Electromyography

The benefit of a fully wireless device is that the subject is free to move with little or no restriction. The wireless EMG system transmission needs to be reliable to avoid loss of packets of data and to minimize interference from radio frequencies, Bluetooth, microwaves, etc. The wired systems require a physical connection, usually in the form of a cable that con-nects each EMG sensor to a separate amplifier unit.

The drawbacks of the wired systems are that they restrict the subjects movements and that the cables i need to be well shielded to avoid introduction of electrical noise in the recording. EMG signals are low voltage and need to be amplified, with values for the amplification typically between 1000 and 10 000 times. Amplifiers are often positioned close to or as part of the electrodes, in the case of active electrodes, reducing the length of the connecting wire that can pick up and introduce electrical interference. Most EMG systems are capable of measuring EMG signals at sampling frequencies of 1000 Hz or higher. This is the minimum sampling frequency required to correctly sample the EMG signal and analyse its fre-quency content, which is negligible above 450 Hz. For intramuscular EMG recordings, which have higher frequency content, a sampling rate of 2000 Hz is recommended. When individual motor unit action potentials are being investigated, up to 10 000 Hz or 20 000 Hz (10 kHz or 20 kHz) is generally needed.

For biomechanical applications, or any application involving dynamic contractions, the bandwidth provided by a surface EMG system should ideally be between 20 and 450 Hz. Surface EMG signals are known to have no or little energy at frequencies greater than 450 Hz. At frequencies lower than 20 Hz, the disturbances from noise and motion artefacts are prevalent and complicate the interpretation of the EMG data. This low-frequency cut-off is particularly relevant for biomechanists who work with dynamic con- be tractions where movement artefacts contribute energy components below the 20 Hz range. For more detailed te] information on this point refer to De Luca and col. (2011). The bandwidth should be greater for intramuscular EMG signals, which present considerable energy up to 1000Hz.