Figure 1.
An EMG-based human-computer interface used for virtual balancing.
EMGs derived from TA and MG muscles of subjects were fed into a DSP, which were transformed into active torques for balancing a virtual inverted pendulum in the DSP. A vertically moving horizontal bar representing the tilt angle of the pendulum, as a visual feedback information for subjects, was displayed on an oscilloscope screen in front of subjects. EMG signals, force-platform-based CoP motion of subjects and tilt angle of the pendulum were recorded by a PC.
Table 1.
Parameter values of the virtual inverted pendulum.
Figure 2.
Subject-wise number of failure events within each trial.
Each subject was classified into either continuous, one-sided intermittent, or two-sided intermittent type. Blue, red, green curves are for the two-sided intermittent, one-sided intermittent, and the continuous types, respectively. Two subjects could not achieve three success trials within the limited forty trials (black curves).
Figure 3.
Typical data of failed and successful trials.
In each panel, top, second, third and bottom traces show, respectively, tilt angle of the virtual inverted pendulum , CoP
of subjects, active torque
and active torque
. A and D: Failed (#1) and successful (#14) trials in Subject 4 (continuous type). B and E: Failed (#1) and successful (#27) trials in Subject 2 (one-sided intermittent type). C and F: Failed (#1) and successful (#29) trials in Subject 9 (two-sided intermittent type).
Table 2.
Summary of performance of all subjects.
Figure 4.
Sway trajectories of the pendulum in the -
phase plane.
Configuration and data used for panels A to F were the same as those in Fig. 3. Each trajectory was segmented into four different groups based on the associated combinations of activations in TA and MG muscles. Back curve segments: Both TA and MG were inactivated (Inactive). Blue segments: TA was inactivated but MG was activated (MG-active). Red segments: TA was activated but MG was inactivated (TA-active). Green segments: Both TA and MG were co-activated (Co-active). Dashed and dotted lines represent the stable and the unstable manifolds of the saddle point defined by Eqs. 4 and 5, respectively, for the virtual pendulum with no active torque.
Figure 5.
Average durations of inactive, TA-active, MG-active and co-active intervals.
See caption of Fig. 4 for definitions of four groups of intervals. Each duration was averaged separately over the first five trials (F5) and the successful trials (S) for each subject. Black bars: Inactive durations. Dark gray bars: MG-active durations. Gray bars: TA-active durations. White bars: Co-active durations.
Figure 6.
Time-courses and distribution histograms of Direction Cosine (DC) during successful trials.
A. Trial 14 of Subject 4 (continuous type). B. Trial 27 of Subject 2 (one-sided intermittent type). C. Trial 29 of Subject 9 (two-sided intermittent type).
Figure 7.
Multiple indices characterizing sway of the virtual inverted pendulum for each subject.
In each panel, values of five indices are plotted as thick black bars. From the left-most index, they represent the ratio of inactive interval to the whole trial time span (Inactive-Ratio) denoted as InactR, the ratio of time interval with DC to the whole single trial time span (DC-Ratio) denoted as DCR, the ratio of the number of off-points (an off-point is a state point at a time instant when an inactive interval began) located in specific areas of the 2nd and the 4th quadrants of the phase plane to the total number of off-points (Q-Ratio) denoted as QR, the average of distance between off-points and the
-axis in the phase plane denoted as Dis (15 times of Dis-value for scaling: 15Dis), and the average standard deviation of
-coordinates of off-points denoted as SD (15 times of SD for scaling: 15SD). See text for detailed definitions of indices. Thick black bars are for average values of the indices for successful trials. Thin gray bars are the corresponding values for F5 trials. See text for the meaning of configuration of panels for nine subjects.
Table 3.
Changes in average values of the five indices from F5 trials to successful trials through learning for each subject and for average of each type.
Figure 8.
Distributions of off-points in failed and successful trials.
An off-point was defined as a state point at a time instant when an inactive interval began. Configuration and data used for panels A to F were the same as those in Figs. 3 and 4. Q-Ratio (Quadrant-Ratio) used to characterize the timing strategy of subjects counted the number of off-points located in the gray triangular areas defined in the second and the fourth quadrants.
Figure 9.
Power spectral density (PSD) functions of sway dynamics () of the pendulum during successful trials.
A: Trial 14 of Subject 4 (continuous type). B: Trial 27 of Subject 2 (one-sided intermittent type). C: Trial 29 of Subject 9 (two-sided intermittent type).
Table 4.
Slopes of PSD at LF and HF bands for successful trials and power consumption [W] for F5 and successful trials for each subject and for average of each type.