was effective to detect back disorders. Back disorders have been shown to have a high prevalence in working horses. Caretakers from 17 riding schools (1 caretaker/school, 161 horses) were given a questionnaire about their horses' health status, including back disorders. Out of these 161 horses, 59 were subjected to manual palpation of the spine and 102 were subjected to sEMG examination all along the spine. RESULTS: The results showed that subjective caretaker-reported evaluation via questionnaire survey was not efficient to detect back disorders: only 19 horses (11.8%) were reported as suffering from back pain, whereas the experimenters' evaluation detected 80 of them (49.7%) as suffering from back disorders. While most caretakers under-evaluated back disorders, a few "over-evaluated" it (more horses reported as affected than found via clinical evaluations). Horses were less prone to present back disorders when under the care of these "over-attentive" caretakers. CONCLUSIONS: This study showed that back pain is difficult to evaluate, even for professionals, and that subjective evaluations using a questionnaire is not valid in this case. The results also highlighted the real need for observational training (behaviours, postures) outside and during riding.