Electromyography

Within each muscle, motor units are hierarchically activated in order of increasing size, a property known as the Size Principle (Henneman, 1957). Smaller motor units, characterized by smaller-amplitude and longer-duration force twitches, are recruited at lower contraction forces  i.e. they have a lower recruitment threshold. Larger motor units, characterized by larger-amplitude and shorter-duration force twitches, are recruited at higher contraction forces, i.e. they have a higher recruitment threshold. When a motor unit is activated during a sustained contraction, it fires (contracts) repeatedly. The series of firing times can be represented with a train of bars over time, called the firing train, where each bar represents  one firing or MUAP of the motor unit  The number of pulses that occur over one second is a measure of the motor unit mean firing rate. The mean firing rate is a time-varying function calcu-lated over a unit-area window that slides through the motor unit firing train. The time interval between consecutive firings of a motor unit is called a interpulse interval (IPI) (see picture MU activation)

The force produced by a muscle is modulated by two main processes: 1) by varying the number of active MU, and 2) )varying the their firing rate. When greater amount contraction force is needed, a greater number of motor units are activated, which are progressively larger (higher threshold) and generate larger forces. The firing rates of the active motor units also increase with increasing force and, consequently, their average motor unit IPIs decrease.

These two phenomena translate in an increasing amplitude of the recorded EMG signal during increasing force contractions, due to the increasing number of active motor units and to the increase in their firing rate. In addition, note that the higher-threshold larger motor units are generally characterized by greater amplitude MUAPs in virtue of the greater number of muscle fibres per motor unit.

Over the past five decades, there has been a common acceptance of the notion that higher-threshold moto-neurons are characterized by greater firing rates than lower-threshold ones (Eccles et al., 1958; Kernell, 1965, 2003). This notion stems from the observation that, when the nerves of anaesthetized cats are electrically stimulated, the larger-diameter (higher-threshold) motoneurons exhibit a shorter after-hyperpolarization and greater firing rates than the smaller-diameter (lower-threshold) ones. This firing rate organization would match the motor unit contractile properties: lower-threshold motor units have wider and smaller amplitude force twitches than higher-threshold ones and require lower firing rates to produce tetanic force. This arrangement would conceivably 'optimize' the force-generating capacity of the muscle because motor units would fire at a rate that produces the greatest amount of force. However, we now know that, in humans performing voluntary contractions, motor unit firing rates are organized in an opposite manner: lower-threshold motor units have greater firing rates than higher-threshold ones. This firing rate arrangement is known as the Onion-Skin scheme (De Luca & Erim, 1994) and it demonstrates that the physiological firing behaviour of motor units when they are voluntarily activated in humans is dramatically different than that observed when isolated motoneurons are electrically stimulated in animals. One ostensive shortcoming of the Onion-Skin scheme is that the motor unit firing rates do not match the motor unit contractile properties, so that motor units do not always produce fused force and they do not always take full advantage of their force-generating capacity. Why is that? One reason is that the high-threshold motor units are fast-fatiguing and would not be able to sustain force production for a is long period if they were to fire fast. Thus, the Onion-Skin scheme is not designed to 'optimize' muscle force. Instead it provides a lower maximal force with the capacity to sustain it over longer time (De Luca & Erim, 1994, Contessa & De Luca, 2013).