We have developed three new features of the electromyographic interference pattern (IP), based on the turns and amplitude of the signal, to quantitate some of the features of the IP that are usually assessed subjectively by an electromyographer. The activity measures the fullness of the IP. The upper centile amplitude (UCA) defines the upper limit of the maximum peak-to-peak amplitude of the motor unit action potentials (MUAPs) contained in the IP. The number of small segments (NSS) measures the complexity of the IP, which is a reflection of the polyphasicity of the component MUAPs. The activity and the logarithm of the UCA correlate strongly with the force of muscle contraction at which the IP is measured. The NSS initially increases with the force of contraction and becomes relatively constant at higher force levels. The normal values of these features and the interpretation of their relationships are described in companion papers.