Prolonged recording of bioelectrical activity of a muscle provides a qualitative and quantitative description of its activity over time, thus characterising its ‘behaviour’ during particular manoeuvres. It should be borne in mind that kinesiological EMG does not provide information on the ‘state’ of the muscle (i.e. whether its motore units have been changed due to neuropathy or myopathy). A special analysis of the EMG signal is necessary to provide that information. Meaningful kinesiologic EMG can, of course, only be obtained from innervated muscle.
When we are interested in the pattern of activity of an individual muscle, the EMG should ideally provide a selective recording, uncontaminated by neighboring muscles on one hand, and a faithful detection of any activity within the source muscle on the other hand. Both objectives are difficult to achieve simultaneously. Overall detection from the bulk of a muscle can only be achieved with non-selective electrodes, selective recordings from small muscles can only be made with intramuscular electrodes with small detection surfaces. Non-selective recordings carry the risk of contamination with activity from other muscles; selective recordings may fail to detect activity in all parts of the source muscle. Meaningful recordings from deep muscles can only be accomplished by invasive techniques. Considering the above, truly selective recording from sphincter muscles can probably only be obtained by intramuscular electrodes. In clinical routine the concentric needle electrode is used as a rule. Needle electrodes, however, may produce some pain on movement, and can be dislodged. Instead, two thin isolated bare tip wires (with a hook at the end) can be introduced into the muscle with a cannula, which is then withdrawn, and the wires stay in place.
The advantage of this type of recording is good positional stability and painlessness once the wires are inserted, though their position cannot be much adjusted. To make EMG recording less invasive various surface-type electrodes have been devised – also for special use in the perineum. Small skin-surface electrodes can be applied to the perineal skin. Other special intravaginal, intrarectal or catheter-mounted recording devices have been described. Recordings with surface electrodes are more artefact prone and furthermore the artefacts may be less easily identified. Critical online assessment of the ‘quality of the EMG signal’ is mandatory in kinesiological EMG, and this requires either auditory or oscilloscope monitoring of the raw signal. Integration of high-quality EMG signals by the software of modern recording systems may help
in quantification of results. It should be borne in mind hat kinesiological EMG needs some concomitant event markers to make it a valid indicator of muscle activity correlated with specifi c manoeuvres or other physiologic events (e.g. detrusor pressure).