## Materials and Methods
### Temporal Synchronization Unit and Data Merging  
Since the recording systems (EEG and pressure plate) are independent devices and not coupled or synchronized during recording (see structural design in Figure 2), the time synchronization between the two devices has to be realized with a custom technical solution. As shown in Figure 2, the stimulus PC was used for the triggering of the recording systems. The stimulus PC runs the software Presentation(r) (Neurobehavioral Systems) which is capable of sending various trigger output signals via a series of possible ports. In the present study, the EEG system and stimulus PC were connected via an LPT port to avoid jittering of the marker signal, typically occurring with serial ports that exceed the necessary temporal precision for EEG recordings of less than 1-2 milliseconds (ms). The pressure plate and the IMU system are triggered directly by the stimulus PC. The devices receive start and stop markers from Presentation, the stimulation software. The trigger interfacing between the stimulus PC  
Figure 3: Structural design of the technical solution developed in this study. The solution includes the interfaces between the subsystems and recording devices. The stimulus PC can send triggers to the EEG, the pressure plate (treadmill), and the recording software of the inertial sensors (IMUs). Each system sends the recorded data to a corresponding PC.  
and the pressure plate (and, if included, the inertial sensor system) was realized by a custom-made microcontroller-based USB synchronization device. This device allows for synchronization of the recording devices with the existing set-up. Finally, the different recording files need to be written into one output file for data merging and data fusion. The EEG data file was used for data merging and all other data were written to and synchronized with this output file using MATLAB(r) routines.  
The reliability and validity of the results relies on the precision and accuracy with which the data are recorded by the different devices and software can be temporally synchronized and merged. To minimize the error in the time delay between all recorded datasets, several validation steps were taken into consideration to estimate the different time delays of the different interfaces and the custom-made USB communication device. This synchronization device sends the trigger signals from the stimulus PC to the treadmill at the same time the trigger signal is sent to the EEG system. The time delay was explored using the following method: The analog output of the USB synchronization device comprises an LED. The LED lights up every time an analog trigger signal is created. The LED, as well as the PC display, are simultaneously recorded with a camera. The accuracy of this method depends on the sampling rate of the camera, which in the present study comprised a frame rate of 120 Hz. The estimation and validation of the time delay for later data synchronization were investigated prior to the pilot study based on test signals. The tests were performed 10 times by triggering the devices in different frequencies. During all tests, the time delay between EEG and treadmill triggers was below \(\text{td1}=8.33\) ms, and therefore within one time frame of the treadmill (sampling frequency of 120 Hz). There was no jittering. Therefore, it is possible to correct for the time delay and align the detected gait events as onset markers for ERP analysis during the step of data merging and data fusion (see Section 2.6 for details).