EMG is being successfully used for other MIS procedures [35] and is the most commonly used method for motor monitoring during MI-LIF [6, 35–45]. EMG is advantageous for its ease of use and interpretation, especially during MI-LIF where nerves lie near the field of view and manipulation. As a test for the proximity of a nerve, it has a high sensitivity and low specificity, making it ideal for screening for nearby nerves [25]. Selecting the appropriate muscles for monitoring depends on the surgeon’s ability to access a specific myotome with a percutaneous needle, knowledge of anatomical safe zones during MI-LIF, and which specific nerves are at risk based on surgical anatomy [7–9].

For approaches to the lumbar spine, the nerve roots and distal nerves from T12 to L5 are at risk; it is important to note that even though EMG is normally used for monitoring motor nerves, spontaneous EMG can also be used with triggered EMG to detect antidromic stimulation of sensory nerves [45]. With these two thoughts in mind, Table 10.1 lists the specific nerves, the nerve roots at risk, and the recommended muscles or dermatomes for distal monitoring [43, 45–47].

Free-running EMGs are active whenever a nerve root is compressed, irritated, or injured, which causes spontaneous firing of the nerve and activation of the corresponding myotome. Each morphologic EMG activity reading corresponds to a specific neurotonic discharge: spikes represent individual discharges, bursts represent brief bundles of discharges, train activity represents persistent regularly repeated discharge patterns, and neurotonic discharges represent persistent prolonged bursting. A single burst of activity that occurs during surgical manipulations identifies and localizes a specific nerve and does not usually cause any deficit. However, activity that is sustained over 2 s is alarming and should prompt the surgeon to stop whatever specific procedure is causing the sustained activity.

Triggered EMG allows the surgeon to probe specific tissues that are suspicious for containing a nerve. The stimulator tip is a bipolar probe, which sends current into the area being probed to potentially activate any local nerves. EMG thresholds less than 5 mA indicate direct contact, and 5–10 mA indicates close proximity [43, 45, 48]. If there is a myogenic response, the surgeon knows to avoid that specific area.

Both free-running and stimulated EMGs should be used in tandem during the dissection of an MI-LIF. Great care should be taken during dissection through the psoas muscle as well as during dilation, as these procedures involve blind manipulation of surrounding structures that may cause nerve injury. Particularly during these times, free-running EMG should be activated and triggered EMG used frequently to localize potential nerves. EMGs with discrete thresholds and directional orientations are the most ideal, as they provide proximity and location of the nerve to the surgeon’s working space [45, 47, 48].

Mechanomyography (MMG) is a relatively new intraoperative monitoring technology. The basis for MMG monitoring is the detection of the mechanical oscillations on the muscle surface, which occur during muscle contraction, and it is the complementary mechanical signal to the electrical activity detected during EMG [63, 64]. It is primarily used for the identification of muscle fiber typing, assessment of muscle force, evaluation of muscle fatigue, characterization of the muscle resonance frequency, and assessment of muscle contractile properties [63, 65]. Its application to MI-LIF surgery is similar to that of triggered EMG, where the surgeon uses a probe to stimulate areas of interest; if a nearby nerve is stimulated, the MMG sensor will detect the mechanical signal of muscle contraction, alerting the surgeon of a potentially dangerous area. MMG has a few advantages over EMG. First, MMG sensors are noninvasive stickers, and their placement for detection of signal does not need to be precise nor specific [66]. Second, MMG is a mechanical signal, and so it is not affected by changes in skin impedance, which may occur when the skin is wet (sweat, normal saline, etc.) [67].