Motor unit recruitment when neuromuscular electrical stimulation is applied over a nerve trunk compared with a muscle belly: quadriceps femoris

Abstract

<jats:p>Neuromuscular electrical stimulation (NMES) can be delivered over a nerve trunk or muscle belly and both can generate contractions through peripheral and central pathways. Generating contractions through peripheral pathways is associated with a nonphysiological motor unit recruitment order, which may limit the efficacy of NMES rehabilitation. Presently, we compared recruitment through peripheral and central pathways for contractions of the knee extensors evoked by NMES applied over the femoral nerve vs. the quadriceps muscle. NMES was delivered to evoke 10 and 20% of maximum voluntary isometric contraction torque 2–3 s into the NMES (time<jats:sub>1</jats:sub>) in two patterns: 1) constant frequency (15 Hz for 8 s); and 2) step frequency (15–100-15 Hz and 25–100-25 Hz for 3–2-3 s, respectively). Torque and electromyographic activity recorded from vastus lateralis and medialis were quantified at the beginning (time<jats:sub>1</jats:sub>) and end (time<jats:sub>2</jats:sub>; 6–7 s into the NMES) of each pattern. M-waves (peripheral pathway), H-reflexes, and asynchronous activity (central pathways) during NMES were quantified. Torque did not differ regardless of NMES location, pattern, or time. For both muscles, M-waves were ∼7–10 times smaller and H-reflexes ∼8–9 times larger during NMES over the nerve compared with over the muscle. However, unlike muscles studied previously, neither torque nor activity through central pathways were augmented following 100 Hz NMES, nor was any asynchronous activity evoked during NMES at either location. The coefficient of variation was also quantified at time<jats:sub>2</jats:sub>to determine the consistency of each dependent measure between three consecutive contractions. Torque, M-waves, and H-reflexes were most variable during NMES over the nerve. In summary, NMES over the nerve produced contractions with the greatest recruitment through central pathways; however, considering some of the limitations of NMES over the femoral nerve, it may be considered a good complement to, as opposed to a replacement for, NMES over the quadriceps muscle for maintaining muscle quality and reducing contraction fatigue during NMES rehabilitation.</jats:p>

Electrical Stimulation of Afferent Pathways for the Suppression of Pathological Tremor

“…Since this is not possible to achieve for all muscles with stimulation at superficial locations of mixed nerve trunks, the stimulation was delivered at the muscle belly or intramuscularly (see details in “Section Experimental Procedure”). A potential drawback of this method, however, is that stimulation at the muscle usually allows for activation of a smaller number of afferent fibers than nerve trunk stimulation (Bergquist et al, 2011, 2012). …”

Is the notion of central fatigue based on a solid foundation?

“…In the first case, motor units are activated in random order with uniform probability distribution as indicated by the empirical evidence presented by Knaflitz et al (1990) and Gregory and Bickel (2005). In the second case, motor units are activated in inverse order of physiological recruitment, i.e., in the order of decreasing recruitment threshold , as suggested by the empirical studies of Bergquist et al (2011Bergquist et al ( , 2012.…”

Contributions to muscle force and EMG by combined neural excitation and electrical stimulation

“…We simulated this pattern by recruiting motor units in random order with a uniform probability distribution. In contrast, when stimulating the peripheral nerve at the wrist, recruitment follows the reverse of the physiological size principle, i.e., large axons innervating large, fast motor units are recruited at lower thresholds than small, slow motor units (Bergquist et al, 2011, Bergquist et al, 2012). …”