**4.3. The F-wave**

cord after injury or in neuropathologies [40]. Perhaps, the main concern about the use of this technique in scientific research is to maintain the mechanical stimulus consistent throughout the experiment. Several investigators have used an instrumented hammer [41, 42] designed to apply a somewhat controlled mechanical percussion to the tendon. An alternative approach is to use a powerful electromechanical shaker to achieve tendon mechanical stimulation [43]. The tip of the shaker is lightly pressed against the Achilles tendon to ensure reasonable stimulus reproducibility. The shaker can be controlled via software that provides the desired input waveform shape, amplitude and duration (e.g., a sinusoidal cycle with 10ms duration and excursion of ~3mm). An inbuilt accelerometer is a reliable alternative to provide a feedback

**Figure 13. a)** Schematic of the experimental setup for testing the presynaptic inhibition pathway. The test stimulus was either electrical (Stim2) or mechanical (indicated by the shaker in contact with the SO tendon). The interval be‐ tween conditioning (Stim 1) and test stimuli (CT interval) was 100ms. SO afferent activation is shown in the dashed circle. For the H-reflex a single action potential is generated per Ia fiber, whereas the tendon stimulus evokes a burst of firing in Ia afferents. This difference might be responsible for the lesser sensitivity of the T-reflex to PSI as compared to the H-reflex (see text for details). **b)** Upper panel: Averaged H-reflex waveforms obtained in the SO muscle under different conditionings as compared to the control or no-conditioning case (in dark gray). The trace in red shows an Hreflex conditioned by a 1ms stimulus to the CPN to induce PSI on the afferent terminals of the SO muscle. The green trace represents the H-reflex under homosynaptic depression, HD (stimulus applied to the PTN at 1Hz). When the test and conditioned responses were obtained with interval of 1s (conditioned and test stimuli applied at 1Hz) an addi‐ tional inhibition was observed (blue trace; HD+PSI). The vertical arrow shows the instant of stimulus delivery. Lower panel: the same as for upper panel showing the T-reflex. Note the longer delay as compared to the H-reflex (indicated by a horizontal dashed line). Results similar to those observed in H-reflex were attained for condition PSI+HD. Data

The main difference between both techniques (H and T reflexes) is that in the case of the Hreflex the stimulus bypasses the muscle spindles (it is applied directly to the nerve, see Figure 6). To generate the T-reflex the stimulus is applied distally, on the tendon of the muscle of

based on [36], but figures are unpublished.

from the shaker tip excursion and monitoring stimulus consistency [3, 43].

72 Electrodiagnosis in New Frontiers of Clinical Research

F-waves are recorded routinely in clinical neurophysiological practice [47]. The F wave is a late response that occurs in a muscle following stimulation of its motor nerve, evoked by antidromic activation ("backfiring") of a fraction of the MNs. Typically, F-waves are evoked in response to a strong electrical stimulus (supramaximal stimulation) applied to a peripheral nerve. Action potentials traveling orthodromically reach the muscle fiber, thereby eliciting a strong M-response (MMAX). The action potentials traveling antidromically (see arrows in Figure 6) reach the cell bodies of the MNs making a small fraction of them to fire. This causes orthodromic action potentials to travel back towards the muscle, generating a relatively small amplitude CMAP called F-wave. Several measurements can be done on the F responses, including peak-to-peak amplitude, duration, latency (period between stimulation and F-wave response), and persistency (number of F-waves obtained per number of stimulations). Most electrophysiologists agree that F-wave latency constitutes a valuable parameter that reflects conduction properties of motor axons, being even more reliable than distal motor conduction measurements used to detect mild or early generalized abnormalities [48]. Although the use of F waves for assessing MN excitability is controversial [49], they are sensitive to changes in MN excitability [48] and have been used to assess it in a variety of protocols [50, 51]. In contrast to the H-reflex, which is influenced by presynaptic effects (PSI and HD), the F response is not a reflex (is not elicited by Ia volley), hence its generation is related solely to the MN membrane potential, which depends on the EPSPs and IPSPs the MN is receiving. Figure 14 shows F-wave recordings from the SO muscle (in response to supramaximal stimulation to the PTN) obtained in a subject at rest.

