The study is a crossover randomised controlled trial. Alcohol or cocaine dependent participants will be recruited from inpatient and outpatient psychiatric treatment centres, on the approval of their treating physician. A healthy control group will be recruited using online advertising. All participants will undergo each of three conditions in a randomised order; 1) 20 minutes of cycle ergometry at 50-60% of maximum heart rate; 2) 20 minutes of exercise at 70-80% of maximum heart rate; 3) 20 minutes of quiet reading. Immediately before and after each condition, participants will be asked to complete a computerised Stroop test, watch a film containing substance-related images, and self-report craving levels. During the Stroop test and film viewing, participants' neural activity will be measured via functional near-infrared spectroscopy

The aim of this study is to examine whether, following acute exercise bouts at moderate and high intensities, reduced craving and increased inhibitory control can be observed in alcohol or cocaine dependent individuals. As the focus of this study is the potential of exercise to be integrated into treatment, individuals in treatment for their dependence, who have been abstinent for a short period, will be recruited.

Inclusion Criteria:

For alcohol or cocaine dependent participants:

• meeting three or more of the DSM-IV criteria for substance dependence (as determined by treating physician);
• minimum of 20 days and maximum of 40 days abstinence from substance of dependence;
• able to understand and complete the informed consent;
• able to travel to study site independently, as confirmed by the treating psychiatrist;
• assessed for physical and mental fitness by treating physician and cleared to take part in the study;
• fewer than 3 hours of physical exercise or sport per week;

For healthy controls:

• fewer than 3 hours of physical exercise or sport per week;
• no history of problematic alcohol or illicit drug consumption.

Exclusion Criteria:

• Pregnancy
• Questionable findings during resting ECG screen