OBJECTIVE: To determine the variation in forces used by different therapists during mobilization of the lumbar spine and the repeatability and reproducibility of individual therapists. DESIGN: An instrumented mobilization couch was developed to measure the forces applied to the trunk during spinal mobilization. BACKGROUND: Due to limitations in equipment design and data analysis, previous related studies demonstrate equivocal results. METHODS: The system was used to collect data from a sample of 30 experienced therapists to evaluate variation, repeatability and reproducibility during the application of five mobilization procedures. RESULTS: The variation in forces used by different therapists when performing the same technique was substantial, ranging between 63 and 347 N for one technique. During this procedure, 30% of the therapists were found to be relatively consistent, repeating the magnitude of the force applied at the first session within 5%. Others demonstrated considerable variation, exhibiting a difference as great as 34%. CONCLUSIONS: The inconsistency between experienced therapists has considerable implications for clinical practice. Changes in the magnitude and rate of loading are likely to have different effects due to the inherent viscoelastic behaviour of soft tissues. RELEVANCE: Spinal mobilization and manipulation techniques are frequently used by manual therapists in the treatment of musculoskeletal disorders. Despite the reliance on these techniques in clinical practice, there is little scientific evidence to substantiate their use. Before progress in this area can be made, it is necessary to characterize the forces used during typical mobilization procedures. The results can be used to develop teaching strategies and as a basis for comparative research on the efficacy of these techniques.