In the next section, we will present simulation results regarding the mechanisms behind the

Experimental and Simulated EMG Responses in the Study of the Human Spinal Cord

http://dx.doi.org/10.5772/54870

75

In this section, we will present simulation results that are valuable to better understand some mechanisms underlying the conditioning of muscle activity discussed previously in this chapter. The simulations were carried out in a multi-scale web-based neuromuscular simulator (dubbed ReMoto) that is freely accessible at http://remoto.leb.usp.br. A complete description of the simulator may be found elsewhere [4, 5]. Briefly, the simulator provides a detailed modeling of four spinal motor nuclei that command leg muscles responsible for ankle extension (SO; medial gastrocnemius - MG; lateral gastrocnemius - LG) and ankle flexion (TA). Each nucleus encompasses a MN pool and spinal INs mediating recurrent inhibition (by means of Renshaw cells), RI (by means of inhibitory Ia INs that receive inputs from antagonist muscles), and Ib inhibition. Individual spinal neurons are modeled following biophysical data from both cat MNs and INs, including active ionic channels responsible for the genesis of action potentials (sodium and fast potassium) and afterhyperpolarization (slow potassium). MN dendrites have an L-type calcium channel yielding a persistent inward current that is activated by the presence of neuromodulators in the spinal cord [57]. Ia and Ib afferents are present in ReMoto so as to allow studies on spinal reflexes (e.g., H-reflex) generated by electrical stimulation applied to a nerve (PTN for SO, LG and MG; CPN for TA). Model parameter values (e.g., axon conduction velocity, ionic channel time constants, maximum synaptic conductan‐ ces) and default numbers of elements (i.e. spinal neurons and afferents) are based on experi‐ mental data from cats or humans. Some of the parameter values were adjusted so that the dynamic behavior of an individual model matches those experimentally observed in cats or humans, for example, MN frequency-current (*f-I*) curves, post-synaptic potentials time course,

The MN pool drives muscle units, which generate both electrical (MUAPs) and mechanical activity (force twitches). For each muscle, one output is the EMG, expressed as the sum of all MUAPs, and the other output is force, being the sum of the twitches of all muscle units. Muscle twitches are modeled as the impulse responses of second-order critically-damped systems [58]. MUAPs occurring at the muscle surface are modeled by first- and second-order Hermite-Rodriguez functions [59], which are randomly attributed to each MU. MUAP amplitude and durations are chosen to match intramuscular MUAPs recorded from humans. To model the MUAP recorded by bipolar surface electrodes at the muscle's surface, each intramuscular signal is re-scaled depending on the MU positioning within the muscle cross-section [60], thus representing the spatio-temporal filtering due to the volume conductor (see section 2.2). A white Gaussian noise is added to the resultant surface EMG and this signal is band-pass filtered

genesis of the V-wave.

**5. Results from simulations**

and IN discharge patterns.

to mimic a real EMG signal recorded in experiments.

**5.1. General description of the simulator**

**Figure 14.** Nine superimposed EMG signals from the SO muscle showing stimulus artifacts, M-waves (MMAX) and Fwaves obtained in response to supramaximal stimulation (rectangular pulses with 0.2ms duration) delivered to the PTN of a resting subject (unpublished data). Surface stimulating electrodes were positioned with the cathode (2cm2) on the popliteal fossa and the anode (8cm2) on the patella. The stimulus intensity used to elicit F-waves was above that necessary to elicit MMAX. The same recordings are shown in **a** and **b**, with different amplitude gains (note the cali‐ bration bars).

#### **4.4. The V-wave**

As described in section 4.1, when a supramaximal stimulus is delivered to the nerve of a relaxed muscle, an M-wave is observed in the EMG with short latency and no H-reflex is observed due to the collision (see Figure 8) between antidromic and orthodromic spikes (there could be Fwaves, but they are not our focus here). However, if the subject maintains a steady voluntary contraction, and the same supramaximal stimulus is delivered to the peripheral nerve, a reflex response appears at a latency equal to the H-reflex. This reflex response, frequently referred to as a V-wave (associated with a voluntary drive), is an electrophysiological variant of the Hreflex and is used to measure the level of efferent drive [52-54].

The rationale behind the genesis of this response is that the descending drive activates a subset of MNs in the spinal cord making their axons conduct action potentials orthodromically. These action potentials collide with the antidromic volley generated at the electrical stimulation site by the supramaximal stimulus applied to the peripheral mixed nerve. Thus, this subset of MNs (recruited by the descending command) will be susceptible to be activated by the reflex afferent volley generated by the supramaximal electrical stimulus. Hence, the V-wave amplitude roughly reflects the number of spinal MNs being activated by the volitional drive, as well as the excitability associated with the stretch reflex pathway (previously discussed in section 4.1).

This electrophysiological measure has been used in several human neurophysiology studies, for instance: (1) neuronal plasticity associated with resistance training in healthy subjects [52]; (2) short-term effects of neuromuscular electrical stimulation [55]; (3) multiple sclerosis [56]. In the next section, we will present simulation results regarding the mechanisms behind the genesis of the V-wave.
