Low back pain (LBP) is a major health problem globally, but approaches to prevention are not yet clearly identified because modifiable risk factors are not well established. Although physical activity is one promising modifiable risk factor, it is still not known what types and doses of physical activity are protective or harmful for LBP. The aim of this study is to establish the feasibility of a definitive cohort study that will investigate the effects of different types and doses of physical activity on the risk of developing recurrent LBP while accounting for genetic factors. This will be a pilot longitudinal twin study and twins will be recruited from the Australian Twin Registry, and will be followed over 1 year. Thirty adult complete twin pairs with a history of LBP, but symptom free at recruitment, will be included. Data on physical activity (predictor) will be collected using four questionnaires and an objective measure (accelerometer) at baseline and at 6-month follow-up. Twins will also complete an additional physical activity questionnaire monthly. Data on LBP (outcome) will be collected at baseline and weekly. Data will be collected using short message service (SMS) and email. We will keep records of the recruitment rate, follow-up rate, and completeness of data. Barriers to completing the study will be investigated. The results of this study will inform the design and implementation of a future definitive study, which will help to clarify the effects of different types and doses of physical activity on the risk of developing recurrent LBP.Keywords: low back pain, physical activity, twin study, longitudinal study, risk factorsThe latest Global Burden of Diseases Study ranked low back pain (LBP) as the highest contributor to disability globally, expressed as years of life lived with disability (YLD; Global Burden of Disease Study, 2015). LBP is also a costly condition, with total treatment costs estimated at $4.8 billion a year in Australia (Arthritis and Osteoporosis Victoria, 2013). Implementing preventive strategies for LBP would be a logical method to reduce the burden of this condition on people's lives and on society.Although many preventive strategies for LBP have been proposed, most have failed to substantially reduce the incidence and recurrence rates of LBP (Steffens et al., 2016).A plausible explanation for the lack of effectiveness of these programs is our poor understanding of the causes of this condition, and therefore identifying strong modifiable risk factors is essential. Importantly, most previous studies in Protective and Harmful Effects of Physical Activity for Low back Pain influence of genetic factors, which have been shown to explain up to 67% of the underlying disposition to develop LBP (Ferreira et al., 2013;Nielsen et al., 2012).A recent review has found moderate quality evidence that exercise in combination with education decreases the risk of LBP by half (Steffens et al., 2016). However, given the limited number of trials identified in the literature, the specific effects o...