222

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6 Applications and Methods in Biosignal Processing

time t / s

f / Hz

U / mV

power density / dB/Hz

Fig. 6.7: One-channel EMG sequence from biceps over a period of 2 s (left) and power density spec-

trum (right).

motor units. However, even in a one-channel monitoring via two surface electrodes,

information can be derived about the physiology of the underlying muscle group. A

thorough summary on electromyography can be found in [50].

Figure 6.7 shows a one-channel EMG from the biceps of the upper arm measured

with skin electrode. The temporal signal pattern results from stimulation of multiple

muscle cells. Unlike EMG measurements with needle electrode, the bioelectrical pro-

cesses of individual muscle fibers cannot be investigated here. Rather, the signal car-

ries more global information about the stimulated muscle unit. Thus, the signal amp-

litude changes with the strength of the tension. Or, with sustained tension, the meta-

bolic exhaustion of the muscle unit can be observed by a shift in the frequency posi-

tion of the spectral center of gravity toward lower frequency. Overall, the EMG signal

is composed of the sum of the action potentials of all muscle fibers involved and ac-

cessible via the electrodes, with muscle fibers remote from the electrodes contributing

less to the overall signal strength (cf. Figure 6.6). In skeletal muscles, the action po-

tential lasts only approx. 4 ms. To maintain a force, the action potential of the fibers

is triggered repeatedly at a frequency between 8 Hz and 35 Hz.

6.2.1 Spectral Analysis of the One-Channel EMG

As already mentioned, the progressive exhaustion of a muscle unit as a result of sus-

tained contraction is reflected in the fact, that the action potentials are triggered less

frequently. Therefore, the frequency spectrum of a late EMG sequence, when meta-

7 MU: motor unit