More than that, they could put multiple weight sensors on the platform to make sure an adult is on it in order for it to start and continue operating. As an ad

More than that, they could put multiple weight sensors on the platform to make sure an adult is on it in order for it to start and continue operating. As an added benefit, it could use the signals from the sensors to detect roughly where the user is stepping, how hard they're hitting it, how frequently, etc. That would be useful for gait analysis and other fitness-related stuff. Additionally, a sensor on the motor could detect a sudden increase in power required to keep the belt spinning at a particular speed (such as something getting under it). This would trigger an emergency stop and possibly an audible alarm.