It is widely believed that activity in the primary motor cortex relates only to motor execution. However, the extent to which similar activity occurs when imagining motor movements remains to be determined and, while some researchers report activity in the primary motor cortex during both motor execution and motor imagery tasks (e.g.Solodkin et al., 2004; Sharma et al., 2008), others report no effects of motor imagery (e.g., Binkofski et al., 2000; Hanakawa et al., 2003; Hétu et al., 2013). It is still unknown whether brain activation patterns of motor execution and motor imagery are similar, and whether both tasks activate the primary motor cortex. In addition, it is also unclear about the effect of imagination intensity on the primary motor cortex (this effect has been well established in motor execution tasks). Accordingly, the present research investigated the relationship between the intensity of real and imagined exercise on cortical activity using functional near-infrared spectroscopy (fNIRS), especially in the primary motor cortex. A preliminary assessment used 10 participants (5 male, 5 female), who did not take part in the main experiment, to establish an appropriate level of exercise intensity. For the main experiment, 30 participants (15 male, 15 female) with high imagination ability were selected using the Motor Imagery Questionnaire (Revised). These participants performed a motor execution task in which they actually lifted dumbbells under two levels of exercise intensity (males, 4 pounds and 8 pounds; females, 2 pounds and 4 pounds) and an imagery version of this task in which they imagined lifting dumbbells of these weights. The fNIRS was used to measure cortical changes in oxygen level during the performance of the two tasks. Finally, on completion of the imagery task, the “motor imagery self-assessment questionnaire” was administered to assess the quality of the participants’ imagination. All participants reported that they could imagine dumbbell movement under different levels of exercise intensity. The fNIRS showed that although both motor imagery and motor execution produced increases in cortical oxygen levels, this was greater, and lateralized to the left hemisphere, during motor execution. In addition, effects of exercise intensity were observed during motor execution but not during motor imagery, such that oxygen levels were higher with increased exercise intensity. The indication from the present findings is that both motor execution and motor imagery activate the primary motor cortex. This challenges the conventional view that activity in the primary motor cortex relates only to motor execution and shows that this activity is also connected with motor imagery, plan, and control. The present findings therefore provide an important theoretical basis for the use of motor imagery therapy in the field of neural prosthetics. However, the present findings also reveal important differences in the effects of motor execution and motor imagery, and in particular that effects of exercise intensity on cortical activation may be observed only during motor execution and not motor imagery. However, further research will be required to more fully understand the nature of these differences in cortical activation.