Modifica di Clinic Electromyography (sezione)

====Interference Pattern (IP) Analysis====
The development of muscle contraction is physiologically correlated by two mechanisms: a) recruitment and b) frequency modulation. At minimal voluntary effort, a single motor unit is activated at a frequency of 5-8 Hz. As voluntary force increases, the discharge frequency increases in parallel. When this reaches 8-12 Hz, a new motor unit is recruited at its minimum discharge frequency. As contraction force increases further, both units will further increase their discharge frequency until a third motor unit is recruited, and so on.

The first recruited motor units are small and fatigue-resistant, while those recruited at maximal force are larger, consist of more muscle fibers, and are more prone to fatigue. Some motor units, predominantly type II, are preferentially activated during brief periods of maximal effort. The recruitment order of motor units based on their size is called the "size principle".<ref name="Henneman" />

When more motor units are activated, the EMG signal becomes so complex that the components of the MUPs can no longer be visually identified. This situation is called an interference pattern. It should be noted that the amplitude of the MUP is primarily determined by the muscle fibers closest to the recording electrode, meaning that MUP amplitude is mainly dependent on the electrode’s position and does not reflect the true anatomical cross-section of the MUP. Larger motor units may have larger fibers, which in turn produce larger MUPs and greater interference pattern amplitude. This is the main reason why the IP signal amplitude increases as contraction force increases.

Large amplitude spikes recorded in the IP during maximal effort contraction may also be the result of summation of two individual MUPs, though this happens in a very low percentage (1-10%) of the largest IP spikes. An important factor contributing to increased amplitude due to summation is the increased density of muscle fibers within the motor unit. This mechanism is particularly influential in cases of reinnervation and is indicative of the sprouting mechanism.