Biomedical Engineering Reference
In-Depth Information
FIGURE 10-51
The Utah Arm
showing the
positions of the
surface electrodes.
(Courtesy of Motion
Control.)
the biceps and triceps channels. On relaxation, the elbow locks and control reverts to the
terminal device.
An example of using myoelectric control to flex the elbow and then to extend the
elbow before flexing it again is shown in Figure 10-52. This is achieved by maintaining a
fairly constant contraction on the biceps and varying the contraction of the triceps.
As microcontroller and DSP chips have become less power hungry, new control
schemes based on complex pattern recognition have become possible. Research has shown
that the contribution of a given muscle within a group varies according to the intended
limb action. The sum of the contributions of all of the muscles within the group can be
used to identify actions.
Initially pattern-classification-based control systems were limited to simple-to-
calculate time-domain signal statistics such as variance, zero crossings, and waveform
length to represent the myoelectric signal of interest. Now, more computationally complex
feature sets are being investigated, including autocorrelation coefficients, spectral mea-
sures, time-series model parameters and time-frequency coefficients based on wavelet
and wavelet packet transforms, and higher-order spectral analysis (Hernandez-Arieta,
Dermitzakis et al., 2008; Parker, Englehart et al., 2004).
Problems arise with high-level limb amputees as more degrees of freedom must be
controlled with fewer available muscle sites. In addition, the remaining muscle sites are
not physiologically appropriate as they bear no natural relationship with the lost degrees
of freedom. No sophisticated pattern recognition can overcome this problem, and the
amputee needs to develop a contrived suite of contractions to control the prosthesis.
10.9.4.5 Targeted Muscle Reinnervation
After amputation, the residual nerves often retain the capability of transmitting and re-
ceiving messages but just do not go anywhere. Targeted muscle reinnervation (TMR) is a
