Twenty-one subjects with chronic back pain (CBP) participated in an ambulatory electromyography (EMG) monitoring study to ascertain the relationships between muscle activity, physical activity, psychosocial stress, and pain. A time-series analysis approach was adopted to investigate both immediate and lagged associations between these variables in an attempt to determine potential causal relationships. Results for group relationships showed a significant relationship between physical activity and pain, self-report of stress and pain, but no relationship between EMG activity and pain. A lagged relationship between physical activity and pain was found, suggesting a causal relationship between physical activity and pain. However, no time lag was observed between stress and pain, hence no causal relationship can be elucidated. Analysis at the individual level indicated stronger relationships between several combinations of these variables, highlighting the need to consider the heterogeneity of the CBP population and etiology of CBP. The use of ambulatory monitoring of pain, stress, and EMG is suggested as one avenue to further explore the population's heterogeneity.