EMG Test Results
Visible bulging and palpable hardening of the muscles are not reliable indicators of their use. The adoption of posture, execution of movement and the exertion of force imply the use of muscles but do not imply a unique set of patterns of muscle activity. Active muscle contributes to energy consumption, but the contributions do not reveal themselves until some time afterwards.
Rapidly fluctuating differences of potential within the tissues and across the skin are created when muscles are activated by signals from their nerves; a record of the potential changes between two electrodes placed on the skin surface or embedded in the tissues is called an electromyogram (EMG). Inspection of an EMG test can be linked to viewing the acknowledgement of instructions received by the muscle motor through a window with a restricted view. The ergonomist´s interest in the musculature lies in whether muscles are being used economically and skillfully, and whether avoidable or unacceptable local stress and fatigue are present. Such decisions require value judgements to which the EMG lends support. The EMG test is valuable as a qualitative tool, but must be treated with caution when used quantitatively.
The EMG reflects local muscular activity in the region of the recording electrodes, provided that some anatomical knowledge guided the choice of muscular regions probably involved in a performance. It is a common and serious mistake to equate mechanical with electrical activity. The former is reflected in the EMG as in a distorting mirror whose shape may alter considerably as conditions change.
Registration and quantification of the EMG are achieved in many ways. Full wave rectification followed by smoothing or integration is most common, but voltage excursions and turning points in the waveform, and power spectral analysis have all been used for purposes of description. The intriguing feature is that if the EMG is recorded from part of the musculature which is clearly involved in a movement or an exertion, a close relationship will be found between any measures of the EMG and any measure of the performance providing that the experimental conditions are rigidly controlled. Thus the EMG will relate to the force exerted if the posture does no alter, and to the amplitude or speed of movement if the loading remians the same. Unfortunately, the introduction of another variable, including shifting and electrodes, fatigue, change of limb temperature, change of subject and even subtle changes in the use of muscle groups is likely to alter the quantitative relationship. The mechanical interpretation of an EMG is therefore often problematical and ambiguous as will now be illustrated.
The maximum torque is always, and the maximum intensity of EMG is usually, altered by the posture of the hand. We would expect the former because of the length tension characteristics of muscles responsible for supination. The reasons for the latter are probably associated with the need to send more frequent signals to a muscle in its shortened condition than when its length is greater, in order to maintain full activity.