Background: Motivation is critically important for rehabilitation, exercise, and motor performance, but its neural basis is poorly understood. Recent correlational research suggests that superior frontal gyrus medial area 9 (SFG9m) may be involved in motivation for walking activity. This study experimentally evaluated brain activity changes in periods of additional motivation during walking exercise, and tested how these brain activity changes relate to self-reported exercise motivation and walking speed. Methods: Non-disabled adults (N=26; 65% female; 25 ± 5 years old) performed a vigorous exercise experiment involving 20 trials of maximal speed overground walking. Half of the trials were randomized to include "extra motivation" stimuli (lap timer, tracked best lap time and verbal encouragement). Wearable near-infrared spectroscopy measured oxygenated hemoglobin responses (ΔHbO2) from frontal lobe regions, including the SFG9m, primary motor, dorsolateral prefrontal, anterior prefrontal, supplementary motor and dorsal premotor cortices. Results: Compared with standard trials, participants walked faster during "extra-motivation" trials (2.67 vs. 2.43 m/s; p<0.0001) and had higher ΔHbO2 in all tested brain regions. This extra motivation effect on ΔHbO2 was greatest for SFG9m (+703 uM) compared with other regions (+45 to +354 uM; p≤0.04). Greater SFG9m activity was correlated with more self-determined motivation for exercise and faster walking speed. Conclusions: Simple motivational stimuli during walking exercise seem to upregulate widespread brain regions, especially SFG9m. This could help explain the positive effects of motivational feedback on gait outcomes observed in prior rehabilitation research. Thus, these findings provide a potential biologic basis for the benefits of motivational stimuli, elicited with clinically-feasible methods during walking exercise. Future clinical studies could build on this information to develop prognostic biomarkers and test novel brain stimulation targets for enhancing exercise motivation (e.g. SFG9m).