Medial gastrocnemius myoelectric control of a robotic ankle exoskeleton

Abstract

A previous study from our laboratory showed that when soleus electromyography was used to control the amount of plantar flexion assistance from a robotic ankle exoskeleton, subjects significantly reduced their soleus activity to quickly return to normal gait kinematics. We speculated that subjects were primarily responding to the local mechanical assistance of the exoskeleton rather than directly attempting to reduce exoskeleton mechanical power via decreases in soleus activity. To test this observation we studied ten healthy subjects walking on a treadmill at 1.25 m/s while wearing a robotic exoskeleton proportionally controlled by medial gastrocnemius activation. We hypothesized that subjects would primarily decrease soleus activity due to its synergistic mechanics with the exoskeleton. Subjects decreased medial gastrocnemius recruitment by 12% ( p < 0.05 ) but decreased soleus recruitment by 27% ( p < 0.05). In agreement with our hypothesis, the primary reduction in muscle activity was not for the control muscle (medial gastrocnemius) but for the anatomical synergist to the exoskeleton (soleus). These findings indicate that anatomical morphology needs to be considered carefully when designing software and hardware for robotic exoskeletons.

Fig. 1

Custom fit robotic ankle exoskeleton experimental setup and control algorithm. The exoskeletons were powered with an artificial plantar flexor that expanded and shortened when filled with air, proportional to medial gastrocnemius EMG.

Fig. 2

Ankle, knee, and hip angle kinematic patterns averaged for all subjects (n=10) across the gait cycle. Baseline data (dark grey line), powered minute 1 (black line), and powered minute 30 (grey line) are given for day 1 and day 2.

Fig. 3

Ankle joint Pearson product moment correlation values (r²) for each minute of walking with respect to baseline. Mean (black dots) +/− SD (grey area) of all subjects (n=10). The horizontal black lines show +/− 2 standard deviations of group mean data from the last 15 min of day 2, representing steady state dynamics. These steady state envelopes are calculated from group mean data and are for display purposes only. Individual subject analyses were used for statistical tests.

Fig. 4

Lower leg EMG linear envelopes for day 1 and day 2 (Butterworth low-pass filter with zero lag and cutoff of 10 Hz. Tibialis anterior (TA), lateral gastrocnemius (LG), medial hamstring (MH), vastus medialis (VM) and vastus lateralis (VL), are averaged for the entire gait cycle across subjects. Due to missing data there were between 8 and 10 subjects per muscle group (n=10, 9, 9, 8, 8, 9 respectively.

Fig. 5

(a) Medial gastrocnemius EMG linear envelopes for day 1 and day 2 (Butterworth low-pass filter with zero lag and cutoff of 10 Hz) averaged across all subjects (n=10). Vertical black lines indicate toe-off. (b) Medial gastrocnemius root mean squared data averaged (black dots) for all subjects (grey area is +/− 1 SD) for each minute of walking with the exoskeleton. The horizontal black lines show +/− 2 standard deviations of group mean data from the last 15 min of day 2, representing steady state dynamics. These steady state envelopes are calculated from group mean data and are for display purposes only. Individual subject analyses were used for statistical tests.

Fig. 6

(a) Exoskeleton plantar flexor torque from day 1 (black line) and day 2 (grey line). (b) Plantar flexion torque produced at the ankle during overground walking without wearing an exoskeleton. (c) Averaged exoskeleton power produced at the ankle from day 1 (black line) and day 2 (grey line). (d) Ankle power produced at the ankle during overground walking without wearing an exoskeleton. The bold lines represent averaged data from 9 subjects, and thin lines represent +1 SD.

Fig. 7

(a) Soleus EMG linear envelopes for day 1 and day 2 (Butterworth low-pass filter with zero lag and cutoff of 10 Hz) averaged across 8 subjects. Vertical black lines indicate toe-off. (b) Soleus root mean squared data averaged (black dots) across 8 subjects (grey area is +/− 1 SD) for each minute of walking with the exoskeleton. The horizontal black lines show +/− 2 standard deviations of group mean data from the last 15 min of day 2, representing steady state dynamics. These steady state envelopes are calculated from group mean data and are for display purposes only. Individual subject analyses were used for statistical tests.

Figure 8

Adaptation times for day 1 (black) and day 2 (grey) for exoskeleton positive and negative work, medial gastrocnemius and soleus EMG root mean square, and ankle correlation common variances. Data represent averages across subjects +1 SD.

Figure 9

Positive and negative exoskeleton mechanical work (J/kg) averaged across 9 subjects (black dots) +/− 1 SD (grey area). Positive mechanical work stays constant over day 1 and day 2 while there is a substantial decrease in exoskeleton negative work on both day 1 and day 2. The horizontal black lines show +/− 2 standard deviations of group mean data from the last 15 min of day 2, representing steady state dynamics. These steady state envelopes are calculated from group mean data and are for display purposes only. Individual subject analyses were used for statistical tests.