Functional disability due to lumbar pain should be considered from the biopsychosocial model. There is inconclusive evidence as to whether the key determining factors in this form of disability are psychosocial or physical. Our aim is to identify variables that cause functional disability due to lumbar pain amongst shellfish gatherers in Galicia by means of a cross-sectional survey. Participants (N = 929) completed a self-administered, paper-based questionnaire including sociodemographic and lifestyle issues, as well as the nature of the lumbar pain, the presence of musculoskeletal pain in other regions of the body, the Roland-Morris Disability Questionnaire (RMDQ) and SF-36. Univariate examination, ROC curve and logistic regression analyses were performed. Most of these workers are women (98.7 %), with a mean age of 50.6 years. The point-prevalence of lumbar pain stands at 65.5 %. The RMDQ mean was 4.9 (SD = 4.7). In the logistic regression analysis, the variables associated with disability (RMDQ > median) were age (OR = 1.04), physical exercise (OR = 0.57), pain intensity (OR = 1.16), the number of regions of musculoskeletal pain (OR = 1.24) and mental health (SF-36) (OR = -0.95). Functional disability is determined by the physical nature of the pain and mental health attributes, although the former has a greater impact. In decreasing order of importance, functional disability is attributable to the presence of lower back pain, the number of regions of musculoskeletal pain, the intensity of that pain and age. Regular physical exercise and better mental health have a protective effect on disability.