Sedentary secular, domestic, and recreational behaviour is a major risk factor (RF) for cardiovascular disease (CVD). This study examines the quality of at-home physical activity (PA) and how it relates to physical fitness (PF) before and during 12 weeks of supervised cardiac rehabilitation (CR) in a group of medically referred cardiac patients. PA was measured with an accelerometer (ActiGraph wGT3X-BT), pre and post-CR, to determine if patient at-home PA behaviour changes during supervised CR. Cardiac patients’ (n=27), haemodynamic and morphological measurements were taken. Direct measurement of the volume of oxygen consumption (VO2peak) was done with respiratory gas analysis during a submaximal cycle ergometer test to determine PF. Analysis of covariance (ANCOVA) was used to assess whether exercise-induced improvement in cardiovascular and muscular capacity (CVaM-capacity) influences the relationships between stages of PF (pre vs. post-CR) and PA behaviour. Pre-CR power output and CVaM-capacity correlated moderately with overall at-home caloric expenditure per week (r=0.47 and 0.53). Calculated r2 values indicate that power output and peak oxygen consumption contribute between 22.1% and 28.1% to the variance of weekly PA energy consumption. At-home PA behaviour (volume and intensity) changed significantly (p≤0.001) after 12 weeks of supervised CR, with moderate and vigorous PA increasing, and sedentary, and light PA decreasing. Future CR research should consider how at-home PA behaviour and other RF inter-associations affect a patient’s cardiac health and CR effectiveness.