Motor unit discharge rates of the anconeus muscle during high-velocity elbow extensions

Abstract

Motor unit recruitment and motor unit discharge rate (MUDR) have been widely studied in isometric conditions but minimally during velocity-dependent contractions. For isometric contractions, surface electromyography (EMG) activity of the elbow extensors plateaus at near maximal torques (Le Bozec et al. 1980; Le Bozec and Maton 1982). One study (Maton and Bouisset 1975) recorded single motor unit (MU) activity at maximal velocities; however, only the rate of the first interspike interval (ISI) was reported and likely was not representative of the average MUDR of the MU train. The purpose was to calculate average MUDRs of the anconeus during loaded velocity-dependent contractions from zero velocity (isometric) up to maximal velocity (V(max25)) through a large range of motion. A Biodex dynamometer was used to record elbow extension torque, position, and velocity. Single MU potentials were collected from the anconeus with intramuscular EMG, and surface EMG was sampled from the lateral head of the triceps brachii during maximal voluntary isometric contractions (MVCs) and velocity-dependent contractions loaded at 25% MVC over 120° range of motion at five target velocities (0, 25, 50, 75, 100%V(max25)). Elbow extension velocities ranged from 93 to 494°/s and average MUDR ranged from 11.8 Hz at 25%MVC to 39.0 Hz at 100%V(max25.) Overall average MUDRs increased as a function of velocity, although the root mean square of triceps brachii surface EMG plateaued at 50%V(max25). Piecewise regression analysis revealed two distinct linear ranges each described by a unique equation, suggesting that MUDRs of the anconeus enter a secondary range of firing, characterized by a steeper slope as velocity approaches maximum.

Changes in motor unit recruitment thresholds of the human anconeus muscle during torque development preceding shortening elbow extensions

“…In addition, the relative fascicle length of the anconeus shortens ϳ80% less during elbow extension than the triceps brachii (Harwood et al 2010;Murray et al 2000), and, as a result, there is less potential displacement of the intramuscular electrode across large ranges of motion and at high target velocities. As evidenced by the results of the present study and one other study (Harwood et al 2011), these characteristics present a useful neuromuscular model to study single MU function across a large joint range of motion, for the production of a range of velocities up to maximal velocity, and most importantly, the ability to track successfully MUs throughout repeated contractions.…”

“…However, with an increase in rate of torque development, such as that observed during an increased speed of ramp contraction (Büdin-gen and Freund 1976;Tanji and Kato 1973;Yoneda et al 1986) or during ballistic isometric contractions (Desmedt and Godaux 1977;Yoneda et al 1986), MURTs are significantly reduced (for review see Freund 1983). Models of MU behavior (Nussbaumer et al 2002), and surface EMG (Le Bozec et al 1980) and single-MU studies of the elbow extensors (Harwood et al 2011) suggest that synaptic input to the motoneuron pool is greater during a shortening compared with an isometric elbow extension, which provides the requisite activation for the generation of the high rates of torque development observed preceding high-velocity dynamic contractions (Khamoui et al 2011). Thus it was hypothesized that a reduction in MURTs as a result of either 1) a uniform MURT decrease across all sampled MUs or, alternatively, 2) a compression of the MURT range within the MU sample should occur in order to generate high-velocity elbow extensions.…”

“…The very few (Garland et al 1996;Gydikov et al 1986;Maton and Bouisset 1975) who have successfully recorded human single MU action potentials responsible for producing moderate (225°/s) to higher (ϳ500°/s) velocity contractions did not track the behavior of single MUs throughout a series of contractions, or recorded very few MU action potentials for each contraction. Moreover, none of these studies explored the potential relationship between MURTs and the resultant peak contraction velocity.Recently, Harwood et al (2011) demonstrated the ability to track single MU behavior in the anconeus muscle throughout repeated velocity-dependent elbow extensions over a full range of movement velocities. This model allowed the investigation of MU properties during previously unattainable conditions, specifically, maximal velocity contractions.…”

“…Recently, Harwood et al (2011) demonstrated the ability to track single MU behavior in the anconeus muscle throughout repeated velocity-dependent elbow extensions over a full range of movement velocities. This model allowed the investigation of MU properties during previously unattainable conditions, specifically, maximal velocity contractions.…”

“…The experimental setup and protocol have been described previously (Harwood et al 2011). Briefly, elbow extension torque, position, and velocity measures were recorded with a Biodex System 3 multijoint dynamometer (Biodex Medical Systems, Shirley, NY), while single MU action potentials from the anconeus and surface electromyography (EMG) of the lateral head of the triceps brachii (LTB) were recorded via fine-wire intramuscular and surface electrodes, respectively.…”