Electromyography

EMG signal amplitude and muslce force production  is best studied during isometric contractions, that is, during contractions where the joint angle is maintained fixed so that the length of the muscle does not change. Isometric contractions are better suited to relate EMG amplitude and muscle force because some confounding factors are minimized, such as the changes in the EMG amplitude as the relative position between the sensors and the muscle fibres changes during anisometric contractions. A study by Lawrence and De Luca (1983) investigated the relation between EMG and force for different muscles (biceps, deltoid and first dorsal interosseous), rate of force production, and training level of the subjects (pianists, long-distance swimmers, power lifters and normal subjects). Only the muscle type influenced the relation between EMG signal and muscle force, with the smaller muscle (first dorsal interosseous) showing an almost linear relation with the force, that was instead non-linear for bigger muscles.

The difference among muscles is due to several factors that affect the relation between EMG and force: the motor unit recruitment and firing rate properties; the relative amount and location of slow-twitch and fast-twitch fibres with respect to the recording electrodes, the crosstalk contamination from other muscles; the muscle viscoelastic properties; and the force contributions from agonist and antagonist muscles. This last factor is particularly important because a single muscle may not be solely responsible for producing a moment around a joint. Additionally, there is often a high degree of variation between subjects, not only between different muscles in the same subject but also in how the surface EMG signal translates to muscle force.

For anisometric (dynamic) contractions, the relation between EMG amplitude and muscle force is not linear. When the muscle contracts, the length of the muscle shortens, the joint angle decreases, and the moment arm of the muscle (the distance from the muscle to the centre of rotation of the joint) increases. If we apply a constant force, as the moment arm increases, the force produced by the lengthening muscle must decrease, resulting in a non-linear EMG—force relation. In addition, two other factors contribute to the non-linearity between the force output and the EMG signal amplitude. The first factor is the change in the relative position of the source of the EMG signal and the sensor which remains attached to the skin as the muscle moves below the skin. The second factor is the non-linear relationship between the force produced by the muscle and the varying length of the contracting muscle fibres.